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Photoinduced Oxidation Reactions at the Air–Water Interface
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化学TOP简介JCI 2.66EI检索SCI升级版 化学1区SCI Q1IF 14.4

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Cite this: J. Am. Chem. Soc. 2020, 142, 38, 16140–16155
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https://doi.org/10.1021/jacs.0c06858
Published August 24, 2020
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Abstract

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Chemistry on water is a fascinating area of research. The surface of water and the interfaces between water and air or hydrophobic media represent asymmetric environments with unique properties that lead to unexpected solvation effects on chemical and photochemical processes. Indeed, the features of interfacial reactions differ, often drastically, from those of bulk-phase reactions. In this Perspective, we focus on photoinduced oxidation reactions, which have attracted enormous interest in recent years because of their implications in many areas of chemistry, including atmospheric and environmental chemistry, biology, electrochemistry, and solar energy conversion. We have chosen a few representative examples of photoinduced oxidation reactions to focus on in this Perspective. Although most of these examples are taken from the field of atmospheric chemistry, they were selected because of their broad relevance to other areas. First, we outline a series of processes whose photochemistry generates hydroxyl radicals. These OH precursors include reactive oxygen species, reactive nitrogen species, and sulfur dioxide. Second, we discuss processes involving the photooxidation of organic species, either directly or via photosensitization. The photochemistry of pyruvic acid and fatty acid, two examples that demonstrate the complexity and versatility of this kind of chemistry, is described. Finally, we discuss the physicochemical factors that can be invoked to explain the kinetics and thermodynamics of photoinduced oxidation reactions at aqueous interfaces and analyze a number of challenges that need to be addressed in future studies.

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1. Introduction

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Chemical processes are sensitive to the molecular environments where they take place, including the solvent, the protein active site, or the surface of a material. The environment influences the relative stability of the species along the reaction path. In the case of liquids, the solvation power and the nature of the reaction system determine how different species are stabilized, and the differential stability may retard or accelerate the reaction process. Moreover, environmental factors can also modify the regioselectivity or stereoselectivity, the endo-/exothermicity, or even the mechanism. (1)
When reactions occur at the surface of liquid water or at the interface of liquid water with a hydrophobic medium, the asymmetry of the interface, the presence of free water OH groups, and the distinctive properties of interfacial water often lead to unexpected solvation effects on chemical reactions, and the resulting properties are not necessarily intermediate between the two bulk phases. (2) For instance, a variety of thermal reactions occurring at aqueous interfaces have been found to be significantly accelerated compared to the bulk reactions; this effect is referred to as “on-water catalysis”. (3) The reaction rate acceleration can stem from an increase in reactant concentration (molecular crowding) or the stabilization of a transition state (or a combination of both); however, a full understanding of this mechanism remains lacking. The enhancement of redox reactions has been broadly studied in this context, with prominent examples including H-abstraction by OH, (4) ozonolysis (5−11) and ozonations, (12) Fenton and Fenton-like reactions, (13) Dakin and Baeyer–Villiger oxidations, (14) halide anion oxidation by OH and O3, (15) and the spontaneous formation of hydrogen peroxide (14) and spontaneous reduction of organic molecules (16) at the neat water surface.
This Perspective deals with the special case of photoinduced oxidation reactions at aqueous interfaces. Photoredox chemistry covers a broad range of processes with great atmospheric and environmental relevance. (17,18) These processes are also key in the fast-developing field of photocatalysis, (19) which in a nonrestrictive sense connects multiple disciplines such as synthetic organic chemistry to biochemistry, electrochemistry, water purification technologies, and solar energy conversion. A review of the literature related to all these fields obviously goes beyond the scope of the present report; however, the roles of liquid water interfaces in reaction kinetics, dynamics, and thermodynamics are highly relevant in all the above fields.
In the atmosphere, gas-phase photochemistry is fundamentally associated with photooxidation by OH radicals and other reactive oxygen species (ROS). (20,21) The main source of tropospheric OH is the photolysis of ozone via the absorption of ultraviolet (UV) light in the presence of water vapor, although several other routes can lead, directly or indirectly, to the formation of OH radicals. (22) ROS react with other compounds to initiate and propagate most tropospheric chemical reactions. A simplified scheme illustrating the complexity of the tropospheric chemistry of ROS in the gas phase is shown in Figure 1. It shows the set of coupled reactions that constitute the sources and sinks of reactive oxygen species and the crucial role played by ozone and hydroxyl radicals. On the other hand, the aqueous-phase chemistry of ROS is of cross-cutting importance in biology, environmental science, and water treatment, for instance. (23)

Figure 1

Figure 1. Gas-phase atmospheric reactions involving reactive oxygen species (ROS). Reproduced with permission from ref (23). Copyright 2015 American Chemical Society.
图 1.涉及活性氧 （ROS） 的气相大气反应。经参考文献（23）许可转载。版权所有 2015 美国化学学会。

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Interest in the direct photochemistry of organic compounds in the atmosphere has grown significantly in recent years in parallel with the development of photocatalysis. Most compounds directly emitted to the atmosphere (e.g., saturated hydrocarbons, monoterpenes, and alcohols) do not absorb sunlight available in the troposphere (i.e., beyond the cutoff of 290 nm) and therefore do not undergo significant direct photochemical reactions. However, their oxidation products can be photochemically active. For instance, methyl hydroperoxide (CH3OOH) is an important trace gas in Earth’s atmosphere that is mainly produced by the oxidation of methane, the ozonolysis of alkenes, and biomass burning. CH3OOH has an electronic transition band that is centered at 200 nm but extends beyond the tropospheric cutoff, leading to photodissociation into OH and CH3O radicals; the latter can react with O2 to form HO2 and formaldehyde. (24,25) Other species produced via the oxidation of primary organic compounds [e.g., pyruvic acid (PYA) and certain imidazole derivatives] and compounds emitted to the atmosphere through combustion processes (e.g., polycyclic aromatic hydrocarbons and aromatic ketones) are also photoactive. The excited states of these species can initiate chemical processes through interactions with other organic compounds that are not themselves photoactive. In these photoinduced processes, a chromophore called a photosensitizer absorbs light and transfers the excess energy to a nearby molecule that is not naturally photoactive, eventually initiating the oxidation chain. Commonly, the photosensitizer absorbs light to a singlet excited state, which then evolves to a triplet state via intersystem crossing (ISC) with a large quantum yield. The higher lifetime of the triplet compared to the singlet facilitates encounters and reaction with other molecules. Overall, two main types of photosensitization processes occur in the atmosphere (18) depending on how the excited photosensitizer is quenched. The quencher can be an organic molecule (Type I) or triplet oxygen (Type II), as shown in Figure 2. In the first case, an electron-transfer or H-abstraction reaction results in the creation of a new organic radical that propagates the oxidation process. In the second case, the photosensitizer reacts with In the first case, an electron-transfer or H-abstraction reaction results in the creation of a new organic radical that propagates the oxidation process. In the second case, the photosensitizer reacts with 3O2, either by transferring the excess energy to form the reactive excited singlet , either by transferring the excess energy to form the reactive excited singlet 1O2 or by electron transfer to form the superoxide anion Oor by electron transfer to form the superoxide anion O2–. In principle and according to the IUPAC definition, a photosensitization process is one in which the photosensitizer is not consumed in the reaction. From this point of view, photosensitization and photocatalysis can be considered synonyms. However, there are processes in which the photosensitizer molecule is also a reactant; in such a case, the molecule is referred to as a photoinitiator.. In principle and according to the IUPAC definition, a photosensitization process is one in which the photosensitizer is not consumed in the reaction. From this point of view, photosensitization and photocatalysis can be considered synonyms. However, there are processes in which the photosensitizer molecule is also a reactant; in such a case, the molecule is referred to as a photoinitiator.

Figure 2

Figure 2. (a) Schematic representation of a Type I sensitization process involving the degradation of phenols via a reaction with an excited carbonyl compound as photosensitizer. (b) Comparison of Type I and Type II photosensitization processes. Reproduced with permission from ref (18). Copyright 2012 American Chemical Society.
图2.（a） I型敏化过程的示意图，该过程涉及通过与作为光敏剂的激发羰基化合物反应来降解酚类物质。（b） I型和II型光敏过程的比较。经参考文献（18）许可转载。版权所有 2012 美国化学学会。

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While all the above processes can occur in the gas phase, the importance of multiphase chemistry involving surfaces and interfaces of different natures (e.g., mineral dust, liquid water and ice surfaces, aerosols, and urban surfaces) has also been established. (17,18,26,27) This Perspective first presents some studies that found enhancements in these photoinduced processes at the air–water interface, which is illustrated by some selected examples. We then discuss the different factors that can lead to modifications of the reaction properties. Finally, we present some perspectives in the field.

2. Photochemistry of OH Precursors

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In this section, we present a few select examples to describe the photochemistry of simple precursors that generate OH radicals (and possibly other ROS), which then trigger the oxidation of more complex compounds present in the environment. These reaction pathways are key in atmospheric and environmental chemistry, and the examples given below refer essentially to these fields. The production of OH originates from the direct photolysis of the precursor (hydroperoxides, HNO3) or from hydrogen abstraction from water molecules, either by an appropriate precursor excited state (SO2*, NO2*) or one of its photolytic products (O3).

2.1. ROS

Ozone, hydrogen peroxide, and methyl hydroperoxide are among the main ROS. (23) They are strong oxidants with relevance in atmospheric chemistry, biological processes, and wastewater treatment in peroxone chemistry. (17,21,23,24,28−37) In the stratosphere, ozone absorbs solar radiation, protecting living organisms from harmful UV radiation; in the troposphere, ozone acts as a greenhouse gas and a pollutant, damaging the respiratory tissues of animals. (38) Hydrogen peroxide and methyl hydroperoxide are prototypical hydroperoxides that are mainly formed via the self-reaction of HO2 radicals and the oxidation of methane, respectively. (24,25,39) These hydroperoxides are reservoirs of hydroxyl radicals and are found in rainwater and aerosols. (40−42)
One of the main roles of tropospheric ozone is to serve as an important source of hydroxyl radicals. Ozone photolysis produces O(1D), which reacts with water vapor to form two OH radicals (reactions 1 and 2). (43) In the gas phase, the measured photolysis rate constant for these reactions is 3.8 × 10–5 s–1, (44) while calculations produced a value of 3.2 × 10–5 s–1. (45) It should be noted that both of these values assume sunlight actinic flux, as will be considered from now on in this work.
(1)
(2)
(3)
Anglada and co-workers (27,45) carried out quantum mechanics/molecular mechanics molecular dynamics (QM/MM-MD) simulations to investigate the spectrum of ozone at the air–water interface and found a red shift and shape broadening compared to the gas phase. They also predicted the photolysis rate constant to be 1 order of magnitude greater at the interface than in the gas phase ((1.1–7.6) × 10–4 s–1) when the contribution of reaction 3 was taken into account. (45) Moreover, simulations (45−47) have shown that ozone pile up at the air–water interface. For instance, for a typical gas-phase concentration of 7.5 × 1011 molecules·cm–3, an average concentration of 9.6 × 1012 molecules·cm–3 in the interfacial layer is estimated. This corresponds to a predicted increase of approximately 4 orders of magnitude in the rate of OH radical formation at the interface, which has important atmospheric implications. (45)
In contrast to ozone, the photolytic process of hydroperoxides (reactions 4 and 5) produces OH radicals directly. In the case of alkyl derivatives like CH3OOH, OH radicals are also produced by the reaction of alkoxy radicals with water (reaction 6).
(4)
(5)
(6)
The absorption cross-sections and quantum yields of H2O2 and CH3OOH have been reported in the gas phase, (48−52) in aqueous solution, and in snow ices. (53−58) The reported photolysis rate constants for H2O2 are 4.8 × 10–6 s–1 in the gas phase and 1.5 × 10–5 s–1 in aqueous solution. (59) For CH3OOH, the rate constants are 1.3 × 10–6 and 1.1 × 10–6 s–1 in the gas phase (51) and 4.5 ± 1.0 × 10–5 s–1 in aqueous solution. (58)
Hydrogen peroxide is much more soluble in water than methyl hydroperoxide (the Henry’s law coefficients are 1.0 × 105 and 3.0 × 102 mol·dm–3·atm–1, respectively (59)). However, according to molecular dynamics (MD) and quantum mechanics/molecular mechanics (QM/MM) calculations, (46,60−62) both species accumulate at the air–water interface, leading to an increase in concentration that can enhance the importance of interfacial photochemistry. (46,62) However, few works have focused on the photochemistry of these species at the air–water interface, (27,62,63) and the results are contrasting. On one hand, Nissenson and co-workers (63) studied the photochemistry of H2O2 in water with a focus on cage effects on the photolysis quantum yields upon the addition of 2-propanol as an OH scavenger. The authors estimated the photolysis rate constant at the surface to be 9.3 × 10–8 s–1, much smaller than the experimental values reported by other authors for the gas phase and in the bulk, (59) although no discussion on this issue was provided. On the other hand, Martins-Costa and co-workers (62) conducted QM/MM MD calculations to study the photolysis of CH3OOH at the air–water interface. The calculated UV–visible spectra reveal a significant blue shift of the main absorption band corresponding to the interface (0.42 eV). This blue shift is attributed to the hydrogen-bond interactions between methyl hydroperoxide and the water molecules. While one would expect this blue shift to significantly decrease the photolytic rate constant, the calculations also show a broadening of the absorption band corresponding to the air–water interface that results in comparable cross-sections in the red part of the band compared to the gas phase (Figure 3Figure 3b), the photolysis rate constant at the interface (JCH3OOH = 0.76 × 10–6 s–1) is only slightly smaller than that in the gas phase (JCH3OOH = 1.05 × 10–6 s–1) All in all, the rate of OH production rate has been shown to be much higher at the interface than in the gas phase, primarily due to the higher concentration of methyl hydroperoxide in this environment and, to a minor extent, the contribution of reaction 6.
过氧化氢比甲基氢过氧化物更易溶于水（亨利定律系数分别为 1.0 × 105 和 3.0 × 102 mol·dm–3·atm–1 (59)）。然而，根据分子动力学（MD）和量子力学/分子力学（QM/MM）计算，（46,60−62）两种物质都在空气-水界面处积累，导致浓度增加，从而增强了重要性界面光化学。 (46,62) 然而，很少有工作关注这些物种在空气-水界面的光化学，(27,62,63) 并且结果是相反的。一方面，Nissenson 和同事 (63) 研究了水中 H2O2 的光化学，重点关注添加 2-丙醇作为 OH 清除剂后笼效应对光解量子产率的影响。作者估计表面的光解速率常数为 9.3 × 10–8 s–1，远小于其他作者报告的气相和本体的实验值，(59)，尽管没有对此问题进行讨论假如。另一方面，Martins-Costa 和同事 (62) 进行了 QM/MM MD 计算，以研究 CH3OOH 在空气-水界面的光解作用。计算的紫外-可见光谱显示与界面相对应的主吸收带发生显着的蓝移（0.42 eV）。这种蓝移归因于甲基氢过氧化物和水分子之间的氢键相互作用。虽然人们预计这种蓝移会显着降低光解速率常数，但计算还显示，与空气-水界面相对应的吸收带变宽，导致与气体相比，吸收带红色部分的横截面相当阶段（图3a）。结果（见图3b），界面处的光解速率常数（CH3OOH = 0.76 × 10–6 s–1）仅略小于气相中的光解速率常数（CH3OOH = 1.05 × 10–6 s–1）总而言之，界面处的 OH 生成速率比气相中的要高得多，这主要是由于该环境中甲基氢过氧化物的浓度较高，并且在较小程度上是由于反应 6 的贡献。关闭

Figure 3

Figure 3. (a) Calculated absorption cross-sections of CH3OOH in the gas phase and at the air–water interface along with different experimental values reported for the gas phase. (b) Calculated partial photolysis rates for CH3OOH. Integrated total rates are indicated. Reproduced from ref (62) with permission from the PCCP Owner Societies.

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2.2. Reactive Nitrogen Species (RNS)

RNS are key compounds in atmospheric and biological chemistry. Like ozone, nitrogen dioxide is a target compound for measuring atmospheric pollution worldwide. Nitrogen dioxide may cause serious health problems by reducing immunity to lung infections. Reactions 7–17 and Figure 1 summarize the most relevant redox processes involving oxygenated nitrogen species; these processes are strongly interwoven. (22,25,43,64)
(7)
(8)
(9)
(10)
(11)
(12)
(13)
(14)
(15)
(16)
(17)
Reaction 7 generates atomic oxygen, which reacts with molecular oxygen (reaction 8) producing atmospheric ozone in gas phase, while reaction 9 regenerates NO2. Reactions 10 and 11 are precursors of OH radicals in the gas phase, which is formed via reaction 12. (65) The hydrolysis of NO2 (reaction 13) has been suggested to occur via heterogeneous processes (64) that produce HONO and HNO3, and the photolysis of nitrous acid is a major source of hydroxyl radicals in the atmosphere (reaction 14). (22) The reaction of nitrogen dioxide with peroxy radicals (reaction 15) produces RO2NO2, an atmospheric reservoir of NO2 that can be transported and released by photolysis through reaction 16. (66) Finally, nitrogen dioxide can react with hydroxyl radicals to produce nitric acid (reaction 17). (25)
The photochemistry of nitrogen dioxide adsorbed on different interfaces has been studied. QM/MM MD simulations at the air–water interface predict an excess concentration of approximately ten times relative to the gas phase (67,68) along with a significant enhancement in the absorption cross-section, (68) as plotted in Figure 4a). The calculated photolysis rate constants for reaction 7 are J7 = 10.8 × 10–3 s–1 in the gas phase (the experimental value (69) is 8.8 × 10–3 s–1) and J7 = 15.3 × 10–3 s–1 at the air–water interface. For reaction 10, the calculated photolysis rate constants are similar for both environments (J10 = 2.2 × 10–2 s–1 in the gas phase and 2.5 × 10–2 s–1 at the air–water interface), and the estimated OH production rate at the air–water interface as a function of the NO2 concentration (Figure 4b) is approximately 2 orders of magnitude larger than in the gas phase (for details, see ref (68)).
研究了吸附在不同界面上的二氧化氮的光化学。空气-水界面处的 QM/MM MD 模拟预测相对于气相 (67,68) 的浓度过量约十倍，同时吸收截面显着增强 (68)，如图 4a) 所示。计算得出的反应 7 的气相光解速率常数为 7 = 10.8 × 10–3 s–1（实验值（69）为 8.8 × 10–3 s–1）和 7 = 15.3 × 10–3 s–1 1 空气-水界面。对于反应 10，两种环境下计算的光解速率常数相似（气相中 10 = 2.2 × 10–2 s–1，空气-水界面处 2.5 × 10–2 s–1），并且估计的 OH空气-水界面处的生成速率与 NO2 浓度的关系（图 4b）比气相中的生成速率大约大 2 个数量级（详细信息请参见参考文献（68））。关闭

Figure 4

Figure 4. (a) Experimental UV spectrum of NO2 in the gas phase and calculated spectra in the gas phase and at the air–water interface. (b) Estimated upper and lower limits of the OH production rate (molecule·cm–3·s–1) for different gas-phase concentrations of NO2 (molecule·cm–3). Calculations in the gas phase (light gray) and at the air–water interface (dark gray) using k12 from ref (70). The plain and dashed red lines respectively correspond to the gas-phase and interfacial values using k12 from ref (71). The gas-phase values assume a relative humidity of 20%. The horizontal plain line represents a typical OH production rate from ozone photolysis. Reproduced with permission from ref (68).
图4.（a） NO2在气相中的实验紫外光谱以及在气相和空气-水界面处计算的光谱。（b） 不同气相浓度NO2（分子·cm–3）下OH产速率（molecule·cm–3·s–1）的估计上限和下限。在气相（浅灰色）和空气-水界面（深灰色）使用参考文献（70）中的k12进行计算。红线和红色虚线分别对应于参考文献（71）中使用k12的气相值和界面值。气相值假设相对湿度为 20%。水平素线代表臭氧光解产生的典型 OH 率。经参考文献（68）许可转载。

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Further interesting results are available regarding the photochemistry of NO2 adsorbed on other surfaces. Adsorbed NO2 releases significant amounts of HONO. Thus, the irradiation of NO2 in the range of 300–700 nm on organic surfaces containing phenols, aromatic ketones, humic acid, aromatic and polycyclic aromatic hydrocarbons, or even urban grime (72−77) results in HONO formation in both the UV-A spectral region and in the visible region under atmospheric conditions. (73) The observed flux densities of HONO in these processes range between 3.2 × 109 and 5 × 1010 molecules·cm–2·s–1. (73,77) In these processes, radiation is not directly absorbed by NO2. Instead, the radiation is absorbed by a photosensitizer (e.g., 4-benzoylbenzoic acid), which then reacts with an organic compound on the surface (e.g., phenols) to form a reductant (e.g., phenolate) capable of converting NO2 to NO2–. Finally, the protonation of NO2– produces HONO. (72,73)
The photochemistry of nitric acid at the air–water interface is also an interesting process that has raised many questions. In the troposphere, HNO3 has long been considered as a sink of NOx species; however, recent studies have stressed its role in chemical (78) and photochemical (79−86) renoxification processes that generate HONO, NO, and NO2. Thus, nitric acid may be a reservoir of nitrogen oxides as well as a sink. Here, we focus on its photolysis, which directly produces hydroxyl radicals in the gas phase (reaction 18). When nitric acid is dissolved in water, it mainly exists in the form of nitrate, and its photolysis produces O– radicals, which react with water to form OH (reactions 19–21).
(18)
(19)
(20)
(21)
The rate constants for HNO3 photolysis in the gas phase (reaction 18) and nitrate photolysis in bulk water (reactions 19 and 20) are very small (J18 = 8.2 × 10–7 s–1, (69)J19 = 8.5 × 10–7 s–1, J20 = (4.5–8.5) × 10–8 s–1). (53,54,69,83,87,88) Note that the NO2– formed in reaction 20 can either be protonated, releasing HONO, or photolyzed (J21 = 1 × 10–4 s–1). (63,87,89,90) However, photolysis at different interfaces is far more important. On one hand, field observations have shown that the photolysis of HNO3 adsorbed on the forest canopy surface is an important source of HONO. (82) On the other hand, experiments in thin water films containing nitrate, chlorine, and bromine salts have shown that the photolysis of NO3– occurs much faster at interfaces than in the bulk, (63,81,90−94) with J values ranging between 1.2 × 10–3 and 3 × 10–6 s–1, depending on the features of the interface. (63,83,95,96) A potential explanation for this enhancement is associated with the solvation of nitrate anion. In bulk water, nitrate is surrounded by a solvent cage that facilitates both the recombination of the NO2 and OH radicals produced by reaction 19 and their deactivation by collision with solvent molecules, thus decreasing the photolysis quantum yield. (63,87,89,90) MD simulations have shown that when halide anions (especially bromide) are present, nitrate ions are dragged closer to the interface so that the water solvent cage surrounding them is reduced, (90,97) arguably making the escape of NO2 to the gas phase easier. (63,81,90−94) Nevertheless, despite the large amount of work done on nitrate photochemistry, open issues remain that are not yet completely understood. For instance, it was recently shown that an important part of nitric acid remains undissociated at the air–water interface, (81,98−106) and the relevance of the corresponding photochemistry is still undetermined.

2.3. Sulfur Dioxide (SO2)

The ground-state chemistry of SO2 has been broadly studied. (107−110) Generally, SO2 is oxidized in the atmosphere to higher sulfur oxides or sulfuric acid. SO2 is emitted by the burning of fossil fuels, industrial processes, ships and other vehicles, and volcanic eruptions. It is eliminated by dry and wet deposition, which contributes to particulate matter and aerosol formation along with acid rain. Exposure to SO2 or particulate matter harms the human respiratory system and makes breathing difficult. (69,111−113) The atmospheric concentrations of SO2 are extremely variable, with mixing ratios ranging from 20 ppt to over 1 ppb in continental background air, 20–50 ppt in the unpolluted marine boundary layer, several hundreds of ppb in urban areas, and much higher in volcanic gases. (111)
Recent studies have shown that SO2 can act as an oxidizer via the photochemistry of its triplet excited state. Upon the UV irradiation of a mixture of SO2 and water vapor without any other oxidants, Donaldson and co-workers (114) observed the formation of new particles, which Kroll and co-workers (115) ascribed to the formation of HOSO and OH radicals according to reactions 22–26. UV light excites SO2 into its first singlet electronic state that evolves to a triplet electronic state via ISC. This triplet electronic state is quite reactive and abstracts one hydrogen atom from water, producing HOSO and OH radicals. In this mechanism, SO2 behaves as a photoinitiator.
(22)
(23)
(24)
(25)
(26)
The reaction between the excited triplet electronic states of SO2 and H2O (reaction 24) may follow two different reaction paths, (116) for which the potential energy surfaces are schematized in Figure 5a. These pathways correspond to either a conventional hydrogen atom transfer (HAT) or a proton-coupled electron transfer (PCET), which requires lower activation energy.

Figure 5

Figure 5. (a) Schematic of the free energy surface at 298 K for the reaction of SO2(a 3B1) with one water molecule. (b) Schematic free energy surface at 298 K for the reaction of SO2(a 3B1) with a cluster of four water molecules. Reproduced from ref (116) with permission from the PCCP Owner Societies.
图5.（a） SO2（a 3B1）与一个水分子反应的298 K自由能表面示意图。（b） SO2（a 3B1）与四个水分子簇反应时在298 K下的自由能表面示意图。转载自参考文献（116），经PCCP业主协会许可。

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The most relevant electronic features describing these processes are collected in Figure 6. The PCET mechanism is described by a two-center three-electron structure, where the single-occupied 8a1 orbital of SO2 interacts with the lone pair [lp(O)] of water in a way that the electron density in the doubly occupied orbital 20a is shared between the oxygen atom of the water moiety and the terminal oxygen atom of SO2. In contrast, in the single-occupied orbital 22a, the antibonding combination is formed. This situation depicts an electron transfer from the oxygen atom of water to one oxygen atom of SO2; at the same time, one proton jumps from water to the other oxygen atom of SO2.

Figure 6

Figure 6. Orbital diagram for the PCET and HAT mechanisms for the reaction SO2(a 3B1) + H2O. Reproduced from ref (116) with permission from the PCCP Owner Societies.
图6.反应 SO2（a 3B1） + H2O. 的 PCET 和 HAT 机理的轨道图，经 PCCP 所有者协会许可，转载自参考文献 （116）。

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In the HAT mechanism, the single-occupied 8a1 orbital of SO2 interacts with one of the σ(O–H) bonds of water to form the doubly occupied orbital 20a with bonding character and the single-occupied orbital 22a with antibonding character. This situation also corresponds to a three-center three-electron system; however, in this case, the electronic density lies over the O–H–O moiety, indicating the simultaneous breaking and forming of the covalent bonds (OS)O–H–O(H). Indeed, elementary reactions occurring through a PCET mechanism in the oxidation of organic and inorganic species by radicals have been found to be more favorable than oxidation processes occurring through the HAT mechanism. (117−127)
The reaction of SO2 (3B1) with up to four water molecules (116) (Figure 5b) sheds light on the microsolvation effect on this process and can be considered as a model for reactivity at the air–water interface, where, as for the other species discussed above, SO2 accumulates. (128−133) Indeed, the free energy barrier of the PCET mechanism is quite small (0.70 kcal·mol–1), suggesting that the reaction could be spontaneous at the interface, making this photochemical mechanism a significant source of OH radicals, as confirmed by further QM/MM simulations. (133) For instance, for a relative humidity of 100% and a gas-phase SO2 concentration of 1012 molecules·cm–3, which is typical of urban areas, the rate of formation of OH radicals is estimated to be approximately 105 molecules·cm–3·s–1 in the gas phase. In comparison, the rate reaches 7 × 108 molecules·cm–3·s–1 when the process occurs at the air–water interface for the same gas-phase concentration. (133)
Beyond the formation of OH radicals, another outcome of the interfacial photochemistry of SO2 is the acidification of the medium because HOSO is quite acidic (pKa = −1) and undergoes fast ionic dissociation at the interface, as shown by QM/MM MD simulations (see Figure 7). (134) Moreover, the formed SO2– ions can be further oxidized by H2O2, O3, OH, or HO2 to produce sulfuric acid. (134)

Figure 7

Figure 7. Different steps of the photo-oxidation of SO2 leading to sulfate. QM/MM MD simulations at the air–water interface reveal that as HOSO radical is formed, it rapidly ionizes. This is shown in the bottom part of the figure by the time evolution of OH distances in the H2O···HOSO system (bottom, right). The structure of the transition structure for proton transfer is also displayed (bottom, left). Adapted with permission from ref (134).
图7.SO2光氧化导致硫酸盐的不同步骤。在空气-水界面的QM/MM MD模拟表明，随着HOSO自由基的形成，它迅速电离。这在图的底部通过H2O···HOSO系统（右下）。还显示了质子转移的过渡结构的结构（底部，左侧）。经参考文献（134）许可改编。

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3. Direct Photooxidation of Organic Species

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The photooxidation of organic species at the aqueous interface represents a vast field of research and turns out to be much more complex than the chemistry of OH precursors. To some extent, the role of organic radicals formed in the early stages of the photochemical process can be compared to the role played by the oxidizing OH radicals in overall atmospheric chemistry. As noted in the introduction, this photochemistry generally involves the presence of photoinitiators or photosensitizers. Such reactions are also enormously important in atmospheric and environmental chemistry because organic matter accumulates at the surfaces of cloud water droplets and aerosols as well as on surface waters, lakes, rivers, and, most importantly, oceanic surface microlayers, which are enriched with organic matter such as humic acid, fatty acids, amino acids, proteins, lipids, and other biogenic materials. (135−139) To illustrate these processes, we have chosen two examples that have received considerable attention in recent publications.

3.1. Pyruvic Acid (PYA), a Versatile Photoinitiator

An intriguing case study is PYA, which has major biological significance and is also relevant in atmospheric chemistry as a product of the atmospheric oxidation of isoprene. (140) PYA is detected in clouds and aerosols (141) and is important in prebiotic chemistry. (139) The photochemistry of PYA has been investigated in different environments as a model of α-dicarbonyl compounds. A unique feature of PYA is that its photochemistry is totally different in the gas phase than in aqueous environments.
In the gas phase, the main photolytic process starts with absorption to the first excited singlet electronic state (n→π* transition, λmax = 350 nm), which induces PYA cleavage into carbon dioxide and methyl hydroxycarbene (reaction 27) via hydrogen transfer from the acidic group to the keto oxygen atom. (142,143) Excitation at higher energies (up to λ = 300 nm) leads to the formation of acetyl and carboxyl radicals (reaction 28). (144)
(27)
(28)
In the aqueous phase, the photolysis is far more intricate than in the gas phase, and the mechanisms are totally different. A first challenge is the coexistence of different forms in aqueous solution, namely PYA and its conjugated base pyruvate (pKa = 2.45), the corresponding hydrated gem-diols (PYT), and oligomeric species such as parapyruvic acid (PPA) and zymonic acid (ZYA; see Figure 8). PYA may also undergo keto–enol equilibria. (145)

Figure 8

Figure 8. Chemical structures of pyruvic acid, pyruvate, the corresponding gem-diols, parapyruvic acid, and zymonic acid.
图8.丙酮酸、丙酮酸、相应的 gem-diols、副丙酮酸和酶酸的化学结构。

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Several works have investigated PYA photochemistry in water under highly acidic conditions (pH = 1–2.5). (139,146−156) Absorption to the single excited state [1S(n,π*)] is blue-shifted (λmax = 320 nm) with respect to gas phase. (139) The excited singlet then undergoes ISC with the lowest-lying triplet excited state [3T(n,π*)], which becomes the reactive species (see Figure 2). This triplet state can react with another PYA or PYT molecule in its ground electronic state through a mechanism that involves either the abstraction of the acidic hydrogen atom via proton-coupled electron transfer (146) or a concerted hydrogen atom abstraction. (147) This initiates a chain of reactions that cause decarboxylation, the formation of oligomers, dimethyltartaric acid, acetoin, acetic and lactic acids, and oxo-C8 and C7 products. (26,146,147,157) However, controversy exists regarding the entire mechanism and the formation of several products. (147−152) As the pH increases, the gem-diol form diminishes, and the pyruvate concentration grows, resulting in an increase in absorption intensity and a moderate blue shift (λmax = 316 nm). (158) Another mechanism then becomes competitive: the triplet excited state of PYA abstracts one of the hydrogen atoms of the methyl group (instead of the acidic group) from the ground electronic state of another PYA molecule. The main oligomeric species formed in this mechanism are PPA and ZYA. (153) Some of the newly formed compounds (PPA in particular) can act as secondary photosensitizers that contribute to or initiate the formation of oligomers (26,153,158) and participate in cross-reactions with glyoxylic acids, fatty acids, and fatty alcohols. (153,154,159) Interestingly, PYA has been shown to behave as a photosensitizer, affecting the atmospheric multiphase reactions of ozone with oxalic acid. (160)
The enormous versatility of this bulk photochemistry is also expected at the air–water interface. However, the interfacial mechanisms remain poorly understood. Using vibrational sum-frequency spectroscopy, Gordon and co-workers (161) identified the oligomers PPA and ZYA at the air–water interfaces of PYA solutions, demonstrating that these compounds are surface active. Additionally, they performed MD simulations in the bulk and at the air–water interface; the corresponding density profiles are collected in Figure 9. Interestingly, the results indicate the depletion of PYT and the accumulation of PYA, PPA, and ZYA at the air–water interface, which suggests that the enhanced photoreactivity of the interface might boost oligomerization reactions. Moreover, Eugen and co-workers (162) demonstrated a reduction of 1.8 units in the effective pKa of PYA at the air–water interface, which would increase the undissociated fraction of PYA and further enhance the photoreactivity at the air–water interface.

Figure 9

Figure 9. Density profiles from MD simulations of a mixed aqueous solution containing PYA (red), PYT (blue), PPA (green), and ZYA (yellow). The concentration of each species was ∼0.25 M. The vertical dotted gray line indicates the approximate interface boundary. Reproduced with permission from ref (161).

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Very recently, Fu and co-workers (163) employed time-of-flight secondary ion mass spectrometry (SIMS) to investigate the products of PYA photolysis at the air–water interface and in bulk water. The SIMS analyses were carried out with dry samples, and inevitable drying effects may have influenced the results of the study. However, the authors established a reaction flowchart, which is displayed in Figure 10. The radical, decarboxylation, esterification, and anhydride formation reactions have been identified. In addition, at the interface, oxygen from the air forms other oxidation products. As a rule, it is observed that radical reactions and the formation of high-molecular-weight compounds are enhanced at the air–water interface.

Figure 10

Figure 10. Reaction flowchart showing reactions in bulk water and at the air–water interface. Numbers on arrows indicate reaction types: (1) radical reactions, (2) decarboxylation reactions, (3) anhydride formation, and (4) esterification. Red squares indicate new products identified by the authors, and chemical formulas correspond to undetermined or to too-large structures. Reproduced with permission from ref (163).
图 10.反应流程图显示了在散装水中和空气-水界面上的反应。箭头上的数字表示反应类型：（1）自由基反应，（2）脱羧反应，（3）酸酐形成和（4）酯化反应。红色方块表示作者确定的新产品，化学式对应于未确定或太大的结构。经参考文献（163）许可转载。

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3.2. Fatty Acids

Another illustrative example is the photoinduced reactivity of fatty acids and fatty alcohols initiated by photosensitizers in chromophoric dissolved organic matter (CDOM). (164−166) Imidazole 2-carboxaldehyde, 4-benzoylbenzoic acid, and humic acids have been widely studied as suitable proxies for CDOM photosensitizers. This represents an important environmental photooxidation chemistry, especially at the sea surface microlayer and interfacial bubbles at the sea surface, which are enriched by many amphiphilic organic compounds with biogenic origins. Fatty acids have been identified in these organic layers, and photosensitizers such as those cited above show a propensity for partitioning to them. (165)
The starting mechanism involves the formation of a triplet electronic state via ISC from the excited singlet state reached by the absorption of sunlight by the photosensitizer. The triplet state then reacts with the organic species at the air–water interface, usually via the abstraction of one hydrogen atom or through electron transfer. In this way, two radicals are formed and initiate a chain of complex chemical reactions that has been thoroughly described for nonanoic acid as a fatty acid proxy. (165−167) The proposed mechanism with humic acid as the photosensitizer is displayed in Figure 11. (166)

Figure 11

Figure 11. Proposed mechanism for the photochemical degradation of nonanoic acid at the air–water interface in the presence of humic acid as photosensitizer. Reproduced with permission from ref (166). Copyright 2016 American Chemical Society.
图 11.提出了在腐殖酸作为光敏剂存在下壬酸在空气-水界面上光化学降解的机理.经参考文献（166）许可转载。版权所有 2016 美国化学学会。

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Figure 11 shows that the triplet excited state of humic acid (3HA) abstracts one hydrogen atom of nonanoic acid via the photosensitizer, unleashing a chain of processes involving dimerization, disproportionation, cross-reactions, and decarboxylation. Moreover, the availability of O2 from the gas phase and water molecules at the air–water interface promotes reactions such as the addition of O2 to form peroxide radicals, allowing the formation of other oxidized products such as hydroxyacids.
Despite organic acids does not absorb light in the actinic range, recent experiments (167) have indicated the presence of radical reactions when a monolayer of nonanoic acid at the water surface is irradiated in the region of 280–330 nm in the absence of any additional photosensitizer. Although a low rate was observed, this study suggests that nonanoic acid itself acts as a photosensitizer at the air–water interface. (167) According to this study, the absorption of radiation by nonanoic acid leads to a triplet state that can dissociate into radicals or react with another fatty acid molecule to from a diol radical. However, other studies on this reaction with different experimental setups could not confirm this mechanism and concluded that if this mechanism exists, it is of minor relevance compared to processes triggered by photoinitiators. (138,159,164−166)

4. Discussion

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In the preceding section, we briefly described a few representative examples of photooxidation reactions at the air–water interface. These and other studies (17,18,27,168) on related systems reveal that the photochemical reactivity at the interface may differ considerably from those in both the gas phase and the bulk. Despite this large body of work, a full understanding of the solvation effects on interfacial photochemistry is not yet available. Hereafter, we briefly discuss some of the thermodynamic and kinetic factors (see Scheme 1) that can be advanced to explain the observed effects but that still require verification through further experiments and calculations.

Scheme 1

Scheme 1. Factors Affecting Photoinduced Reactions at the Air–Water Interface
方案 1.影响空气-水界面光致反应的因素
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The prominent role played by aqueous interfaces in atmospheric and environmental photochemistry has several explanations. The most obvious one is the amphiphilic character of a large fraction of organic matter and chromophoric compounds. The biogenic surfactants mentioned earlier are an emblematic example as they are ubiquitous in environmental interfaces. (137,168,169) Indeed, the photochemistry of microlayers at the ocean surface is a major source of volatile organic compounds, which compete with emissions from marine biology. (170) In addition, the accumulation of organic matter at the ocean’s surface acts as a solar filter that changes the intensity and spectral distribution of the radiation that penetrates into the inner water layers, thereby modifying the photochemistry there. (168) However, accumulation at aqueous interfaces is not reserved to surfactants, and both simulations and experiments have clearly demonstrated that many small polar molecules and even soft anions, including important oxidants from the ROS/RNS chemical families, display affinity for interfaces. (45,46,171) The accumulation of organic matter, chromophores, and oxidants in the same location therefore represents an important driving force of photooxidation chemistry at environmental aqueous interfaces.
Nevertheless, the solvation power of the air–water interface is still incompletely understood. Attempts to establish scales of interfacial polarity (172) have faced challenging problems (173) due to the asymmetry of the interfaces and preferential stabilization of some solute orientations, resulting in quite different solvent responses for compounds bearing similar groups but having different stereochemistries. (174) The enthalpic and entropic contributions to the interfacial free energy of solvation have been discussed for some organic molecules showing that the stability at the surface is mainly driven by the enthalpic component (Figure 12). (175) Similar studies have been carried out for ions. (176) However, in this case, the interface affinity remains incompletely understood, as discussed in many previous reviews. (177−186) Briefly, hard and multiply charged ions are repelled from the interface, while soft and large polarizable anions (e.g., I– and Br–) tend to accumulate at the air–water interface. (177,178) In any case, the difference in solvation energy between the interface and the bulk has direct consequences for the equilibrium constants of conformational, tautomeric, and acid/base equilibria, which remains a subject of intense debate in the literature (see, for instance, refs (187and188)). Changes in equilibrium constants imply changes in speciation and thus changes in photochemical behavior. The photochemistries of nitric acid and PYA described above are good examples illustrating these issues. For similar reasons, one expects variations in the effective redox potentials at the interface with respect to bulk values, particularly for ROS/RNS couples. (171,189) Estimations for the O2/O2– couple indicate a decrease as large as 0.32 V due to the poor solvation of the anion at the interface. (171)
然而，空气-水界面的溶剂化能力仍不完全清楚。由于界面的不对称性和某些溶质取向的优先稳定，建立界面极性尺度（172）的尝试面临着具有挑战性的问题（173），导致具有相似基团但具有不同立体化学的化合物的溶剂响应截然不同。 (174) 对于某些有机分子，已经讨论了焓和熵对溶剂化界面自由能的贡献，表明表面稳定性主要由焓分量驱动（图 12）。 (175) 对离子也进行了类似的研究。 (176) 然而，在这种情况下，界面亲和力仍然不完全被理解，正如许多以前的评论中所讨论的那样。 (177−186) 简而言之，硬离子和多电荷离子被界面排斥，而软离子和大的可极化阴离子（例如 I- 和 Br-）倾向于在空气-水界面积聚。 (177,178) 无论如何，界面和本体之间溶剂化能的差异对构象、互变异构和酸/碱平衡的平衡常数有直接影响，这仍然是文献中激烈争论的主题（参见，例如，参考文献（187和188））。平衡常数的变化意味着形态的变化，从而导致光化学行为的变化。上述硝酸和 PYA 的光化学是说明这些问题的好例子。出于类似的原因，人们预计界面处的有效氧化还原电位相对于体积值会发生变化，特别是对于 ROS/RNS 配对。 (171,189) 对 O2/O2- 电对的估计表明，由于界面处阴离子的溶剂化不良，电压下降幅度高达 0.32 V。 (171)关闭

Figure 12 图12

Figure 12. Free energy profiles for water accommodation of six polar and apolar organic compounds (A), and the corresponding enthalpic (B) and entropic components (C) as a function of position in a water droplet. The gray dashed line shows the Gibbs dividing surface. Reproduced with permission from ref (175).

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Interface solvation not only controls the concentration of the species, it also affects the electronic properties (i.e., the reactivity). This happens in the initiation steps (absorption, photolysis, and ISC) as well as in the subsequent propagation steps (electron transfer, hydrogen transfer, and reaction with oxygen). As for bulk solvation, interfacial solvation may be the origin of solvatochromism effects, producing blue shifts (hydroperoxides) or red shifts (ozone) in the gas-phase absorption bands. On the other hand, large fluctuations of the solvation shell influence the shapes of the bands, and both effects (band shifts and broadening) eventually modify the photochemical rate constants. (45) Moreover, solvation effects modulate ISC in photosensitizers, as shown by experiments (190,191) and calculations, (192,193) and in the more general case of conical intersections. (193−197) In contrast, interfacial solvent effects on quantum yields have not yet been fully investigated. The case of PYA represents a limiting case in which the photochemistry at the aqueous interface differs from that in the gas phase. (146−148,153) The case of nitrate illustrates the change in quantum yield due to solvent cage effects. (63,87,89,90)
界面溶剂化不仅控制物质的浓度，还影响电子性质（即反应性）。这发生在起始步骤（吸收、光解和 ISC）以及随后的传播步骤（电子转移、氢转移和与氧反应）中。至于本体溶剂化，界面溶剂化可能是溶剂变色效应的起源，在气相吸收带中产生蓝移（氢过氧化物）或红移（臭氧）。另一方面，溶剂化壳层的动会影响能带的形状，这两种效应（能带位移和展宽）最终都会改变光化学速率常数。（45） 此外，溶剂化效应调节光敏剂中的 ISC，如实验 （190,191） 和计算 （192,193） 以及更一般的锥形交叉点情况所示。（193−197） 相比之下，界面溶剂对量子产率的影响尚未得到充分研究。PYA的情况代表了一种极限情况，其中水界面的光化学与气相中的光化学不同。（146−148,153）硝酸盐的情况说明了由于溶剂笼效应导致的量子产率变化。(63,87,89,90)
After the photochemical formation of the initial radicals, reactions proceed via energy or electron transfer, HAT or PCET mechanisms, reaction with oxygen, etc. The relaxation of the photosensitizer triplet state via phosphorescence is also conceivable and may compete with other processes. Electron transfer at interfaces has a long history that is justified by a broad applicability ranging from electrochemistry to biology and solar energy technologies (see ref (198) for a recent review on these processes). Likewise, in bulk solution, Marcus theory (199−202) can account for the observed solvent effects on electron transfer at interfaces. (203,204) A study of photoinduced electron transfer at water/dimethylaniline interfaces (205,206) concluded that the interfacial processes are faster than the bulk processes, possibly due to faster solvation dynamics at the interface. (207) The stability of solvated electrons at aqueous interfaces has also been studied, (208−210) and simulations have predicted that the water surface electron affinity is ∼0.6 eV higher than in the bulk, and that the conduction band edge is deeper in energy. Electron transfer is also relevant in PCET mechanisms and can be significantly affected by aqueous solvation, as shown for the 3SO2 + H2O reaction (116) and the H2CO + HO2 reaction. (189) One should note that the kinetics of such bimolecular reactions are often controlled by differences in intermolecular energy differences between the highest occupied and lowest unoccupied molecular orbitals. At the interface, these differences depend strongly on the electric field created by the polarized water surface, which in turn varies with the proton donor or acceptor character of the reactants. (189)
在初始自由基的光化学形成之后，反应通过能量或电子转移、HAT 或 PCET 机制、与氧反应等进行。通过磷光使光敏剂三重态的松弛也是可以想象的，并且可能与其他过程竞争。电子在界面上的转移有着悠久的历史，从电化学到生物学和太阳能技术的广泛适用性证明了这一点（参见参考文献（198）关于这些过程的最新综述）。同样，在本体溶液中，Marcus理论（199−202）可以解释观察到的溶剂对界面处电子转移的影响。（203,204） 一项关于水/二甲基苯胺界面光生电子转移的研究 （205,206） 得出结论，界面过程比本体过程更快，可能是由于界面处的溶剂化动力学更快。（207）还研究了溶剂化电子在水界面上的稳定性，（208−210）和模拟预测，水表面电亲和力比本体高∼0.6 eV，并且导带边缘的能量更深。电子转移与PCET机制也有关，并且可能受到水溶剂化的显着影响，如3SO2 + H2O反应（116）和H2CO + HO2反应所示。（189） 人们应该注意到，这种双分子反应的动力学通常由最高占据和最低未占据的分子轨道之间的分子间能量差异控制。在界面上，这些差异很大程度上取决于极化水表面产生的电场，而极化水表面又随反应物的质子供体或受体特性而变化。(189)

5. Conclusions 5. 结论

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Similarly to thermal reactions, many photoinduced oxidation reactions at the air–water interface are significantly enhanced in comparison with the corresponding gas-phase or bulk processes. However, despite a considerable amount of experimental and theoretical work, the precise mechanisms by which this effect is achieved remain incompletely understood. Enthalpic and entropic factors may result in the accumulation of photoactive species at the interface, where partial hydration modify the electronic absorption energies, as well as the reactivity of the radicals formed in the early stages of the photo-oxidation processes. Interfacial dynamic effects are important as well. They have been found to be responsible for band broadening and the increase of photolytic rate constants. Likewise, solvent cage effects reducing the quantum yields turn out to be less important at the interface that in the bulk and this also contributes to enhance the photoreactivity. Various other effects have been briefly presented in the discussion above: changes in equilibrium constants (tautomeric, acid/base), effective redox potentials, rate constants for electron of hydrogen transfer, etc. All of them deserve further examination through a combination of new experiments and theoretical simulations in order to achieve a deeper understanding of these challenging phenomena, which have implications in many areas. For instance, incorporation of photo-oxidation mechanisms on cloud water surfaces would represent a considerable improvement in atmospheric models. Accounting for enhanced interfacial photochemistry is also relevant to design novel applications in water treatment and disinfection technologies, which already use the photolysis of O3 and H2O2 to trigger the oxidation of organic matter. Finally, these processes also open avenues in synthetic organic chemistry. On-water reactions are now broadly studied in microdroplets, thin films and microfluidic systems thanks to the development of innovative setups, and in many cases processes have been found to be dramatically accelerated in comparison to bulk reactions. Photoinduced oxidation on-water is likely to experience similar success in the coming years.
与热反应类似，与相应的气相或本体过程相比，空气-水界面处的许多光生氧化反应显着增强。然而，尽管进行了大量的实验和理论工作，但实现这种效应的确切机制仍然不完全清楚。焓因子和熵因子可能导致光活性物质在界面处的积累，其中部分水合改变了电子吸收能，以及在光氧化过程的早期阶段形成的自由基的反应性。界面动态效应也很重要。已发现它们负责谱带展宽和光解速率常数的增加。同样，降低量子产率的溶剂笼效应在界面上不如在本体界面上重要，这也有助于增强光反应性。在上面的讨论中已经简要介绍了各种其他效应：平衡常数（互变异构、酸/碱）、有效氧化还原电位、氢转移电子的速率常数等的变化。所有这些都值得通过新的实验和理论模拟的结合来进一步研究，以便更深入地理解这些具有挑战性的现象，这些现象在许多领域都有影响。例如，在云水面上加入光氧化机制将代表大气模型的相当大的改进。考虑到增强的界面光化学也与设计水处理和消毒技术中的新应用有关，这些技术已经利用 O3 和 H2O2 的光解来触发有机物的氧化。 最后，这些过程也为合成有机化学开辟了道路。由于创新装置的发展，现在在微滴、薄膜和微流体系统中对水上反应进行了广泛的研究，在许多情况下，与本体反应相比，水上反应被发现大大加快了速度。未来几年，水上光诱导氧化可能会取得类似的成功。

Author Information 作者信息

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Acknowledgments 确认

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J.M.A. thanks the Generalitat de Catalunya (Grant No. 2017SGR348) for financial support. M.F.R.-L. and M.T.C.M.-C. are grateful to the French CINES (project lct2550) for providing computational resources.
J.M.A. 感谢加泰罗尼亚政府（拨款编号：2017SGR348）的财政支持。M.F.R.-L.和 M.T.C.M.-C.感谢法国CINES（lct2550项目）提供的计算资源。

References 引用

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This article references 210 other publications.
本文引用了210篇其他文献。

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    Extensive H-atom abstraction from benzoate by OH-radicals at the air-water interface
    Enami, Shinichi; Hoffmann, Michael R.; Colussi, Agustin J.
    Physical Chemistry Chemical Physics (2016), 18 (46), 31505-31512CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Much is known about OH-radical chem. in the gas-phase and bulk water. Important atm. and biol. processes, however, involve little investigated OH-radical reactions at aq. interfaces with hydrophobic media. Here, we report the online mass-specific identification of the products and intermediates generated on the surface of aq. (H2O, D2O) benzoate-h5 and -d5 microjets by ∼8 ns OH(g) pulses in air at 1 atm. Isotopic labeling lets us unambiguously identify the phenylperoxyl radicals that ensue H-abstraction from the arom. ring and establish a lower bound (>26%) to this process as it takes place in the interfacial water nanolayers probed by our expts. The significant extent of H-abstraction vs. its negligible contribution both in the gas-phase and bulk water underscores the unique properties of the air-water interface as a reaction medium. The enhancement of H-atom abstraction in interfacial water is ascribed, in part, to the relative destabilization of a more polar transition state for OH-radical addn. vs. H-abstraction due to incomplete hydration at the low water densities prevalent therein.
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    Enami, S.; Colussi, A. J. Efficient scavenging of Criegee intermediates on water by surface-active cis-pinonic acid. Phys. Chem. Chem. Phys. 2017, 19, 17044– 17051,  DOI: 10.1039/C7CP03869K
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    Efficient scavenging of Criegee intermediates on water by surface-active cis-pinonic acid
    Enami, Shinichi; Colussi, A. J.
    Physical Chemistry Chemical Physics (2017), 19 (26), 17044-17051CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Cis-Pinonic acid (CPA), the main product of the atm. oxidn. of biogenic α-pinene emissions and a major component of secondary org. aerosol (SOA), is a potentially key species en route to extremely low volatility compds. Here, we report that CPA is an exceptionally efficient scavenger of Criegee intermediates (CIs) on aq. surfaces. Against expectations, millimolar CPA (a surface-active C10 keto-carboxylic acid possessing a rigid skeleton) is able to compete with 23 M bulk water for the CIs produced in the ozonolysis of sesquiterpene solutes by O3(g) on the surface of a water:acetonitrile solvent. The significance of this finding is that CPA reactions with sesquiterpene CIs on the surface of aq. org. aerosols would directly generate C25 species. The finding that competitive reactions at the air-liq. interface depend on interfacial rather than bulk reactant concns. should be incorporated in current chem. models dealing with SOA formation, growth and aging.
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    Enami, S.; Colussi, A. J. Reactions of Criegee Intermediates with Alcohols at Air-Aqueous Interfaces. J. Phys. Chem. A 2017, 121, 5175– 5182,  DOI: 10.1021/acs.jpca.7b04272
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    Reactions of Criegee Intermediates with Alcohols at Air-Aqueous Interfaces
    Enami, Shinichi; Colussi, A. J.
    Journal of Physical Chemistry A (2017), 121 (27), 5175-5182CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    The authors show, via online electrospray mass spectrometric detection, that Criegee intermediates (CIs) generated in situ in the fast ozonolysis of sesquiterpenes (C15H24) on the surface of water:acetonitrile microjets react with n ≥ 4 linear alcs. CnH2n+1OH to produce high mol. wt. C15+n ethers in one step. The OH group of 1-octanol proved to be ∼25 times less reactive than that of n-octanoic toward CIs at the same bulk molar concn., revealing that the reactivity of hydroxylic species depends on both acidities and interfacial affinities. CI interfacial reactions with surface-active hydroxylic species, by bypassing water, represent shortcuts to mol. complexity in atm. aerosols.
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    Enami, S.; Hoffmann, M. R.; Colussi, A. J. Criegee Intermediates React with Levoglucosan on Water. J. Phys. Chem. Lett. 2017, 8, 3888– 3894,  DOI: 10.1021/acs.jpclett.7b01665
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    Criegee Intermediates React with Levoglucosan on Water
    Enami, Shinichi; Hoffmann, Michael R.; Colussi, A. J.
    Journal of Physical Chemistry Letters (2017), 8 (16), 3888-3894CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Levoglucosan (Levo), a C6-anhydrosaccharide produced during combustion of cellulosic materials, is the major component of aerosols produced from biomass burning over vast regions worldwide. Levo is long considered chem. inert and has been used as a biomass burning source tracer; however, this work showed sugars (Levo, glucose, arabitol, mannitol) rapidly react with Criegee intermediates (CI) generated during ozonolysis of sesquiterpenes on the surface of water:acetonitrile micro-jets. Hydrophilic Levo reacts faster with CI than with water or surface-active 1-octanol at air/aq. interfaces. This unexpected phenomenon is likely assocd. with the relatively low water d. at air/aq. interfaces coupled to a higher gas-phase acidity of saccharide hydroxyl groups (i.e., -OH) vs. n-alkanols. Presented results showed aerosol saccharides are reactive toward CI. Given the abundance of saccharides in the atm., they may be important contributors to secondary org. aerosols growth and mass loads.
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    Qiu, J. T.; Ishizuka, S.; Tonokura, K.; Colussi, A. J.; Enami, S. Reactivity of Monoterpene Criegee Intermediates at Gas-Liquid Interfaces. J. Phys. Chem. A 2018, 122, 7910– 7917,  DOI: 10.1021/acs.jpca.8b06914
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    Reactivity of Monoterpene Criegee Intermediates at Gas-Liquid Interfaces
    Qiu, Junting; Ishizuka, Shinnosuke; Tonokura, Kenichi; Colussi, Agustin J.; Enami, Shinichi
    Journal of Physical Chemistry A (2018), 122 (39), 7910-7917CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    Biogenic monoterpenes are major sources of Criegee intermediates (CI) in the troposphere. Recent studies underscored the importance of their heterogeneous chem. The work, which assessed monoterpene CI reactions on liq. surfaces, was challenging due to a lack of suitable probes. The first mass spectrometric detection of intermediates and products, including labile hydroperoxides, from representative monoterpene (α-terpinene, γ-terpinene, terpinolene, D-limonene, α-pinene) CI reactions with water, cis-pinonic acid (CPA), and octanoic acid (OA) on liq. micro-jet surfaces are reported. Significantly, relative yields of α-hydroxy-hydroperoxides produced from CI hydration at gas/liq. interfaces (α-terpinene [1.00] » D-limonene [0.18] > γ-terpinene [0.11] ∼ terpinolene [0.10] » α-pinene [0.01]) do not track the rate consts. of their gas-phase ozonolyzes. In contrast with the inertness of other CI, α-terpinene ozonolysis-derived CI readily reacted with CPA and OA to produce C20 and C18 ester hydroperoxides, resp. These results showed hitherto unknown structural effects on CI reactivity at aq. interfaces.
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    Qiu, J. T.; Ishizuka, S.; Tonokura, K.; Enami, S. Reactions of Criegee Intermediates with Benzoic Acid at the Gas/Liquid Interface. J. Phys. Chem. A 2018, 122, 6303– 6310,  DOI: 10.1021/acs.jpca.8b04995
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    Reactions of Criegee Intermediates with Benzoic Acid at the Gas/Liquid Interface
    Qiu, Junting; Ishizuka, Shinnosuke; Tonokura, Kenichi; Enami, Shinichi
    Journal of Physical Chemistry A (2018), 122 (30), 6303-6310CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    Secondary org. aerosol (SOA) found in polluted mega-cities contains benzoic acid (BA) as a major org. acid in addn. to a variety of species including alkenes. In polluted air, ozone could be a major oxidizer for SOA and induces subsequent reactions involving Criegee intermediates (CIs, carbonyl oxide, RR'C•-O-O•/RR'C=O+-O-) formed by the -C=C- + O3 reaction at the gas/liq. interface. The possibility that abundant BA could be an effective scavenger of CIs at the interface remains to be investigated by direct expts. Here, we showed that amphiphilic BA is able to compete with water mols. for the CIs produced in the prompt ozonolysis of β-caryophyllene on the surface of a water/acetonitrile solvent microjet by generating hitherto uncharacterized C22 ester hydroperoxide products. Competition between BA vs octanoic acid vs cis-pinonic acid toward CIs reveals that BA is a much less-efficient scavenger of CIs on aq. org. surfaces. We attribute it to the surface-specific orientation of BA at the gas/liq. interface, where the reactive -C(O)OH group is fully hydrated and not available for CIs generated at the topmost layers.
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    Qiu, J. T.; Ishizuka, S.; Tonokura, K.; Enami, S. Interfacial vs Bulk Ozonolysis of Nerolidol. Environ. Sci. Technol. 2019, 53, 5750– 5757,  DOI: 10.1021/acs.est.9b00364
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    Interfacial vs. Bulk Ozonolysis of Nerolidol as air pollution
    Qiu, Junting; Ishizuka, Shinnosuke; Tonokura, Kenichi; Enami, Shinichi
    Environmental Science & Technology (2019), 53 (10), 5750-5757CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Ozone readily reacts with olefins with the formation of more reactive Criegee intermediates (CIs). The transient CIs impact HOx cycles, and they play a role in new particle formation in the troposphere. Oxidn. by O3 occurs both in the gas-phase, in the liq. phase, and at air-H2O and air-aerosol interfaces. In light of the importance of O3 in environmental and engineered chem. transformations, we have studied the ozonolysis mechanisms of a triolefin C15-alc., nerolidol (Nero, a biogenic sesquiterpene), at the air-H2O interface in the presence of MeCN. Surface-sensitive pneumatic ionization mass spectrometric detection of α-hydroxy-hydroperoxides and functionalized carboxylates, generated by the hydration and isomerization of CIs, resp., enables one to evaluate the relative reactivity of each C=C toward O3. We compare bulk-phase ozonolysis chem. to similar reactions taking place at the air-H2O interface. O3 reacts primarily with the Me2C=CH- and -MeC=CH- moieties (>∼98%), while the O3 attack on the terminal -HC=CH2 site (<∼2%) is a minor pathway during both interfacial and bulk ozonolysis. The presence of functionalized-carboxylates on interfaces but not in bulk-phase reactions with O3 indicates that the isomerization of the CIs is not hindered at the air-H2O interface due to the lower availability of H2O.
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  11. 11
    Qiu, J. T.; Ishizuka, S.; Tonokura, K.; Sato, K.; Inomata, S.; Enami, S. Effects of pH on Interfacial Ozonolysis of alpha-Terpineol. J. Phys. Chem. A 2019, 123, 7148– 7155,  DOI: 10.1021/acs.jpca.9b05434
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    Effects of pH on Interfacial Ozonolysis of α-Terpineol
    Qiu, Junting; Ishizuka, Shinnosuke; Tonokura, Kenichi; Sato, Kei; Inomata, Satoshi; Enami, Shinichi
    Journal of Physical Chemistry A (2019), 123 (32), 7148-7155CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    Acidity changes the phys. properties of atm. aerosol particles and the mechanisms of reactions that occur therein and on the surface. Here, the authors used surface-sensitive pneumatic ionization mass spectrometry to investigate the effects of pH on the heterogeneous reactions of aq. α-terpineol (C10H17OH), a representative monoterpene alc., with gaseous ozone. Rapid (≤10 μs) ozonolysis of α-terpineol produced Criegee intermediates (CIs, zwitterionic/diradical carbonyl oxides) on the surface of water microjets. The authors studied the effects of microjet bulk pH (1-11) on the formation of functionalized carboxylate and α-hydroxy-hydroperoxide chloride adduct (HH-Cl-) products generated by isomerization and hydration of α-terpineol CIs, resp. Compared with the signal at pH ≈6, the mass spectral signal of HH-Cl- was less intense under both basic and more acidic conditions, whereas the intensity of the functionalized carboxylate signal increased with increasing pH up to 4 and then remained const. The decrease of HH-Cl- signals at bulk pH values of >6 is attributable to the accumulation of OH- at the air-water interface that suppresses the relative abundance of hydrophilic HH and Cl-. The present study suggests that α-terpineol in ambient aq. org. aerosols will be converted into much lower volatile and potentially toxic org. hydroperoxides during the heterogeneous ozonolysis.
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    Mmereki, B. T.; Donaldson, D. J.; Gilman, J. B.; Eliason, T. L.; Vaida, V. Kinetics and products of the reaction of gas-phase ozone with anthracene adsorbed at the air-aqueous interface. Atmos. Environ. 2004, 38, 6091– 6103,  DOI: 10.1016/j.atmosenv.2004.08.014
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    Kinetics and products of the reaction of gas-phase ozone with anthracene adsorbed at the air-aqueous interface
    Mmereki, Baagi T.; Donaldson, D. J.; Gilman, J. B.; Eliason, T. L.; Vaida, V.
    Atmospheric Environment (2004), 38 (36), 6091-6103CODEN: AENVEQ; ISSN:1352-2310. (Elsevier B.V.)
    Many atmospherically important chem. processes occur at the interface between the air and aq. phases, e.g., surfaces of oceans, lakes, and atm. aerosols. Kinetics and products of a reaction between gas-phase O3 and anthracene adsorbed at the air-aq. interface were measured. The intensity of laser-induced fluorescence from adsorbed anthracene was used to follow reaction kinetics; gas chromatog.-mass spectrometry identified 9,10-anthraquinone as the major product. Reactions at a clean air-water interface and at an interface consisting of a monolayer of various amphiphilic org. compds. were studied. In all cases, reactions follows a Langmuir-Hinshelwood mechanism, in which O3 first adsorbed to the air-aq. interface then reacted with already adsorbed anthracene. For typical atm. O3 concns., the estd. gas-surface reaction probability was from 2 × 10-8 to 3 × 10-7, depending on the nature of the air-aq. interface. Small carboxylic acids at the interface inhibited the reaction (vs. a clean water surface); 1-octanol enhanced the reaction. In some circumstances, anthracene oxidn. by O3 on aq. surfaces may be of comparable importance in the atm. to gas-phase oxidn. by OH-.
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    Enami, S.; Sakamoto, Y.; Colussi, A. J. Fenton chemistry at aqueous interfaces. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 623– 628,  DOI: 10.1073/pnas.1314885111
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    Fenton chemistry at aqueous interfaces
    Enami, Shinichi; Sakamoto, Yosuke; Colussi, Agustin J.
    Proceedings of the National Academy of Sciences of the United States of America (2014), 111 (2), 623-628CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    In a fundamental process throughout nature, reduced Fe unleashes the oxidative power of H2O2 into reactive intermediates. However, notwithstanding much work, the mechanism by which Fe2+ catalyzes H2O2 oxidns. and the identity of the participating intermediates remain controversial. Here the authors report the prompt formation of OFeCl3- and chloride-bridged di-Fe O=FeIV·Cl·FeIICl4- and O=FeIV·Cl·FeIIICl5- ferryl species, in addn. to FeIIICl4-, on the surface of aq. FeCl2 microjets exposed to gaseous H2O2 or O3 beams for <50 μs. The unambiguous identification of such species in situ via online electrospray mass spectrometry let the authors study their individual dependences on Fe2+, H2O2, O3, and H+ concns., and their responses to tert-BuOH (an ·OH scavenger) and DMSO (an O-atom acceptor) cosolutes. (i) mass spectra are not affected by excess tert-BuOH, i.e., the detected species are primary products whose formation does not involve ·OH radicals, and (ii) the di-Fe ferryls, but not O=FeIVCl3-, can be fully quenched by DMSO under present conditions. The authors infer that interfacial Fe(H2O)n2+ ions react with H2O2 and O3 >103 times faster than Fe(H2O)62+ in bulk H2O via a process that favors inner-sphere two-electron O-atom over outer-sphere 1-electron transfers. The higher reactivity of di-Fe ferryls vs. O=FeIVCl3- as O-atom donors implicates the electronic coupling of mixed-valence Fe centers in the weakening of the FeIV-O bond in poly-Fe ferryl species.
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    Gao, D.; Jin, F.; Lee, J. K.; Zare, R. N. Aqueous microdroplets containing only ketones or aldehydes undergo Dakin and Baeyer-Villiger reactions. Chem. Sci. 2019, 10, 10974– 10978,  DOI: 10.1039/C9SC05112K
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    Aqueous microdroplets containing only ketones or aldehydes undergo Dakin and Baeyer-Villiger reactions
    Gao, Dan; Jin, Feng; Lee, Jae Kyoo; Zare, Richard N.
    Chemical Science (2019), 10 (48), 10974-10978CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    The Dakin and Baeyer-Villiger (BV) reactions occurred in modest yields within milliseconds in aq. microdroplets at room-temp. without addn. of external peroxides and catalysts. H2O2 Generation was the result of special environment of microdroplet surface, which promotes water autoionization. It was found that increasing content of water and decreasing droplet size improved product yield of Dakin and BV reactions, supporting contention that amt. of H2O2 generated in aq. microdroplets could induce two reactions and reactions occurred at or near air-water interface of microdroplet surface.
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    Thomas, J. L.; Jimenez-Aranda, A.; Finlayson-Pitts, B. J.; Dabdub, D. Gas-phase molecular halogen formation from NaCl and NaBr aerosols: When are interface reactions important?. J. Phys. Chem. A 2006, 110, 1859– 1867,  DOI: 10.1021/jp054911c
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    Gas-Phase Molecular Halogen Formation from NaCl and NaBr Aerosols: When Are Interface Reactions Important?
    Thomas, Jennie L.; Jimenez-Aranda, Angel; Finlayson-Pitts, Barbara J.; Dabdub, Donald
    Journal of Physical Chemistry A (2006), 110 (5), 1859-1867CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    Unique interface reactions at the surface of sea-salt particles have been suggested as an important source of photolyzable gas-phase halogen species in the troposphere. Many factors influence the relative importance of interface chem. compared to aq.-phase chem. The Model of Aerosol, Gas, and Interfacial Chem. (MAGIC 2.0) is used to study the influence of interface reactions on gas-phase mol. halogen prodn. from pure NaCl and NaBr aerosols. The main focus is to identify the relative importance of bulk compared to interface chem. and to det. when interface chem. dominates. The results show that the interface process involving Cl-(surf) and OH(g) is the main source of Cl2(g). For the analogous oxidn. of bromide by OH, gaseous Br2 is formed mainly in the bulk aq. phase and transferred across the interface. However, the reaction of Br-(surf) with gaseous O3 at the interface is the primary source of Br2(g) under dark conditions. The effect of aerosol size is also studied. Potential atm. implications and effects of interface processes on aerosol pH are discussed.
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    Lee, J. K.; Samanta, D.; Nam, H. G.; Zare, R. N. Micrometer-sized water droplets induce spontaneous reduction. J. Am. Chem. Soc. 2019, 141, 10585– 10589,  DOI: 10.1021/jacs.9b03227
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    Micrometer-Sized Water Droplets Induce Spontaneous Reduction
    Lee, Jae Kyoo; Samanta, Devleena; Nam, Hong Gil; Zare, Richard N.
    Journal of the American Chemical Society (2019), 141 (27), 10585-10589CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Bulk water serves as an inert solvent for many chem. and biol. reactions. Here, we report a striking exception. We observe that in micrometer-sized water droplets (microdroplets), spontaneous redn. of several org. mols. occurs, pyruvate to lactate, lipoic acid to dihydrolipoic acid, fumarate to succinate, and oxaloacetate to malate. This redn. proceeds in microdroplets without any added electron donors or acceptors and without any applied voltage. In three of the four cases, the redn. efficiency is 90% or greater when the concn. of the dissolved org. species is less than 0.1 μM. None of these reactions occurs spontaneously in bulk water. One example demonstrating the possible broad application of redn. in water microdroplets to org. mols. is the redn. of acetophenone to form 1-phenylethanol. Taken together, these results show that microdroplets provide a new foundation for green chem. by rendering water mols. to be highly electrochem. active without any added reducing agent or applied potential. In this manner, aq. microdroplets might have provided a route for abiotic redn. reactions in the prebiotic era, thereby providing org. mols. with a reducing power before the advent of biotic reducing machineries.
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    George, C.; Ammann, M.; D’Anna, B.; Donaldson, D. J.; Nizkorodov, S. A. Heterogeneous Photochemistry in the Atmosphere. Chem. Rev. 2015, 115, 4218– 4258,  DOI: 10.1021/cr500648z
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    Heterogeneous Photochemistry in the Atmosphere
    George, Christian; Ammann, Markus; D'Anna, Barbara; Donaldson, D. J.; Nizkorodov, Sergey A.
    Chemical Reviews (Washington, DC, United States) (2015), 115 (10), 4218-4258CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review concerning advances in heterogeneous photochem. occurring at the atm. surface/air and other atm. environmental interfaces, including future developments, is given. Topics covered include: background (types and significance of condensed matter and surfaces in the atm. environment, atm. photochem. principles, gas phase vs. condensed phase); photo-phys. properties of obsd. atm. particles and interfaces (primary chromophores, secondary org. chromophores, humic-like substances of primary and secondary origins); mineral dust (NOx, gaseous H2O2/HOx loss and prodn., SO2, O3 loss and prodn., org. compds., field observations of dust photochem.); photo-phys. processes at liq. interfaces; org. aerosol photochem. (general considerations, smog chamber and aerosol flow tube-based expts., photodegrdn. of bulk materials mimicking org. aerosols, photo-sensitized reactions involving carbonyl compds.); heterogeneous photochem. at ice interfaces (inorg. chromophores, org. mol. photolysis, photo-sensitized chem. with org. chromophores, indirect photochem. induced by mineral dust in ice); heterogeneous photochem. on urban surfaces (outdoor surfaces and urban grime, indoor surfaces); and looking ahead.
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    Gomez Alvarez, E.; Wortham, H.; Strekowski, R.; Zetzsch, C.; Gligorovski, S. Atmospheric Photosensitized Heterogeneous and Multiphase Reactions: From Outdoors to Indoors. Environ. Sci. Technol. 2012, 46, 1955– 1963,  DOI: 10.1021/es2019675
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    Atmospheric Photosensitized Heterogeneous and Multiphase Reactions: From Outdoors to Indoors
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    Environmental Science & Technology (2012), 46 (4), 1955-1963CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    The impact of photosensitized phenomena on atm. reactivity is explored and the importance of atm. photosensitization processes is evaluated. The study of such processes could provide a better understanding of ·OH radical formation pathways in the atm. and in consequence, of a more accurate prediction of the oxidative capacity of the atm. Compds. that readily absorb in the tropospheric actinic window (ionic org. complexes, PAHs, arom. carbonyl compds.) acting as potential photosensitizers of atm. relevant processes are considered. The impact of photosensitization on relevant systems which could act as powerful atm. reactors, i.e. interface ocean-atm., urban and forest surfaces and indoor air environments is also discussed.
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    Kozlowski, M.; Yoon, T. Editorial for the Special Issue on Photocatalysis. J. Org. Chem. 2016, 81, 6895– 6897,  DOI: 10.1021/acs.joc.6b01717
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    Editorial for the Special Issue on Photocatalysis
    Kozlowski, Marisa; Yoon, Tehshik
    Journal of Organic Chemistry (2016), 81 (16), 6895-6897CODEN: JOCEAH; ISSN:0022-3263. (American Chemical Society)
    There is no expanded citation for this reference.
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    Gligorovski, S.; Strekowski, R.; Barbati, S.; Vione, D. Environmental Implications of Hydroxyl Radicals (OH). Chem. Rev. 2015, 115, 13051– 13092,  DOI: 10.1021/cr500310b
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    Environmental Implications of Hydroxyl Radicals (•OH)
    Gligorovski, Sasho; Strekowski, Rafal; Barbati, Stephane; Vione, Davide
    Chemical Reviews (Washington, DC, United States) (2015), 115 (24), 13051-13092CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review concerning the formation, occurrence, reactivity, scavenging, and detection of OH- in natural environmental compartments (natural water and indoor and outdoor atm.) and lab. systems is given. A comprehensive understanding of OH- sources and sinks and their implications in natural water and in the atm. is critically important, including the way surface water irradiated chromophoric dissolved org. matter yields OH- via the H2O2-independent pathway, and including an assessment of the relative importance of gas- vs aq.-phase OH- reactions with many atm. components. Topics discussed include: introduction; OH- formation and scavenging under different conditions (aq. environment, atm., indoor atm.); OH- generation under controlled lab. conditions (aq. phase, gas phase); OH- prodn. and prodn.-detection in the lab. and gaseous and aq. phases; OH- kinetic properties in aq. soln.; OH- reaction mechanisms in aq. soln. and in gas phase; and conclusions and outlook.
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    Gas-phase radical chemistry in the troposphere
    Monks, Paul S.
    Chemical Society Reviews (2005), 34 (5), 376-395CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. Atm. free radicals are low concn., relatively fast reacting species whose influence is felt throughout the atm. Reactive radicals have a key role in maintaining a balanced atm. compn. through their central function in controlling the oxidative capacity of the atm. In this tutorial review, the chem. of three main groups of atm. radicals HOx, NOx and XOx (X = Cl, Br, I) are examd. in terms of their sources, interconversions and sinks. Key examples of the chem. are given for each group of radicals in their atm. context.
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    Anglada, J. M.; Martins-Costa, M.; Francisco, J. S.; Ruiz-Lopez, M. F. Interconnection of Reactive Oxygen Species Chemistry across the Interfaces of Atmospheric, Environmental, and Biological Processes. Acc. Chem. Res. 2015, 48, 575– 583,  DOI: 10.1021/ar500412p
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    Interconnection of Reactive Oxygen Species Chemistry across the Interfaces of Atmospheric, Environmental, and Biological Processes
    Anglada, Josep M.; Martins-Costa, Marilia; Francisco, Joseph S.; Ruiz-Lopez, Manuel F.
    Accounts of Chemical Research (2015), 48 (3), 575-583CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    A review is presented. Oxidn. reactions are ubiquitous and play key roles in the chem. of the atm., in water treatment processes, and in aerobic organisms. Ozone (O3), hydrogen peroxide (H2O2), hydrogen polyoxides (H2Ox, x > 2), assocd. hydroxyl and hydroperoxyl radicals (HOx = OH and HO2), and superoxide and ozonide anions (O2- and O3-, resp.) are the primary oxidants in these systems. They are commonly classified as reactive oxygen species (ROS). Atm. chem. is driven by a complex system of chain reactions of species, including nitrogen oxides, hydroxyl and hydroperoxide radicals, alkoxy and peroxy radicals, and ozone. HOx radicals contribute to keeping air clean, but in polluted areas, the ozone concn. increases and creates a neg. impact on plants and animals. Indeed, ozone concn. is used to assess air quality worldwide. Clouds have a direct effect on the chem. compn. of the atm. On one hand, cloud droplets absorb many trace atm. gases, which can be scavenged by rain and fog. On the other hand, ionic species can form in this medium, which makes the chem. of the atm. richer and more complex. Furthermore, recent studies have suggested that air-cloud interfaces might have a significant impact on the overall chem. of the troposphere. Despite the large differences in mol. compn., concn., and thermodn. conditions among atm., environmental, and biol. systems, the underlying chem. involving ROS has many similarities. In this Account, we examine ROS and discuss the chem. characteristics common to all of these systems. In water treatment, ROS are key components of an important subset of advanced oxidn. processes. Ozonation, peroxone chem., and Fenton reactions play important roles in generating sufficient amts. of hydroxyl radicals to purify wastewater. Biochem. processes within living organisms also involve ROS. These species can come from pollutants in the environment, but they can also originate endogenously, initiated by electron redn. of mol. oxygen. These mols. have important biol. signaling activities, but they cause oxidative stress when dysfunction within the antioxidant system occurs. Excess ROS in living organisms can lead to problems, such as protein oxidn.-through either cleavage of the polypeptide chain or modification of amino acid side chains-and lipid oxidn.
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    Zhang, X.; He, S.; Chen, Z.; Zhao, Y.; Hua, W. Methyl hydroperoxide (CH3OOH) in urban, suburban and rural atmosphere: ambient concentration, budget, and contribution to the atmospheric oxidizing capacity. Atmos. Chem. Phys. 2012, 12, 8951– 8962,  DOI: 10.5194/acp-12-8951-2012
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    Methyl hydroperoxide (CH3OOH) in urban, suburban and rural atmosphere: ambient concentration, budget, and contribution to the atmospheric oxidizing capacity
    Zhang, X.; He, S. Z.; Chen, Z. M.; Zhao, Y.; Hua, W.
    Atmospheric Chemistry and Physics (2012), 12 (19), 8951-8962CODEN: ACPTCE; ISSN:1680-7316. (Copernicus Publications)
    Me hydroperoxide (MHP), one of the most important org. peroxides in the atm., contributes to the tropospheric oxidizing capacity either directly as an oxidant or indirectly as a free radical precursor. In this study we report measurements of MHP from seven field campaigns at urban, suburban and rural sites in China in winter 2007 and summer 2006/2007/2008. MHP was usually present in the order of several hundreds of pptv level, but the av. mixing ratios have shown a wide range depending on the season and measuring site. Primary sources and sinks of MHP are investigated to understand the impact of meteorol. and chem. parameters on the atm. MHP budget. The MHP/(MHP + H2O2) ratio is also presented here to examine different sensitivities of MHP and H2O2 to certain atm. processes. The diurnal cycle of MHP/(MHP + H2O2), which is out of phase with that of both H2O2 and MHP, could imply that MHP prodn. is more sensitive to the ambient NO concn., while H2O2 is more strongly influenced by the wet deposition and the subsequent aq. chem. It is interesting to note that our observation at urban Beijing site in winter 2007 provides evidence for the occasional transport of MHP-contg. air masses from the marine boundary layer to the continent. Furthermore, the contribution of MHP as an atm. oxidant to the oxidizing capacity of an air parcel is assessed based on the "Counter Species" concept.
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    Jacob, D. J. In Handbook of Weather, Climate and Water: Atmospheric Chemistry, Hydrology and Societal Impacts; Potter, T. D., Colman, B. R., Eds.; Wiley-Interscience: Hoboken, NJ, 2003.
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    Reed Harris, A. E.; Pajunoja, A.; Cazaunau, M.; Gratien, A.; Pangui, E.; Monod, A.; Griffith, E. C.; Virtanen, A.; Doussin, J. F.; Vaida, V. Multiphase Photochemistry of Pyruvic Acid under Atmospheric Conditions. J. Phys. Chem. A 2017, 121, 3327– 3339,  DOI: 10.1021/acs.jpca.7b01107
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    Multiphase Photochemistry of Pyruvic Acid under Atmospheric Conditions
    Reed Harris, Allison E.; Pajunoja, Aki; Cazaunau, Mathieu; Gratien, Aline; Pangui, Edouard; Monod, Anne; Griffith, Elizabeth C.; Virtanen, Annele; Doussin, Jean-Francois; Vaida, Veronica
    Journal of Physical Chemistry A (2017), 121 (18), 3327-3339CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    Aerosol and mol. processing in the atm. occur in a complex, variable environment consisting of multiple phases and interfacial zones. The effect of such conditions on chem. system reactivity, an environment simulation chamber assessed the multi-phase photolysis of pyruvic acid, which photo-reacts in the troposphere in aq. particles and as gases. Irradn. of nebulized pyruvic acid rapidly generates acetic acid and CO2, consistent with the literature on bulk phase photolysis reactions. Also., a new C6 product, zymonic acid, was identified, a species not previously reported from pyruvic acid photolysis under any conditions. Its observation here and corresponding spectroscopic signatures indicated it could be formed by heterogeneous reactions at droplet surfaces. Previous studies of pyruvic acid aq. photolysis showed high mol. wt. compds. are formed by radical reactions; however, they are inhibited in the presence of O2, leading to doubt whether this chem. would occur in the atm. Identification of dimethyltartaric acid from atm. multi-phase pyruvic acid photolysis confirmed radical polymn. chem. can compete with O2 reactions to some extent in aerobic conditions. Evidence of addnl. polymn. within particles during irradn. was suggested by increasing viscosity and org. content of the particles. The implications of multi-phase specific processes are discussed within the broader scope of atm. science.
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    Zhong, J.; Kumar, M.; Anglada, J. M.; Martins-Costa, M. T. C.; Ruiz-Lopez, M. F.; Zeng, X. C.; Francisco, J. S. Atmospheric Spectroscopy and Photochemistry at Environmental Water Interfaces. Annu. Rev. Phys. Chem. 2019, 70, 45– 69,  DOI: 10.1146/annurev-physchem-042018-052311
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    Atmospheric Spectroscopy and Photochemistry at Environmental Water Interfaces
    Zhong, J.; Kumar, M.; Anglada, J. M.; Martins-Costa, M. T. C.; Ruiz-Lopez, M. F.; Zeng, X. C.; Francisco, Joseph S.
    Annual Review of Physical Chemistry (2019), 70 (), 45-69CODEN: ARPLAP; ISSN:0066-426X. (Annual Reviews)
    The air-water interface is ubiquitous in nature, as manifested in the form of the surfaces of oceans, lakes, and atm. aerosols. The aerosol interface, in particular, can play a crucial role in atm. chem. The adsorption of atm. species onto and into aerosols modifies their concns. and chemistries. Moreover, the aerosol phase allows otherwise unlikely soln.-phase chem. to occur in the atm. The effect of the air-water interface on these processes is not entirely known. This review summarizes recent theor. investigations of the interactions of atm. species with the air-water interface, including reactant adsorption, photochem., and the spectroscopy of reactants at the water surface, with an emphasis on understanding differences between interfacial chemistries and the chemistries in both bulk soln. and the gas phase. The results discussed here enable an understanding of fundamental concepts that lead to potential air-water interface effects, providing a framework to understand the effects of water surfaces on our atm.
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    Lee, M. H.; Heikes, B. G.; O’Sullivan, D. W. Hydrogen peroxide and organic hydroperoxide in the troposphere: A review. Atmos. Environ. 2000, 34, 3475– 3494,  DOI: 10.1016/S1352-2310(99)00432-X
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    Hydrogen peroxide and organic hydroperoxide in the troposphere: a review
    Lee, Meehye; Heikes, Brian G.; O'Sullivan, Daniel W.
    Atmospheric Environment (2000), 34 (21), 3475-3494CODEN: AENVEQ; ISSN:1352-2310. (Elsevier Science Ltd.)
    A review with many refs. on the current knowledge of gas-phase H2O2 and org. hydroperoxide in the troposphere including chem., kinetics and thermodn., properties, measurement methodol. and tropospheric distribution.
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    Khan, M. A. H.; Cooke, M. C.; Utembe, S. R.; Xiao, P.; Morris, W. C.; Derwent, R. G.; Archibald, A. T.; Jenkin, M. E.; Percival, C. J.; Shallcross, D. E. The global budgets of organic hydroperoxides for present and pre-industrial scenarios. Atmos. Environ. 2015, 110, 65– 74,  DOI: 10.1016/j.atmosenv.2015.03.045
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    The global budgets of organic hydroperoxides for present and pre-industrial scenarios
    Khan, M. A. H.; Cooke, M. C.; Utembe, S. R.; Xiao, P.; Morris, W. C.; Derwent, R. G.; Archibald, A. T.; Jenkin, M. E.; Percival, C. J.; Shallcross, D. E.
    Atmospheric Environment (2015), 110 (), 65-74CODEN: AENVEQ; ISSN:1352-2310. (Elsevier Ltd.)
    The global 3-D chem.-transport model, STOCHEM-CRI (Utembe et al., 2010), has been used to simulate the global distribution of org. hydroperoxides (ROOH) for both present day and pre-industrial scenarios. Globally, the formation of ROOH is solely from the reaction between RO2 and HO2, being more significant under NOx-limited conditions; here the self and cross reactions of RO2 and HO2 radicals dominate over their reaction with NO. The predominant global loss processes for ROOH are reaction with OH (95%) and by photolysis (4.4%) with a minor loss (<1%) by deposition, in the present day scenario. The assocd. global burden of ROOH in our model study is found to be 3.8 Tg. The surface distribution of ROOH shows a peak near the equator corresponding with higher photochem. activity and large (biogenic) VOC emissions. The simulated abundances of ROOH are comparable with those recorded in field campaigns, but generally show a tendency towards underestimation, particularly in the boundary layer. ROOH displayed seasonal cycles with higher concns. during the summer months and lower concns. during the winter months. The effects of including proposed HOx recycling schemes, including isomerisation of isoprene-derived peroxy radicals on the global budget of ROOH have also been investigated for the present and the pre-industrial environment. The present day simulations showed significant increases in CH3OOH and ROOH (up to 80% and 30%, resp.) over tropical forested regions, due to a general increase in HO2 and RO2 levels in isoprene-rich regions at low NOx levels. In the pre-industrial scenario, the increases in CH3OOH and total ROOH abundances are even larger, reflecting the more efficient operation of HOx recycling mechanisms at lower NOx levels. RCO3H species contribute 40-50% of the global burden of ROOH; inclusion of HOx recycling mechanisms leads to an increase in these RCO3H species but there is no discernible change in the remaining ROOH (ROOH-RCO3H) burden.
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    Plant Physiology (2006), 141 (2), 312-322CODEN: PLPHAY; ISSN:0032-0889. (American Society of Plant Biologists)
    A review. Free radicals and other reactive oxygen species (ROS) and the damages caused by them are reviewed. Cell respond to oxygen stress and antioxidant defenses are characterized. The changing role of ROS in defense against infection and signaling during life is discussed. Differences between plants and other organisms are highlighted.
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    Wang, C. X.; Chen, Z. M. Effect of CH3OOH on the atmospheric concentration of OH radicals. Prog. Nat. Sci. 2006, 16, 1141– 1149,  DOI: 10.1080/10020070612330121
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    Effect of CH3OOH on the atmospheric concentration of OH radicals
    Wang, Caixia; Chen, Zhongming
    Progress in Natural Science (2006), 16 (11), 1141-1149CODEN: PNASEA; ISSN:1002-0071. (Science in China Press)
    Me hydroperoxide (CH3OOH, MHP) is known to be a significant sink and reservoir of HOx and ROx radicals in the atm. In order to investigate the impact of MHP on the concn. of atm. OH radicals, two key gas-phase reactions of MHP, i.e. the reactions with OH radicals and with UV photolysis, have been simulated at temp. of 293 ± 2 K and total pressure of 1.01 × 105 Pa, using the long path Fourier transform IR (LP-FTIR) spectrometry. OH radicals are generated by the photolysis of O3 in the presence of water vapor. Combined with the relative rate method, the reaction rate const. of MHP with OH radicals is detd. to be (3.99 ± 0.15) × 10-12 cm3·mol.-1·s-1, and thus the atm. lifetime of MHP is estd. at 2.9 days. Furthermore, from detailed anal. of the UV photolysis of MHP, the yield of OH radicals is obtained to be 0.91 ± 0.04. Based on the MHP atm. lifetime and the yield of OH radicals, it is concluded that MHP plays an essential role in the redistribution of OH radicals in the troposphere.
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    Staehelin, Johannes; Hoigne, Juerg
    Environmental Science and Technology (1982), 16 (10), 676-81CODEN: ESTHAG; ISSN:0013-936X.
    Reactions of OH- and HO2- with O3 initiate radical chain reactions. The kinetic chain length of such reactions depends on the relative rate by which the radicals formed react with O3 compared with reactions with other solutes present in the soln. Such chain reactions can be more important in pure water than in drinking waters or in some types of wastewaters where HCO3- and org. impurities may significantly scavenge OH radicals. The half-lives of O3 measured in different types of natural waters are compared with those measured in model solns.
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    Herrmann, H.; Hoffmann, D.; Schaefer, T.; Bräuer, P.; Tilgner, A. Tropospheric Aqueous-Phase Free-Radical Chemistry: Radical Sources, Spectra, Reaction Kinetics and Prediction Tools. ChemPhysChem 2010, 11, 3796– 3822,  DOI: 10.1002/cphc.201000533
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    Tropospheric aqueous-phase free-radical chemistry: Radical sources, spectra, reaction kinetics and prediction tools
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    ChemPhysChem (2010), 11 (18), 3796-3822CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. The most important radicals which need to be considered for the description of chem. conversion processes in tropospheric aq. systems are the hydroxyl radical (OH), the nitrate radical (NO3) and sulfur-contg. radicals such as the sulfate radical (SO4-). For each of the three radicals their generation and their properties are discussed first in the corresponding sections. The main focus herein is to summarize newly published aq.-phase kinetic data on OH, NO3 and SO4- radical reactions relevant for the description of multiphase tropospheric chem. The data compilation builds up on earlier datasets published in the literature. Since the last review in 2003 more than hundred new rate consts. are available from literature. In case of larger discrepancies between novel and already published rate consts. the available kinetic data for these reactions are discussed and recommendations are provided when possible. As many OH kinetic data are obtained by means of the thiocyanate (SCN-) system in competition kinetic measurements of OH radical reactions this system is reviewed in a subchapter of this review. Available rate consts. for the reaction sequence following the reaction of OH + SCN- are summarized. Newly published data since 2003 have been considered and averaged rate consts. are calcd. Applying competition kinetics measurements usually the formation of the radical anion (SCN)2- is monitored directly by absorption measurements. Within this subchapter available absorption spectra of the (SCN)2- radical anion from the last five decades are presented. Based on these spectra an averaged (SCN)2- spectrum was calcd. In the last years different estn. methods for aq. phase kinetic data of radical reactions have been developed and published. Such methods are often essential to est. kinetic data which are not accessible from the literature. Approaches for rate const. prediction include empirical correlations as well as structure activity relationships (SAR) either with or without the usage of quantum chem. descriptors. Recently published estn. methods for OH, NO3 and SO4- radical reactions in aq. soln. are finally summarized, compared and discussed.
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    Bianco, A.; Passananti, M.; Brigante, M.; Mailhot, G. Photochemistry of the Cloud Aqueous Phase: A Review. Molecules 2020, 25, 423,  DOI: 10.3390/molecules25020423
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    Photochemistry of the cloud aqueous phase: a review
    Bianco, Angelica; Passananti, Monica; Brigante, Marcello; Mailhot, Gilles
    Molecules (2020), 25 (2), 423CODEN: MOLEFW; ISSN:1420-3049. (MDPI AG)
    A review. This review paper describes briefly the cloud aq. phase compn. and deeply its reactivity in the dark and mainly under solar radiation. The role of the main oxidants (hydrogen peroxide, nitrate radical, and hydroxyl radical) is presented with a focus on the hydroxyl radical, which drives the oxidn. capacity during the day. Its sources in the aq. phase, mainly through photochem. mechanisms with H2O2, iron complexes, or nitrate/nitrite ions, are presented in detail. The formation rate of hydroxyl radical and its steady state concn. evaluated by different authors are listed and compared. Finally, a paragraph is also dedicated to the sinks and the reactivity of the hydroxyl radical with the main compds. found in the cloud aq. phase. This review presents an assessment of the reactivity in the cloud aq. phase and shows the significant potential impact that this medium can have on the chem. of the atm. and more generally on the climate.
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    Iriti, M.; Faoro, F. Oxidative stress, the paradigm of ozone toxicity in plants and animals. Water, Air, Soil Pollut. 2007, 187, 285– 301,  DOI: 10.1007/s11270-007-9517-7
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    Jaeglé, L.; Jacob, D. J.; Brune, W. H.; Wennberg, P. O. Chemistry of HOx radicals in the upper troposphere. Atmos. Environ. 2001, 35, 469– 489,  DOI: 10.1016/S1352-2310(00)00376-9
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    Chemistry of HOx radicals in the upper troposphere
    Jaegle, L.; Jacob, D. J.; Brune, W. H.; Wennberg, P. O.
    Atmospheric Environment (2000), 35 (3), 469-489CODEN: AENVEQ; ISSN:1352-2310. (Elsevier Science Ltd.)
    Aircraft observations from three recent missions (STRAT, SUCCESS, SONEX) are synthesized into a theor. anal. of the factors controlling the concns. of HOx radicals (HOx=OH+peroxy) and the larger reservoir family HOy (HOy=HOx+2H2O2+2CH3OOH+HNO2+HNO4) in the upper troposphere. Photochem. model calcns. capture 66% of the variance of obsd. HOx concns. Two master variables det. the variance of the 24 h av. HOx concns.: the primary HOx prodn. rate, P(HOx), and the concn. of nitrogen oxide radicals (NOx=NO+NO2). The authors use these two variables as a coordinate system to diagnose the photochem. of the upper troposphere and map the different chem. regimes. Primary HOx prodn. is dominated by the O(1D)+H2O reaction when [H2O]>100 ppmv, and by photolysis of acetone (and possibly other convected HOx precursors) under drier conditions. For the principally northern mid latitude conditions sampled by the aircraft missions, the HOx yield from acetone photolysis ranges from 2 to 3. Methane oxidn. amplifies the primary HOx source by a factor of 1.1-1.9. Chem. cycling within the HOx family has a chain length of 2.5-7, while cycling between the HOx family and its HOy reservoirs has a chain length of 1.6-2.2. The no. of ozone mols. produced per HOy mol. consumed ranges from 4 to 12, such that ozone prodn. rates vary between 0.3 and 5 ppbv d-1 in the upper troposphere. Three chem. regimes (NOx-limited, transition, NOx-satd.) are identified to describe the dependence of HOx concns. and ozone prodn. rates on the two master variables P(HOx) and [NOx]. Simplified anal. expressions are derived to express these dependences as power laws for each regime. By applying an eigen lifetime anal. to the HOx-NOx-O3 chem. system, the decay of a perturbation to HOy in the upper troposphere (as from deep convection) is represented by four dominant modes with the longest time scale being factors of 2-3 times longer than the steady-state lifetime of HOy.
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    Hewitt, C. N.; Kok, G. L. Formation and Occurence of Organic Hydroperoxides in the Troposphere: Laboratory and Field Observations. J. Atmos. Chem. 1991, 12, 181– 194,  DOI: 10.1007/BF00115779
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    Formation and occurrence of organic hydroperoxides in the troposphere: laboratory and field observations
    Hewitt, C. Nicholas; Kok, Gregory L.
    Journal of Atmospheric Chemistry (1991), 12 (2), 181-94CODEN: JATCE2; ISSN:0167-7764.
    The formation and occurrence of hydroperoxides in the troposphere were studied in lab. expts. and field measurements. Nine alkenes were reacted individually with O3 in a reaction chamber in the presence of excess H2O and the amts. of H2O2 and 9 org. hydroperoxides produced in the gas and aerosol phases and deposited on the chamber walls were detd. by HPLC. The reactions of ethene, propene, 1-butene, and isoprene gave hydroxymethyl hydroperoxide as the major product with no H2O2 obsd. In the case of α- and β-pinene, 2-carene, and limonene the major product was H2O2. cis-2-Butene produced H2O2 and Me hydroperoxide. Preliminary measurements of H2O2 and 5 org. hydroperoxides in ambient air were made at Niwot Ridge, Colorado from 24 July-4 August 1989. The gas-phase species were preconcd. by cryotrapping with subsequent HPLC sepn. The gas-phase concns. of H2O2 ranged from 0.5-2 parts per tryllion-vol. (pptv) with the lowest concns. being measured at night and the highest under conditions of strong photochem. activity. The max. concns. ranged from <50 to 800 pptv and 3 other org. hydroperoxides were detected at concns. <200 pptv. High vol. aerosol samples yielded H2O2 and Me hydroperoxide concns. <10 ng/m3 while H2O2 and 6 org. species were detected in rainwater at concns. in the range <0.01-50 μM.
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    Detection of methyl, hydroxymethyl and hydroxyethyl hydroperoxides in air and precipitation
    Hellpointner, E.; Gaeb, S.
    Nature (London, United Kingdom) (1989), 337 (6208), 631-4CODEN: NATUAS; ISSN:0028-0836.
    During Feb.-Mar., 1988, in the Freising/Munich area, West Germany, snow samples had an av. H2O2 concn. of 0.65 μ mol/L; rain samples from the same period had significantly higher H2O2 concns. (mean 3.5 μ mol/L). Rain samples had an av. concn. of HOCH2OOH (I) of 0.1 μ mol/L, with higher concns. occurring in samples having a pH of <5.5. In contrast, MeCH(OH)OOH (II) and MeOOH were present at low concns. which appeared to be nearly independent of pH. The peroxide concns. in rain samples in Apr. and May, 1988, varied much more widely. After extended periods of fine weather, the 1st rain, typically with high pH values, contained little H2O2, no I or II, and av. amts. of MeOOH; 1-2 h later, however, peak concns. of H2O2 were reached at a pH that was still relatively high, and hydroxyalkyl hydroperoxides could be found only as the pH dropped with prolonged rain. In general, peroxide concns. increased significantly whenever snow turned into rain. Anal. of 17 aq. samples obtained by freezing water out of the air showed the presence of MeOOH; I and II could not be detected in samples taken in the morning. Possible mechanisms of formation of the hydroperoxides and possible biol. effects are discussed.
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    Heterogeneous chemistry and tropospheric ozone
    Jacob, Daniel J.
    Atmospheric Environment (2000), 34 (12-14), 2131-2159CODEN: AENVEQ; ISSN:1352-2310. (Elsevier Science Ltd.)
    A review with many refs. on current knowledge and hypotheses on ozone formation in the troposphere by gas-phase oxidn. of hydrocarbons and CO catalyzed by hydrogen oxide radicals and nitrogen oxide radicals. Effects of heterogeneous chem. involving reactions in aerosol particles and cloud droplets on O3 concns. are regarded. Reaction probability parameterizations for reactive uptake of gases by aq. aerosols and clouds are recommended for chem. mechanisms in std. O3 models. Hypotheses regarding fast O3 loss on soot or in clouds, fast redn. of HNO3 to NOx in aerosols, or heterogeneous loss of CH2O are not supported by evidence. Halogen radical chem. could possibly be significant in the marine boundary layer but more evidence is needed. Recommendations for future research are presented.
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    Anglada, J. M.; Martins-Costa, M.; Ruiz-López, M. F.; Francisco, J. S. Spectroscopic signatures of ozone at the air–water interface and photochemistry implications. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 11618– 11623,  DOI: 10.1073/pnas.1411727111
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    Spectroscopic signatures of ozone at the air-water interface and photochemistry implications
    Anglada, Josep M.; Martins-Costa, Marilia; Ruiz-Lopez, Manuel F.; Francisco, Joseph S.
    Proceedings of the National Academy of Sciences of the United States of America (2014), 111 (32), 11618-11623CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    First-principles simulations suggest that addnl. OH formation in the troposphere can result from ozone interactions with the surface of cloud droplets. Ozone exhibits an affinity for the air-water interface, which modifies its UV and visible light spectroscopic signatures and photolytic rate const. in the troposphere. Ozone cross sections on the red side of the Hartley band (290- to 350-nm region) and in the Chappuis band (450-700 nm) are increased due to electronic ozone-water interactions. This effect, combined with the potential contribution of the O3 + hν → O(3P) + O2(X3Σg-) photolytic channel at the interface, leads to an enhancement of the OH radical formation rate by four orders of magnitude. This finding suggests that clouds can influence the overall oxidizing capacity of the troposphere on a global scale by stimulating the prodn. of OH radicals through ozone photolysis by UV and visible light at the air-water interface.
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    Vácha, R.; Slavíček, P.; Mucha, M.; Finlayson-Pitts, B. J.; Jungwirth, P. Adsorption of Atmospherically relevant Gases at the Air/Water Interface: Free Energy Profiles of Aqueous Solvation of N2, O2, O3, H2O, HO2, and H2O2. J. Phys. Chem. A 2004, 108, 11573– 11579,  DOI: 10.1021/jp046268k
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    Adsorption of atmospherically relevant gases at the air/water interface: Free energy profiles of aqueous solvation of N2, O2, O3, OH, H2O, HO2, and H2O2
    Vacha, Robert; Slavicek, Petr; Mucha, Martin; Finlayson-Pitts, Barbara J.; Jungwirth, Pavel
    Journal of Physical Chemistry A (2004), 108 (52), 11573-11579CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    Free energy profiles assocd. with moving atm. gases or radicals across the air/water interface were calcd. as potentials of mean force by classical mol. dynamics simulations. With the employed force field, the exptl. hydration free energies are satisfactorily reproduced. The main finding is that both hydrophobic gases (N2, O2, and O3) and hydrophilic species (OH, HO2, or H2O2) have a free energy min. at the air/water interface. As a consequence, it is inferred that atm. gases, with the exception of water vapor, exhibit enhanced concns. at surfaces of aq. aerosols. This has important implications for understanding heterogeneous chem. processes in the troposphere.
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    Vieceli, J.; Roeselova, M.; Potter, N.; Dang, L. X.; Garrett, B. C.; Tobias, D. J. Molecular dynamics simulations of atmospheric oxidants at the air-water interface: Solvation and accommodation of OH and O3. J. Phys. Chem. B 2005, 109, 15876– 15892,  DOI: 10.1021/jp051361+
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    Molecular dynamics simulations of atmospheric oxidants at the air-water interface: Solvation and accommodation of OH and O3
    Vieceli, John; Roeselova, Martina; Potter, Nicholas; Dang, Liem X.; Garrett, Bruce C.; Tobias, Douglas J.
    Journal of Physical Chemistry B (2005), 109 (33), 15876-15892CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
    A comparative study of OH, O3, and H2O equil. aq. solvation and gas-phase accommodation on liq. water at 300 K is performed using a combination of ab initio calcns. and mol. dynamics simulations. Polarizable force fields are developed for the interaction potential of OH and O3 with water. The free energy profiles for transfer of OH and O3 from the gas phase to the bulk liq. exhibit a pronounced min. at the surface, but no barrier to solvation in the bulk liq. The calcd. surface excess of each oxidant is comparable to calcd. and exptl. values for short chain, aliph. alcs. Driving forces for the surface activity are discussed in terms of the radial distribution functions and dipole orientation distributions for each mol. in the bulk liq. and at the surface. Simulations of OH, O3, and H2O impinging on liq. water with a thermal impact velocity are used to calc. thermal accommodation (S) and mass accommodation (α) coeffs. The values of S for OH, O3, and H2O are 0.95, 0.90, and 0.99, resp. The approaching mols. are accelerated toward the liq. surface when they are approx. 5 Å above it. The mols. that reach thermal equil. with the surface do so within 2 ps of striking the surface, while those that do not scatter into the gas phase with excess translational kinetic energy in the direction perpendicular to the surface. The time consts. for absorption and desorption range from approx. 35 to 140 ps, and the values of α for OH, O3, and H2O are 0.83, 0.047, and 0.99, resp. The results are consistent with previous formulations of gas-phase accommodation from simulations, in which the process occurs by rapid thermal and structural equilibration followed by diffusion on the free energy profile. The implications of these results with respect to atm. chem. are discussed.
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    Vaghjiani, G. L.; Ravishankara, A. R. Photodissociation of H2O2 and CH3OOH at 248 nm and 298 K: Quantum yields for OH, O(3P) and H(2S). J. Chem. Phys. 1990, 92, 996– 1003,  DOI: 10.1063/1.458081
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    Photodissociation of hydrogen peroxide and methyl hydroperoxide (CH3OOH) at 248 nm and 298 K: quantum yields for hydroxyl, atomic oxygen, and atomic hydrogen (O(3P), and H(2S))
    Vaghjiani, Ghanshyam L.; Ravishankara, A. R.
    Journal of Chemical Physics (1990), 92 (2), 996-1003CODEN: JCPSA6; ISSN:0021-9606.
    The quantum yields of OH, O, and H in H2O2 photolysis (248 nm, 298 K) were 2.09 ± 0.36, <0.002 and <0.0002 for OH, O and H, resp. For CH3OOH photolysis, the quantum yields were detd. as 1 ± 0.18, <0.007 and 0.038 ± 0.007 for OH, O and H, resp. In both H2O2 and CH3OOH photolysis, the obsd. O and H quantum yields showed an apparent dependence on the fluence of the photolysis light, the possible origin of which is discussed. The large quantum yield of OH was consistent with the known continuous and unstructured absorption spectra of these mols. in this wavelength region, where the key process is the dissociative (~A1A ← ~X1A) transition to give OH (X2Π, ν'' = 0) fragment.
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    Vaghjiani, G. L.; Turnipseed, A. A.; Warren, R. F.; Ravishankara, A. R. Photodissociation of H2O2 at 193 and 222 nm: Products and quantum yields. J. Chem. Phys. 1992, 96, 5878– 5886,  DOI: 10.1063/1.462684
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    Photodissociation of hydrogen peroxide at 193 and 222 nm: products and quantum yields
    Vaghjiani, Ghanshyam L.; Turnipseed, Andrew A.; Warren, Rachel F.; Ravishankara, A. R.
    Journal of Chemical Physics (1992), 96 (8), 5878-86CODEN: JCPSA6; ISSN:0021-9606.
    The primary quantum yields of OH (X2 Π), H(2S), and O atoms [O(1D) + O(3P)] produced in the photodissocn. of H2O2 at 193 and 222 nm were measured at 298 K. At 193 nm, the primary quantum yields were 1.51 ± 0.18, 0.16 ± 0.04, and <0.02, for Φ(OH), Φ(H), and the sum of Φ(O) and Φ(O1S), resp. At 222 nm, the OH yield was Φ(OH) = 2.02 ± 0.35, the H atom yield was Φ(H) = 0.024 ± 0.012, and Φ(O) was < 0.002. The errors quoted are 2σ, precision plus estd. systematic errors. The OH product was directly monitored by pulsed laser-induced fluorescence, and the at. species were detected via cw resonance fluorescence. The OH quantum yields reported were measured relative to known product quantum yields in the dissocn. of H2O2 at 248 nm. H(2S) yields were measured relative to those in photolysis of HBr and HCl, (at 193 nm) or CH3SH (at 222 nm), whereas O atom yields were measured relative to O3 photolysis at both wavelengths. The results indicate unit dissocn. of H2O2 at both 222 and 193 nm with only 2 major products OH (∼80% at 193 nm, 98% at 222 nm) and H(2S) (∼20% at 193 nm, 2% at 222 nm). Up to 15% of the OH produced in the 193 nm photolysis may be vibrationally excited; however, no evidence for vibrationally excited OH was obsd. at 222 nm.
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    Blitz, M. A.; Heard, D. E.; Pilling, M. J. Wavelength dependent photodissociation of CH3OOH: Quantum yields for CH3O and OH, and measurement of the OH+CH3OOH rate coefficient. J. Photochem. Photobiol., A 2005, 176, 107– 113,  DOI: 10.1016/j.jphotochem.2005.09.017
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    Wavelength dependent photodissociation of CH3OOH
    Blitz, Mark A.; Heard, Dwayne E.; Pilling, Michael J.
    Journal of Photochemistry and Photobiology, A: Chemistry (2005), 176 (1-3), 107-113CODEN: JPPCEJ; ISSN:1010-6030. (Elsevier B.V.)
    Me hydroperoxide, CH3OOH, has been synthesized with >99.5% purity, confirmed using UV absorption spectroscopy and high-pressure liq. chromatog. (HPLC) followed by post-column derivatization. The UV absorption cross-section for CH3OOH was measured and for < 325 nm was in good agreement with the literature. Laser-flash photolysis combined with laser-induced fluorescence (LIF) spectroscopy has been used to measure both OH and CH3O photofragments following the photolysis of CH3OOH in the wavelength range 223-355 nm. Using the previously measured unity quantum yield for OH at 248 nm as a ref., the LIF signals immediately following photolysis were used to measure wavelength dependent quantum yields for OH and CH3O, taking into account changes in laser pulse energy and absorption cross-section. The quantum yields for both species were unity within exptl. error. The rate coeff. for the reaction of OH with CH3OOH (R1a) to generate CH3O2 + H2O products was measured at 295 K to be k (R1a) = (9.0 ± 0.2) × 10-12 cm3 mol.-1 s-1, considerably higher (by about a factor of two) than previous values measured by Vaghjiani and Ravishankara [G.L. Vaghjiani, A.R. Ravishankara, J. Phys. Chem. 93 (1989) 1948-1959] and Niki et al. [H. Niki, P.D. Maker, C.M. Savage, L.P. Breitenbach, J. Phys. Chem. 87 (1983) 2190-2193].
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    Roehl, C. M.; Marka, Z.; Fry, J. L.; Wennberg, P. O. Near-UV photolysis cross-sections of CH3OOH and HOCH2OOH determined via action spectroscopy. Atmos. Chem. Phys. 2007, 7, 713– 720,  DOI: 10.5194/acp-7-713-2007
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    Near-UV photolysis cross sections of CH3OOH and HOCH2OOH determined via action spectroscopy
    Roehl, C. M.; Marka, Z.; Fry, J. L.; Wennberg, P. O.
    Atmospheric Chemistry and Physics (2007), 7 (3), 713-720CODEN: ACPTCE; ISSN:1680-7316. (European Geosciences Union)
    Knowledge of mol. photolysis cross sections is important for detg. atm. lifetimes and fates of many species. A method and laser app. for measurement of these cross sections in the near-UV region is described. The technique is based on action spectroscopy, where the yield of a photodissocn. product (in this case OH) is measured as a function of excitation energy. For compds. yielding OH, this method can be used to measure near-UV photodissocn. cross section as low as 10-23 cm2 mol.-1. The method is applied to det. the photodissocn. cross sections for Me hydroperoxide (CH3OOH; MHP) and hydroxymethyl hydroperoxide (HOCH2OOH; HMHP) in the 305-365 nm wavelength range. The measured cross sections are in good agreement with previous measurements of absorption cross sections.
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    Matthews, J.; Sinha, A.; Francisco, J. S. The Importance of Weak Absorption Features in Promoting Tropospheric Radical Production. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 7449– 7452,  DOI: 10.1073/pnas.0502687102
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    The importance of weak absorption features in promoting tropospheric radical production
    Matthews, Jamie; Sinha, Amitabha; Francisco, Joseph S.
    Proceedings of the National Academy of Sciences of the United States of America (2005), 102 (21), 7449-7452CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    Atm. field measurement and modeling studies have long noted discrepancies between observation and predictions of OH and HO2 concns. in the atm. Novel photochem. mechanisms have been proposed to explain these differences. Although inclusion of these addnl. sources improves agreement, they are unable to fully account for the observations. The authors report and demonstrate the importance of weak electronic absorption features, normally ignored or not measured, in contributing to significant OH radical prodn. Expts. on Me hydroperoxide, a prototypical org. peroxide in large abundance in the troposphere, highlights how photochem. in the neglected electronic absorption tail makes an important addn. to the tropospheric OH budget. The present results underscore the need to measure absorption cross sections for atm. mols. over a wider dynamic range, esp. over the wavelength regions where the solar flux is high, to fully quantitate their contributions to atm. photochem.
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    Herrmann, H. On the photolysis of simple anions and neutral molecules as sources of O–/OH, SOx– and Cl in aqueous solution. Phys. Chem. Chem. Phys. 2007, 9, 3935– 3964,  DOI: 10.1039/B618565G
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    On the photolysis of simple anions and neutral molecules as sources of O-/OH, SOx- and Cl in aqueous solution
    Herrmann, Hartmut
    Physical Chemistry Chemical Physics (2007), 9 (30), 3935-3964CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    This contribution examines the aq. phase photolysis processes of simple anions such as nitrate, nitrite, peroxodisulfate and neutral mols. such as H2O2. The review includes new results on abs. effective quantum yields for the photodissocn. processes of NO3-, NO2-, S2O82-, HSO5-, S2O62-, HOCl, and chloroacetone in an aq. soln. The quantum yields for the photolysis of nitrate and nitrite have also been detd. as a function of temp. Models to interpret the wavelength and the temp. dependencies of the quantum yields for the different systems are discussed and a simple model treatment is developed to quantify the effects of (i) impulse conservation, (ii) electrostatic interaction (e.g., ion-dipole, dipole-dipole and Coulomb interaction between the photofragments directly after photolytic fragmentation), and (iii) diffusion and recombination. The combined impulse-interaction-diffusion (IID) model is compared to the exptl. obsd. effective radical formation quantum yields and reasonable agreement is found for a no. of systems. It is shown that the temp. dependencies for effective quantum yields of photolysis processes in aq. soln. are not only governed by the temp. dependence of the viscosity of water but also detd. by the temp. dependence of the rate consts. of the photofragment recombination reactions.
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    Zellner, R.; Exner, M.; Herrmann, H. Absolute OH quantum yields in the laser photolysis of nitrate, nitrite and dissolved H2O2 at 308 and 351 nm in the temperature-range 278–353 K. J. Atmos. Chem. 1990, 10, 411– 425,  DOI: 10.1007/BF00115783
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    Absolute hydroxyl quantum yields in the laser photolysis of nitrate, nitrite and dissolved hydrogen peroxide at 308 and 351 nm in the temperature range 278-353 K
    Zellner, R.; Exner, M.; Herrmann, H.
    Journal of Atmospheric Chemistry (1990), 10 (4), 411-25CODEN: JATCE2; ISSN:0167-7764.
    Abs. quantum yields for the formation of OH radicals in the laser photolysis of aq. solns. of NO3-, NO2-, and H2O2 at 308 and 351 nm and as a function of pH and temp. were measured. A scavenging technique involving the reaction between OH and SCN- ions and the time resolved detection by visible absorption of the (SCN)2- radical ion was used to det. the abs. OH yields. Together with the absorption coeffs. and an assumed actinic flux within atm. droplets of twice the clear air value, the partial photolytic lifetimes (τOH) of these species at 298 K are estd. as 10.5 days, 5.4 h, and 30.3 h for NO3-, NO2-, and H2O2, resp. These lifetimes will increase by a factor of 2 (NO3-, NO2-) and by 15% (H2O2) at 278 K. Using av. ambient concns. in tropospheric aq. droplets, the photolytic OH source strengths from these species are calcd. to be 2.8 × 10-11, 1.3 × 10-11, and 1.4 × 10-11 mol/L s for NO3-, NO2-, and H2O2, resp.
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    Anastasio, C.; Robles, T. Light absorption by soluble chemical species in Arctic and Antarctic snow. J. Geophys. Res. 2007, 112, 2217,  DOI: 10.1029/2007JD008695
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    Kamboures, M. A.; Nizkorodov, S. A.; Gerber, R. B. Ultrafast photochemistry of methyl hydroperoxide on ice particles. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 6600– 6604,  DOI: 10.1073/pnas.0907922106
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    Ultrafast photochemistry of methyl hydroperoxide on ice particles
    Kamboures, M. A.; Nizkorodov, S. A.; Gerber, R. B.
    Proceedings of the National Academy of Sciences of the United States of America (2010), 107 (15), 6600-6604CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    Simulations show that photodissocn. of Me hydroperoxide, CH3OOH, on water clusters produces a surprisingly wide range of products on a subpicosecond time scale, pointing to the possibility of complex photodegrdn. pathways for org. perox- ides on aerosols and water droplets. Dynamics are computed at several excitation energies at 50 K using a semiempirical PM3 potential surface. CH3OOH is found to prefer the exterior of the cluster, with the CH3O group sticking out and the OH group immersed within the cluster. At atmospherically relevant photodissocn. wavelengths the OH and CH3O photofragments remain at the surface of the cluster or embedded within it. However, none of the 25 completed trajectories carried out at the atmospherically relevant photodissocn. energies led to recombination of OH and CH3O to form CH3OOH. Within the limited statistics of the available trajectories the predicted yield for the recombination is zero. Instead, various reactions involving the initial fragments and water promptly form a wide range of stable mol. products such as CH2O, H2O, H2, CO, CH3OH, and H2O2.
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    Epstein, S. A.; Shemesh, D.; Tran, V. T.; Nizkorodov, S. A.; Gerber, R. B. Absorption Spectra and Photolysis of Methyl Peroxide in Liquid and Frozen Water. J. Phys. Chem. A 2012, 116, 6068– 6077,  DOI: 10.1021/jp211304v
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    Absorption Spectra and Photolysis of Methyl Peroxide in Liquid and Frozen Water
    Epstein, Scott A.; Shemesh, Dorit; Tran, Van T.; Nizkorodov, Sergey A.; Gerber, R. Benny
    Journal of Physical Chemistry A (2012), 116 (24), 6068-6077CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    Me peroxide (MeOOH) is commonly found in atm. waters and ices in significant concns. It is the simplest org. peroxide and an important precursor to hydroxyl radical. Many studies have examd. the photochem. behavior of gaseous MeOOH; however, the photochem. of liq. and frozen H2O solns. is poorly understood. The authors present expts. and theor. calcns. designed to elucidate the photochem. behavior of MeOOH dissolved in liq. H2O and ice over a range of temps. The molar absorptivities of aq. MeOOH are different from the gas phase, and they do not change upon freezing. Between -12 and 43°, the quantum yield of MeOOH photolysis is described by the following equation: Φ(T) = exp(((-2175 ± 448)1/T) + 7.66 ± 1.56). The authors use on-the-fly ab initio mol. dynamics simulations to model structures and absorption spectra of a bare MeOOH mol. and a MeOOH mol. immersed inside 20 H2O mols. at 50, 200, and 220 K. The simulations predict large sensitivity in the absorption spectrum of MeOOH to temp., with the spectrum narrowing and shifting to the blue under cryogenic conditions because of constrained dihedral motion around the O-O bond. The shift in the absorption spectrum is not obsd. in the expt. when the MeOOH soln. is frozen suggesting that MeOOH remains in a liq. layer between the ice grains. Using the extinction coeffs. and photolysis quantum yields obtained under conditions with low temps., in the presence of clouds with a high liq.-H2O content and large solar zenith angles, the loss of MeOOH by aq. photolysis is responsible for up to 20% of the total loss of MeOOH due to photolysis. Gas phase photolysis of MeOOH dominates under all other conditions.
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    Monod, A.; Chevallier, E.; Durand Jolibois, R.; Doussin, J. F.; Picquet-Varrault, B.; Carlier, P. Photooxidation of methylhydroperoxide and ethylhydroperoxide in the aqueous phase under simulated cloud droplet conditions. Atmos. Environ. 2007, 41, 2412– 2426,  DOI: 10.1016/j.atmosenv.2006.10.006
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    Photooxidation of methylhydroperoxide and ethylhydroperoxide in the aqueous phase under simulated cloud droplet conditions
    Monod, A.; Chevallier, E.; Durand Jolibois, R.; Doussin, J. F.; Picquet-Varrault, B.; Carlier, P.
    Atmospheric Environment (2007), 41 (11), 2412-2426CODEN: AENVEQ; ISSN:1352-2310. (Elsevier Ltd.)
    The photooxidn. of methylhydroperoxide (MHP) and ethylhydroperoxide (EHP) was studied in the aq. phase under simulated cloud droplet conditions. The kinetics and the reaction products of direct photolysis and OH-oxidn. were studied for both compds. The photolysis frequencies obtained were JMHP = 4.5 (±1.0) × 10-5 s-1 and JEHP = 3.8 (±1.0) × 10-5 s-1 for MHP and EHP resp. at 6°C. The rate consts. of OH-oxidn. of MHP at 6°C were 6.3 (±2.6) × 108 M-1 s-1 and 5.8 (±1.9) × 108 M-1 s-1 relative to ethanol and 2-propanol resp., and the rate const. of OH-oxidn. of EHP was 2.1 (±0.6) × 109 M-1 s-1 relative to 2-propanol at 6°C. The reaction products obtained were not only the corresponding aldehydes, but also the corresponding acids, and hydroxyhydroperoxides as primary reaction products. The yields for these products were sensitive to the pH value. The carbon balance was higher than 85% for all expts., showing that most reaction products were detected. A chem. mechanism was proposed for each reaction, and the atm. implications were discussed.
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    Warneck, P. The relative importance of various pathways for the oxidation of sulfur dioxide and nitrogen dioxide in sunlit continental fair weather clouds. Phys. Chem. Chem. Phys. 1999, 1, 5471– 5483,  DOI: 10.1039/a906558j
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    The relative importance of various pathways for the oxidation of sulfur dioxide and nitrogen dioxide in sunlit continental fair weather clouds
    Warneck, Peter
    Physical Chemistry Chemical Physics (1999), 1 (24), 5471-5483CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    A simple box model of a sunlit small cumulus cloud has been used to explore the efficiency of various chem. reactions contributing to the oxidn. of sulfur dioxide and nitrogen dioxide in clouds. The principal aq.-phase processes of sulfur(IV) oxidn. are reactions with ozone, with hydroperoxides, with OH radicals, and catalytic reactions involving transition metals. The last two oxidants initiate chain oxidn. processes, which were analyzed in detail. The results indicate that chain reactions are not very effective, partly because the chain carriers are scavenged, and partly because chain termination overrides chain propagation. Hydrogen peroxide is the most effective oxidant in S(IV) oxidn., contributing about 80% to the total rate. Peroxynitric acid also contributes appreciably, in addn. to ozone. The oxidn. of nitrogen dioxide to nitric acid occurs to 60% in the gas phase by reaction with OH radicals. In the aq. phase, the reaction of peroxynitric acid with hydrogensulfite is most important, contributing 20-30% to the total rate.
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    Martins-Costa, M. T. C.; Ruiz-Lopez, M. F. Highly accurate computation of free energies in complex systems through horsetail QM/MM molecular dynamics combined with free-energy perturbation theory. Theor. Chem. Acc. 2017, 136, 50,  DOI: 10.1007/s00214-017-2078-y
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    Martins-Costa, M. T. C.; Ruiz-Lopez, M. F. Reaching Multi-Nanosecond Timescales in Combined QM/MM Molecular Dynamics Simulations through Parallel Horsetail Sampling. J. Comput. Chem. 2017, 38, 659– 668,  DOI: 10.1002/jcc.24723
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    Reaching multi-nanosecond timescales in combined QM/MM molecular dynamics simulations through parallel horsetail sampling
    Martins-Costa, Marilia T. C.; Ruiz-Lopez, Manuel F.
    Journal of Computational Chemistry (2017), 38 (10), 659-668CODEN: JCCHDD; ISSN:0192-8651. (John Wiley & Sons, Inc.)
    We report an enhanced sampling technique that allows to reach the multi-nanosecond timescale in quantum mechanics/mol. mechanics mol. dynamics simulations. The proposed technique, called horsetail sampling, is a specific type of multiple mol. dynamics approach exhibiting high parallel efficiency. It couples a main simulation with a large no. of shorter trajectories launched on independent processors at periodic time intervals. The technique is applied to study hydrogen peroxide at the water liq.-vapor interface, a system of considerable atm. relevance. A total simulation time of a little more than 6 ns has been attained for a total CPU time of 5.1 years representing only about 20 days of wall-clock time. The discussion of the results highlights the strong influence of the solvation effects at the interface on the structure and the electronic properties of the solute. © 2017 Wiley Periodicals, Inc.
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    Martins-Costa, M. T. C.; Anglada, J. M.; Francisco, J. S.; Ruiz-Lopez, M. F. Impacts of cloud water droplets on the OH production rate from peroxide photolysis. Phys. Chem. Chem. Phys. 2017, 19, 31621– 31627,  DOI: 10.1039/C7CP06813A
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    Impacts of cloud water droplets on the OH production rate from peroxide photolysis
    Martins-Costa, M. T. C.; Anglada, J. M.; Francisco, J. S.; Ruiz-Lopez, Manuel F.
    Physical Chemistry Chemical Physics (2017), 19 (47), 31621-31627CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Understanding the difference between obsd. and modeled concns. of HOx radicals in the troposphere is a current major issue in atm. chem. It is widely believed that existing atm. models miss a source of such radicals and several potential new sources have been proposed. In recent years, interest has increased on the role played by cloud droplets and org. aerosols. Computer modeling of ozone photolysis, for instance, has shown that atm. aq. interfaces accelerate the assocd. OH prodn. rate by as much as 3-4 orders of magnitude. Since methylhydroperoxide is a main source and sink of HOx radicals, esp. at low NOx concns., it is fundamental to assess what is the influence of clouds on its chem. and photochem. In this study, computer simulations for the photolysis of methylhydroperoxide at the air-water interface have been carried out showing that the OH prodn. rate is severely enhanced, reaching a comparable level to ozone photolysis.
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    Nissenson, P.; Dabdub, D.; Das, R.; Maurino, V.; Minero, C.; Vione, D. Evidence of the water-cage effect on the photolysis of and FeOH2+. Implications of this effect and of H2O2 surface accumulation on photochemistry at the air–water interface of atmospheric droplets. Atmos. Environ. 2010, 44, 4859– 4866,  DOI: 10.1016/j.atmosenv.2010.08.035
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    Evidence of the water-cage effect on the photolysis of NO3- and FeOH2+. Implications of this effect and of H2O2 surface accumulation on photochemistry at the air-water interface of atmospheric droplets
    Nissenson, P.; Dabdub, D.; Das, R.; Maurino, V.; Minero, C.; Vione, D.
    Atmospheric Environment (2010), 44 (38), 4859-4866CODEN: AENVEQ; ISSN:1352-2310. (Elsevier Ltd.)
    Expts. are conducted to det. the effect of a cage of water mols. on the photolysis quantum yields of nitrate, FeOH2+, and H2O2. Results suggest that the quantum yields of nitrate and FeOH2+ are decreased by the recombination of photo-fragments (·OH + ·NO2 and Fe2+ + ·OH, resp.) before they leave the surrounding cage of water mols. However, no evidence is found for an enhanced quantum yield for H2O2. Therefore, the photolysis of nitrate and FeOH2+ could be enhanced if the cage of the solvent mols. is incomplete, as is the case at the air-water interface of atm. droplets. The photolysis rate const. distribution within nitrate, FeOH2+, and H2O2 aerosols is calcd. by combining the expected quantum yield data in the bulk and at the interface with Mie theory calcns. of light intensity. The photolysis rate const. of nitrate and FeOH2+ would be significantly higher at the surface than in the bulk if quantum yields are enhanced at the surface. In the case of H2O2, the photolysis rate const. would be enhanced by surface accumulation. The results concerning the expected rates of photolysis of these photoactive species are applied to the assessment of the reaction between benzene and ·OH in the presence of ·OH scavengers in an atmospherically relevant scenario. For a droplet of 1 μm radius, a large fraction of the total ·OH-benzene reaction (15% for H2O2, 20% for nitrate, and 35% for FeOH2+) would occur in the surface layer, which accounts for just 0.15% of the droplet vol.
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    Finlayson-Pitts, B. J.; Wingen, L. M.; Sumner, A. L.; Syomin, D.; Ramazan, K. A. The heterogeneous hydrolysis of NO2 in laboratory systems and in outdoor and indoor atmospheres: An integrated mechanism. Phys. Chem. Chem. Phys. 2003, 5, 223– 242,  DOI: 10.1039/b208564j
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    The heterogeneous hydrolysis of NO2 in laboratory systems and in outdoor and indoor atmospheres: An integrated mechanism
    Finlayson-Pitts, B. J.; Wingen, L. M.; Sumner, A. L.; Syomin, D.; Ramazan, K. A.
    Physical Chemistry Chemical Physics (2003), 5 (2), 223-242CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The heterogeneous reaction of NO2 with water on the surface of lab. systems has been known for decades to generate HONO, a major source of OH that drives the formation of ozone and other air pollutants in urban areas and possibly in snowpacks. Previous studies have shown that the reaction is first order in NO2 and in water vapor, and the formation of a complex between NO2 and water at the air-water interface has been hypothesized as being the key step in the mechanism. We report data from long path FTIR studies in borosilicate glass reaction chambers of the loss of gaseous NO2 and the formation of the products HONO, NO and N2O. Further FTIR studies were carried out to measure species generated on the surface during the reaction, including HNO3, N2O4 and NO2+. We propose a new reaction mechanism in which we hypothesize that the sym. form of the NO2 dimer, N2O4, is taken up on the surface and isomerizes to the asym. form, ONONO2. The latter autoionizes to NO+NO3-, and it is this intermediate that reacts with water to generate HONO and surface-adsorbed HNO3. Nitric oxide is then generated by secondary reactions of HONO on the highly acidic surface. This new mechanism is discussed in the context of our exptl. data and those of previous studies, as well as the chem. of such intermediates as NO+ and NO2+ that is known to occur in soln. Implications for the formation of HONO both outdoors and indoors in real and simulated polluted atmospheres, as well as on airborne particles and in snowpacks, are discussed. A key aspect of this chem. is that in the atm. boundary layer where human exposure occurs and many measurements of HONO and related atm. constituents such as ozone are made, a major substrate for this heterogeneous chem. is the surface of buildings, roads, soils, vegetation and other materials. This area of reactions in thin films on surfaces (SURFACE = Surfaces, Urban and Remote: Films As a Chem. Environment) has received relatively little attention compared to reactions in the gas and liq. phases, but in fact may be quite important in the chem. of the boundary layer in urban areas.
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    Crowley, J. N.; Carl, S. A. OH formation in the photoexcitation of NO2 beyond the dissociation threshold in the presence of water vapor. J. Phys. Chem. A 1997, 101, 4178– 4184,  DOI: 10.1021/jp970319e
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    OH Formation in the Photoexcitation of NO2 beyond the Dissociation Threshold in the Presence of Water Vapor
    Crowley, John N.; Carl, Shaun A.
    Journal of Physical Chemistry A (1997), 101 (23), 4178-4184CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    The pulsed-laser-excitation/resonance fluorescence technique was used to assess the efficiency of OH formation following photoexcitation of NO2 at discrete wavelengths beyond the photodissocn. threshold in the presence of water vapor: NO2* + H2O → HONO + OH. Excitation at wavelengths 432-449 nm led to OH prodn. via a facile sequential 2-photon absorption by NO2, leading to O(1D) and thus to OH in the presence of H2O, i.e., NO2 + hν → NO2*, NO2* + hν → NO2**, NO2** → NO + O(1D), O(1D) + H2O → 2OH. The cross section for the transition NO2** ← NO2* was similar to that for the NO2* ← NO2 transition at 435 nm. At 532 nm, the 2-photon process is not sufficiently energetic to form O(1D), and OH is not obsd. An upper limit of ∼7 × 10-5 was obsd. for the reactive quenching of NO2* by water vapor relative to collisional quenching. The atm. relevance of OH formation via NO2 excitation is discussed.
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    Morel, O.; Simonaitis, R.; Heicklen, J. Ultraviolet absorption spectra of HO2NO2, CCl3O2NO2, CCl2FO2NO2, and CH3O2NO2. Chem. Phys. Lett. 1980, 73, 38– 42,  DOI: 10.1016/0009-2614(80)85197-9
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    Ultraviolet absorption spectra of peroxynitric acid, peroxynitric acid trichloromethyl ester, peroxynitric acid dichlorofluoromethyl ester, and peroxynitric acid methyl ester
    Morel, Oscar; Simonaitis, R.; Heicklen, Julian
    Chemical Physics Letters (1980), 73 (1), 38-42CODEN: CHPLBC; ISSN:0009-2614.
    The UV absorption spectra of HO2NO2, CCl3O2NO2, CCl2FO2NO2, and CH3O2NO2 were measured from 210 to 280 nm. The spectra are similar, and the absorption intensity increases as the wavelength decreases with a shoulder at ≈ 255 nm. Assuming that every absorption leads to photodissocn., the photodissocn. lifetimes will be .ltorsim.1 day in the lower stratosphere and <1 h at the stratopause.
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    Murdachaew, G.; Varner, M. E.; Phillips, L. F.; Finlayson-Pitts, B. J.; Gerber, R. B. Nitrogen dioxide at the air-water interface: trapping, absorption, and solvation in the bulk and at the surface. Phys. Chem. Chem. Phys. 2013, 15, 204– 212,  DOI: 10.1039/C2CP42810E
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    Nitrogen dioxide at the air-water interface: trapping, absorption, and solvation in the bulk and at the surface
    Murdachaew, Garold; Varner, Mychel E.; Phillips, Leon F.; Finlayson-Pitts, Barbara J.; Gerber, R. Benny
    Physical Chemistry Chemical Physics (2013), 15 (1), 204-212CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The interaction of NO2 with water surfaces in the troposphere is of major interest in atm. chem. We examd. an initial step in this process, the uptake of NO2 by water through the use of mol. dynamics simulations. An NO2-H2O intermol. potential was obtained by fitting to high-level ab initio calcns. We detd. the binding of NO2-H2O to be about two times stronger than that previously calcd. From scattering simulations of an NO2 mol. interacting with a water slab we obsd. that the majority of the scattering events resulted in outcomes in which the NO2 mol. became trapped at the surface or in the interior of the water slab. Typical surface-trapped/adsorbed and bulk-solvated/absorbed trajectories were analyzed to obtain radial distribution functions and the orientational propensity of NO2 with respect to the water surface. We obsd. an affinity of the nitrogen atom for the oxygen in water, rather than hydrogen-bonding which was rare. The water solvation shell was less tight for the bulk-absorbed NO2 than for the surface-adsorbed NO2. Adsorbed NO2 demonstrated a marked orientational preference, with the oxygens pointing into the vacuum. Such behavior is expected for a mildly hydrophobic and surfactant mol. like NO2. Ests. based on our results suggest that at high NO2 concns. encountered, for example, in some sampling systems, adsorption and reaction of NO2 at the surface may contribute to the formation of gas-phase HONO.
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    Martins-Costa, M. T. C.; Anglada, J. M.; Francisco, J. S.; Ruiz-Lopez, M. F. Theoretical Investigation of the Photoexcited NO2+H2O reaction at the Air-Water Interface and Its Atmospheric Implications. Chem. - Eur. J. 2019, 25, 13899– 13904,  DOI: 10.1002/chem.201902769
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    Theoretical Investigation of the Photoexcited NO2+H2O reaction at the Air-Water Interface and Its Atmospheric Implications
    Martins-Costa, Marilia T. C.; Anglada, Josep M.; Francisco, Joseph S.; Ruiz-Lopez, Manuel F.
    Chemistry - A European Journal (2019), 25 (61), 13899-13904CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The atm. role of photochem. processes involving NO2 beyond its dissocn. limit (398 nm) is controversial. Recent expts. have confirmed that excited NO2* beyond 420 nm reacts with water according to NO2*+H2O→HONO+OH. However, the estd. kinetic const. for this process in the gas phase is quite small (k≈10-15-3.4×10-14 cm3 mol.-1/s) suggesting minor atm. implications of the formed radicals. In this work, ab initio mol. dynamics simulations of NO2 adsorbed at the air-water interface reveal that the OH prodn. rate increases by ∼2 orders of magnitude with respect to gas phase, attaining ozone ref. values for NO2 concns. corresponding to slightly polluted rural areas. This finding substantiates the argument that chem. on clouds can be an addnl. source of OH radicals in the troposphere and suggests directions for future lab. exptl. studies.
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    Warneck, P.; Williams, J. The Atmospheric Chemist’s Companion; Springer: The Netherlands, 2012.
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    Dillon, T. J.; Crowley, J. N. Reactive quenching of electronically excited NO2* and NO3* by H2O as potential sources of atmospheric HOx radicals. Atmos. Chem. Phys. 2018, 18, 14005– 14015,  DOI: 10.5194/acp-18-14005-2018
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    Reactive quenching of electronically excited NO*2 and NO*3 by H2O as potential sources of atmospheric HOx radicals
    Dillon, Terry J.; Crowley, John N.
    Atmospheric Chemistry and Physics (2018), 18 (19), 14005-14015CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
    Pulsed laser excitation of NO2 (532-647 nm) or NO3 (623-662 nm) in the presence of H2O was used to initiate the gas-phase reaction NO*2 + H2O → products (Reaction R5) and NO*3 + H2O → products (Reaction R12). No evidence for OH prodn. in Reactions (R5) or (R12) was obsd. and upper limits for OH prodn. of k5b/k5 < 1 × 10-5 and k12b/k12 < 0.03 were assigned. The upper limit for k5b/k5 renders this reaction insignificant as a source of OH in the atm. and extends the studies which demonstrate that the previously reported large OH yield by Li et al. (2008) was erroneous. The upper limit obtained for k12b/k12 indicates that non-reactive energy transfer is the dominant mechanism for Reaction (R12), though generation of small but significant amts. of atm. HOx and HONO cannot be ruled out. In the course of this work, rate coeffs. for overall removal of NO*3 by N2 (Reaction R10) and by H2O (Reaction R12) were detd.: k10 = (2.1 ± 0.1) × 10-11 cm3 mol.-1 s-1 and k12 = (1.6 ± 0.3) × 10-10 cm3 mol.-1 s-1. Our value of k12 is more than a factor of 4 smaller than the single previously reported value.
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  71. 71
    Li, S.; Matthews, J.; Sinha, A. Atmospheric hydroxyl radical production from electronically excited NO2 and H2O. Science 2008, 319, 1657– 1660,  DOI: 10.1126/science.1151443
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    Atmospheric Hydroxyl Radical Production from Electronically Excited NO2 and H2O
    Li, Shuping; Matthews, Jamie; Sinha, Amitabha
    Science (Washington, DC, United States) (2008), 319 (5870), 1657-1660CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    OH- are often called the detergent of the atm. because they control its capacity to remove pollutants. The reaction of electronically-excited NO2 with water was shown to be an important source of tropospheric OH-. Using measured rate data in conjunction with available solar flux and atm. mixing ratios, the authors demonstrated the tropospheric OH- contribution from this source can be a substantial fraction (50%) of that from the traditional, O(1D) + water reaction in the boundary layer region for high solar zenith angles. Including this chem. is expected to affect urban air quality modeling, where sunlight interactions with emitted NOx species, volatile org. compds., and OH- are central in detg. the O3 formation rate.
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    George, C.; Strekowski, R. S.; Kleffmann, J.; Stemmler, K.; Ammann, M. Photoenhanced uptake of gaseous NO2 on solid-organic compounds: a photochemical source of HONO?. Faraday Discuss. 2005, 130, 195– 210,  DOI: 10.1039/b417888m
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    Photoenhanced uptake of gaseous NO2 on solid organic compounds: A photochemical source of HONO?
    George, C.; Strekowski, R. S.; Kleffmann, J.; Stemmler, K.; Ammann, M.
    Faraday Discussions (2005), 130 (Atmospheric Chemistry), 195-210CODEN: FDISE6; ISSN:1359-6640. (Royal Society of Chemistry)
    In several recent field campaigns the existence of a strong daytime source of nitrous acid was demonstrated. The mechanism of this source remains unclear. Accordingly, in the present lab. study, the effect of light (in the range 300-500 nm) on the uptake kinetics of NO2 on various surfaces taken as proxies for org. surfaces encountered in the troposphere (as org. aerosol but also ground surfaces) was investigated. In this collaborative study, the uptake kinetics and product formation rate were measured by different flow tube reactors in combination with a sensitive HONO instrument. Uptake on light absorbing arom. compds. was significantly enhanced when irradiated with light of 300-420 nm, and HONO was formed with high yield when the gas was humidified. Esp. org. substrates contg. a combination of electron donors, such as phenols, and of compds. yielding excited triplet states, such as arom. ketones, showed a high reactivity towards NO2. Based on the results reported a mechanism is suggested, in which photosensitized electron transfer is occurring. The results show that HONO can be efficiently formed during the day in the atm. at much longer wavelengths compared to the recently proposed nitrate photolysis.
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    Stemmler, K.; Ammann, M.; Donders, C.; Kleffmann, J.; George, C. Photosensitized reduction of nitrogen dioxide on humic acid as a source of nitrous acid. Nature 2006, 440, 195– 198,  DOI: 10.1038/nature04603
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    Photosensitized reduction of nitrogen dioxide on humic acid as a source of nitrous acid
    Stemmler, Konrad; Ammann, Markus; Donders, Chantal; Kleffmann, Joerg; George, Christian
    Nature (London, United Kingdom) (2006), 440 (7081), 195-198CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    Nitrous acid is a significant photochem. precursor of the hydroxyl radical, the key oxidant in the degrdn. of most air pollutants in the troposphere. The sources of nitrous acid in the troposphere, however, are still poorly understood. Recent atm. measurements revealed a strongly enhanced formation of nitrous acid during daytime via unknown mechanisms. Here we expose humic acid films to nitrogen dioxide in an irradiated tubular gas flow reactor and find that redn. of nitrogen dioxide on light-activated humic acids is an important source of gaseous nitrous acid. Our findings indicate that soil and other surfaces contg. humic acid exhibit an org. surface photochem. that produces reductive surface species, which react selectively with nitrogen dioxide. The obsd. rate of nitrous acid formation could explain the recently obsd. high daytime concns. of nitrous acid in the boundary layer, the photolysis of which accounts for up to 60% of the integrated hydroxyl radical source strengths. We suggest that this photo-induced nitrous acid prodn. on humic acid could have a potentially significant impact on the chem. of the lowermost troposphere.
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    Stemmler, K.; Ndour, M.; Elshorbany, Y.; Kleffmann, J.; D’Anna, B.; George, C.; Bohn, B.; Ammann, M. Light induced conversion of nitrogen dioxide into nitrous acid on submicron humic acid aerosol. Atmos. Chem. Phys. 2007, 7, 4237– 4248,  DOI: 10.5194/acp-7-4237-2007
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    Light induced conversion of nitrogen dioxide into nitrous acid on submicron humic acid aerosol
    Stemmler, K.; Ndour, M.; Elshorbany, Y.; Kleffmann, J.; D'Anna, B.; George, C.; Bohn, B.; Ammann, M.
    Atmospheric Chemistry and Physics (2007), 7 (16), 4237-4248CODEN: ACPTCE; ISSN:1680-7316. (Copernicus Publications)
    The interactions of aerosols consisting of humic acids with gaseous NO2 were investigated under different light conditions in aerosol flow tube expts. at ambient pressure and temp. The results show that NO2 is converted on the humic acid aerosol into HONO, which is released from the aerosol and can be detected in the gas phase at the reactor exit. The formation of HONO on the humic acid aerosol is strongly activated by light: in the dark, the HONO-formation was below the detection limit, but it was increasing with the intensity of the irradn. with visible light. Under simulated atm. conditions with respect to the actinic flux, relative humidity and NO2-concn., reactive uptake coeffs. γrxn for the NO2 → HONO conversion on the aerosol between γrxn <10-7 (in the dark) and γrxn = 6 × 10-6 were obsd. The obsd. uptake coeffs. decreased with increasing NO2-concn. in the range from 2.7 to 280 ppb and were dependent on the relative humidity (RH) with slightly reduced values at low humidity (<20% RH) and high humidity (>60% RH). The measured uptake coeffs. for the NO2 → HONO conversion are too low to explain the HONO-formation rates obsd. near the ground in rural and urban environments by the conversion of NO2 → HONO on org. aerosol surfaces, even if one would assume that all aerosols consist of humic acid only. It is concluded that the processes leading to HONO formation on the Earth surface will have a much larger impact on the HONO-formation in the lowermost layer of the troposphere than humic materials potentially occurring in airborne particles.
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    Ammann, M.; Rossler, E.; Strekowski, R.; George, C. Nitrogen dioxide multiphase chemistry: Uptake kinetics on aqueous solutions containing phenolic compounds. Phys. Chem. Chem. Phys. 2005, 7, 2513– 2518,  DOI: 10.1039/b501808k
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    Nitrogen dioxide multiphase chemistry: Uptake kinetics on aqueous solutions containing phenolic compounds
    Ammann, Markus; Roessler, Elfriede; Strekowski, Rafal; George, Christian
    Physical Chemistry Chemical Physics (2005), 7 (12), 2513-2518CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The uptake coeffs. of NO2 on aq. solns. contg. guaiacol, syringol and catechol were detd. over the pH range from 1 to 13 using the wetted wall flowtube technique. The measured uptake coeffs. were used to det. the rate coeffs. for the reaction of the phys. dissolved NO2 with the neutral and deprotonated forms of phenolic compds. listed above. These org. compds. are ubiquitous not only in biomass burning plumes but also in soils, where they form part of the building blocks of humic acids. The NO2 uptake kinetics on solns. contg. guaiacol, syringol or catechol are strongly pH dependent with uptake coeffs. increasing from <10-7, under acidic conditions, to >10-5 at pH values >10. This behavior illustrates the difference of reactivity between the neutral phenolic species and the phenoxide ions. The corresponding 2nd order rate coeffs. were typically obsd. to increase from 105 M-1 s-1 for the neutral compds. to a min. of 108 M-1 s-1 for the phenoxide ions.
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    Cazoir, D.; Brigante, M.; Ammar, R.; D’Anna, B.; George, C. Heterogeneous photochemistry of gaseous NO2 on solid fluoranthene films: A source of gaseous nitrous acid (HONO) in the urban environment. J. Photochem. Photobiol., A 2014, 273, 23– 28,  DOI: 10.1016/j.jphotochem.2013.07.016
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    Heterogeneous photochemistry of gaseous NO2 on solid fluoranthene films: A source of gaseous nitrous acid (HONO) in the urban environment
    Cazoir, David; Brigante, Marcello; Ammar, Rachid; D'Anna, Barbara; George, Christian
    Journal of Photochemistry and Photobiology, A: Chemistry (2014), 273 (), 23-28CODEN: JPPCEJ; ISSN:1010-6030. (Elsevier B.V.)
    Using a coating process flow tube equipped with several near-UV emitting lamps, (range 300-420 nm), we examd. the effect of actinic radiation on the heterogeneous loss kinetics of gaseous nitrogen dioxide on solid Polycyclic Arom. Hydrocarbon (PAH) films deposited on a Pyrex substrate. The PAH studied was mainly fluoranthene, with addnl. tests on phenanthrene and pyrene. No dependence of the uptake coeff. (γ) was obsd. either with temp., or with relative humidity. In the dark, the reaction was very slow but was greatly enhanced by increasing the UV-A light intensity. A linear dependency of the reaction kinetics with the photonic flux was obsd. Under atmospherically-relevant NO2 concns. (20 ppbv), the uptake coeff. was about 1 × 10-6. The uptake coeff. variation as a function of the NO2 concn. suggests a Langmuir-Hinshelwood (L-H) type mechanism. This is characterized by the adsorption of NO2 on the solid surface followed by a chem. reaction. The corresponding equil. const. (K') and the surface reaction rate const. (k1s) were found to be 3 × 10-2 ppbv-1 and 5 × 10-5 s-1 resp. for the photo-enhanced uptake of NO2 on the fluoranthene substrate. Particular attention was given to the detection of the gas-phase products showing the photo-enhanced redn. of NO2 to HONO and NO via a photosensitized reaction involving excited states of the PAH. Addnl. we investigated the reactivity of PAH in the presence of nitrates in order to better understand if HONO generation mechanism could be explained by a first deposition of nitrates (generated via NO2 hydrolysis) on the solid surface.
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    Liu, J. P.; Li, S.; Mekic, M.; Jiang, H. Y.; Zhou, W. T.; Loisel, G.; Song, W.; Wang, X. M.; Gligorovski, S. Photoenhanced Uptake of NO2 and HONO Formation on Real Urban Grime. Environ. Sci. Technol. Lett. 2019, 6, 413– 417,  DOI: 10.1021/acs.estlett.9b00308
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    Photoenhanced Uptake of NO2 and HONO Formation on Real Urban Grime
    Liu, Jiangping; Li, Sheng; Mekic, Majda; Jiang, Haoyu; Zhou, Wentao; Loisel, Gwendal; Song, Wei; Wang, Xinming; Gligorovski, Sasho
    Environmental Science & Technology Letters (2019), 6 (7), 413-417CODEN: ESTLCU; ISSN:2328-8930. (American Chemical Society)
    Nitrous acid (HONO) is one of the most important photochem. precursors of the hydroxyl radical in the sunlit urban atm. The sources of HONO, however, are still poorly characterized, yet there is a disagreement between the field observations and the model results. Here, we show that light-induced NO2 heterogeneous chem. on authentic urban grime can make an important contribution to the total HONO levels in the urban atm. The effective uptake coeffs. of NO2 on urban grime in the presence of UV light [2.6 × 1015 photons cm-2/s (300 nm < λ < 400 nm)] increased markedly from (1.1 ± 0.2) × 10-6 at 0% relative humidity (RH) to (5.8 ± 0.7) × 10-6 at 90% RH, exhibiting the following linear correlation with RH: γ(NO2) = (7.4 ± 3.3) × 10-7 + (5.5 ± 0.6) × 10-8 × RH%. The flux densities of HONO mediated by light-induced heterogeneous conversion of NO2 (46 ppb) on urban grime were enhanced by ∼1 order of magnitude from (2.3 ± 0.2) × 109 mols. cm-2/s at 0% RH to (1.5 ± 0.01) × 1010 mols. cm-2/s at 90% RH. This study promotes light-induced NO2 chem. on urban grime being an important source of HONO and suggests that further expts. be performed in the future.
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    Knipping, E. M.; Dabdub, D. Modeling surface-mediated renoxification of the atmosphere via reaction of gaseous nitric oxide with deposited nitric acid. Atmos. Environ. 2002, 36, 5741– 5748,  DOI: 10.1016/S1352-2310(02)00652-0
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    Modeling surface-mediated renoxification of the atmosphere via reaction of gaseous nitric oxide with deposited nitric acid
    Knipping, Eladio M.; Dabdub, Donald
    Atmospheric Environment (2002), 36 (36-37), 5741-5748CODEN: AENVEQ; ISSN:1352-2310. (Elsevier Science Ltd.)
    Air quality models consider the formation and deposition of nitric acid (HNO3) on surfaces to be an irreversible sink of atm. nitrogen oxides (NOx) and therefore an effective termination step in the ozone formation cycle. However, exptl. evidence suggests that the reaction of gaseous nitric oxide with nitric acid on surfaces may convert HNO3 to photochem. active NOx. A first-order simulation of this surface-mediated renoxification process is performed using an air quality model of the South Coast Air Basin of California. Peak ozone concns. are predicted closer to obsd. values in regions regularly underpredicted by base case models. In certain regions, ozone predictions are enhanced by as much as ∼30 ppb or ∼20% compared to the baseline simulation. These results suggest that renoxification processes may be a key to resolving long-standing shortcomings of air quality models, in addn. to reconciling [HNO3]/[NOx] ratios in remote regions. This study also illustrates that the surface terrain may play a more active chem. role than hitherto considered in air quality models.
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    Rivera-Figueroa, A. M.; Sumner, A. L.; Finlayson-Pitts, B. J. Laboratory Studies of Potential Mechanisms of Renoxification of Tropospheric Nitric Acid. Environ. Sci. Technol. 2003, 37, 548– 554,  DOI: 10.1021/es020828g
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    Laboratory Studies of Potential Mechanisms of Renoxification of Tropospheric Nitric Acid
    Rivera-Figueroa, A. M.; Sumner, A. L.; Finlayson-Pitts, B. J.
    Environmental Science and Technology (2003), 37 (3), 548-554CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Lab. studies of the heterogeneous reactions between HNO3 in thin water films on silica surfaces and gaseous NO, CO, CH4, and SO2, proposed as potential "renoxification" (i.e., the redn. of HNO3 back to photochem. active N, such as NO, NO2, etc.) mechanisms in the atm., are reported. Transmission Fourier-transform IR spectroscopy (FTIR) was used to monitor reactants and products on the silica surface and in the gas phase as a function of time. No reaction of CO, CH4, or SO2 was obsd.; upper limits to the reaction probabilities (γrxn) are ≤10-10 for CO and SO2 and ≤10-12 for CH4. However, the reaction of HNO3 with NO does occur with a lower limit for the reaction probability of γNO ≥(6 ±2) × 10-9 (2s). The exptl. evidence shows that the chem. is insensitive to whether the substrate is pure silica or borosilicate glass. Nitric acid in its mol. form, and not the nitrate anion form, was shown to be the reactive species, and NH4NO3 was shown not to react with NO. The HNO3-NO reaction could be a significant means of renoxification of nitric acid on the surfaces of buildings and soils in the boundary layer of polluted urban atmospheres. This chem. may help to resolve some discrepancies between model-predicted ozone and field observations in polluted urban atmospheres.
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    Handley, S. R.; Clifford, D.; Donaldson, D. J. Photochemical loss of nitric acid on organic films: A possible recycling mechanism for NOx. Environ. Sci. Technol. 2007, 41, 3898– 3903,  DOI: 10.1021/es062044z
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    Photochemical Loss of Nitric Acid on Organic Films: a Possible Recycling Mechanism for NOx
    Handley, Susannah R.; Clifford, Daniel; Donaldson, D. J.
    Environmental Science & Technology (2007), 41 (11), 3898-3903CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    The films coating urban impervious surfaces have been found to be comprised of about 7 % inorg. nitrate and ∼10 % org. compds. (by mass). A simple steady-state anal. of the lifetime of the nitrate in the film suggests the existence of a loss process(es) in addn. to washout by rainfall. We show here that gas-phase nitric acid can be taken up in org. films and lower the film pH. Photolysis of nitrated films using actinic illumination causes loss both of protons and of nitrate anion. We argue that this is possibly due to a combination of direct and indirect (photosensitized) photochem. involving nitrate ions, yielding gas-phase HONO and/or NO2.
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    Finlayson-Pitts, B. J. Reactions at surfaces in the atmosphere: integration of experiments and theory as necessary (but not necessarily sufficient) for predicting the physical chemistry of aerosols. Phys. Chem. Chem. Phys. 2009, 11, 7760– 7779,  DOI: 10.1039/b906540g
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    Reactions at surfaces in the atmosphere: integration of experiments and theory as necessary (but not necessarily sufficient) for predicting the physical chemistry of aerosols
    Finlayson-Pitts, Barbara J.
    Physical Chemistry Chemical Physics (2009), 11 (36), 7760-7779CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    A review. While particles have significant deleterious impacts on human health, visibility and climate, quant. understanding of their formation, compn. and fates remains problematic. Indeed, in many cases, even qual. understanding is lacking. One area of particular uncertainty is the nature of particle surfaces and how this dets. interactions with gases in the atm., including water, which is important for cloud formation and properties. The focus in this Perspective article is on some chem. relevant to airborne particles and esp. to reactions occurring on their surfaces. The intent is not to provide a comprehensive review, but rather to highlight a few selected examples of interface chem. involving inorg. and org. species that may be important in the lower atm. This includes sea salt chem., nitrate and nitrite ion photochem., orgs. on surfaces and heterogeneous reactions of oxides of nitrogen on proxies for airborne mineral dust and boundary layer surfaces. Emphasis is on the mol. level understanding that can only be gained by fully integrating expt. and theory to elucidate these complex systems.
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    Zhou, X.; Zhang, N.; TerAvest, M.; Tang, D.; Hou, J.; Bertman, S.; Alaghmand, M.; Shepson, P. B.; Carroll, M. A.; Griffith, S.; Dusanter, S.; Stevens, P. S. Nitric acid photolysis on forest canopy surface as a source for tropospheric nitrous acid. Nat. Geosci. 2011, 4, 440– 443,  DOI: 10.1038/ngeo1164
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    Nitric acid photolysis on forest canopy surface as a source for tropospheric nitrous acid
    Zhou, Xianliang; Zhang, Ning; TerAvest, Michaela; Tang, David; Hou, Jian; Bertman, Steve; Alaghmand, Marjan; Shepson, Paul B.; Carroll, Mary Anne; Griffith, Stephen; Dusanter, Sebastien; Stevens, Philip S.
    Nature Geoscience (2011), 4 (7), 440-443CODEN: NGAEBU; ISSN:1752-0894. (Nature Publishing Group)
    Photolysis of nitrous acid generates hydroxyl radicals-a key atm. oxidant-in the lower atm. Significant concns. of nitrous acid have been reported in the rural atm. boundary layer during the day, where photolysis of nitrous acid accounts for up to 42% of sunlight-induced radical prodn. The obsd. concns. of nitrous acid are thought to be sustained by heterogeneous reactions involving precursors such as nitrogen oxides and nitric acid. Here, we present direct measurements of nitrous acid flux over a rural forest canopy in Michigan, together with surface nitrate loading at the top of the canopy. We report a significant upward flux of nitrous acid during the day, with a peak around noontime. Daytime nitrous acid flux was pos. correlated with the product of leaf surface nitrate loading and the rate const. of nitrate photolysis. We suggest that the photolysis of nitric acid on forest canopies is a significant daytime source of nitrous acid to the lower atm. in rural environments, and could serve as an important pathway for the remobilization of deposited nitric acid.
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    Baergen, A. M.; Donaldson, D. J. Photochemical Renoxification of Nitric Acid on Real Urban Grime. Environ. Sci. Technol. 2013, 47, 815– 820,  DOI: 10.1021/es3037862
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    Photochemical renoxification of nitric acid on real urban grime
    Baergen, Alyson M.; Donaldson, D. J.
    Environmental Science & Technology (2013), 47 (2), 815-820CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    The fate of NOx (=NO + NO2) is important to understand because NOx is a significant player in air quality detn. through its role in O3 formation. Renoxification of the urban atm. may occur through the photolysis of HNO3 deposited onto urban grime. The photolysis occurs 4 orders of magnitude faster than in water with J values at noon on July 1 in Toronto of 1.2 × 10-3 s-1 for nitrate on urban grime and 1.0 × 10-7 s-1 for aq. nitrate. Photolysis of nitrate present on urban grime probably follows the same mechanism as aq. nitrate photolysis, involving the formation of NO2, OH, and possibly HONO. The NOx may be rapidly returned to the atm. rather than being ultimately removed from the atm. through film wash off.
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    Ye, C.; Zhou, X.; Pu, D.; Stutz, J.; Festa, J.; Spolaor, M.; Tsai, C.; Cantrell, C.; Mauldin, R. L.; Campos, T.; Weinheimer, A.; Hornbrook, R. S.; Apel, E. C.; Guenther, A.; Kaser, L.; Yuan, B.; Karl, T.; Haggerty, J.; Hall, S.; Ullmann, K.; Smith, J. N.; Ortega, J.; Knote, C. Rapid cycling of reactive nitrogen in the marine boundary layer. Nature 2016, 532, 489– 491,  DOI: 10.1038/nature17195
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    Rapid cycling of reactive nitrogen in the marine boundary layer
    Ye, Chunxiang; Zhou, Xianliang; Pu, Dennis; Stutz, Jochen; Festa, James; Spolaor, Max; Tsai, Catalina; Cantrell, Christopher; Mauldin, Roy L.; Campos, Teresa; Weinheimer, Andrew; Hornbrook, Rebecca S.; Apel, Eric C.; Guenther, Alex; Kaser, Lisa; Yuan, Bin; Karl, Thomas; Haggerty, Julie; Hall, Samuel; Ullmann, Kirk; Smith, James N.; Ortega, John; Knote, Christoph
    Nature (London, United Kingdom) (2016), 532 (7600), 489-491CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    NOx are essential for the formation of secondary atm. aerosols and of atm. oxidants such as ozone and the hydroxyl radical, which controls the self-cleansing capacity of the atm. Nitric acid, a major oxidn. product of NOx, has traditionally been considered to be a permanent sink of NOx. However, model studies predict higher ratios of nitric acid to NOx in the troposphere than are obsd. A renoxification process that recycles nitric acid into NOx has been proposed to reconcile observations with model studies, but the mechanisms responsible for this process remain uncertain. We present data from an aircraft measurement campaign over the North Atlantic Ocean and find evidence for rapid recycling of nitric acid to nitrous acid and NOx in the clean marine boundary layer via particulate nitrate photolysis. Lab. expts. further demonstrate the photolysis of particulate nitrate collected on filters at a rate >2 orders of magnitude greater than that of gaseous nitric acid, with nitrous acid as the main product. Box model calcns. based on the Master Chem. Mechanism suggest that particulate nitrate photolysis mainly sustains the obsd. levels of nitrous acid and NOx at midday under typical marine boundary layer conditions. Given that oceans account for >70% of Earth's surface, we propose that particulate nitrate photolysis could be a substantial tropospheric NOx source. Recycling of NOx in remote oceanic regions with minimal direct NOx emissions could increase the formation of tropospheric oxidants and secondary atm. aerosols on a global scale.
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  85. 85
    Zhou, X.; Gao, H.; He, Y.; Huang, G.; Bertman, S. B.; Civerolo, K.; Schwab, J. Nitric acid photolysis on surfaces in low-NOx environments: Significant atmospheric implications. Geophys. Res. Lett. 2003, 30, 2217  DOI: 10.1029/2003GL018620
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    Schuttlefield, J.; Rubasinghege, G.; El-Maazawi, M.; Bone, J.; Grassian, V. H. Photochemistry of adsorbed nitrate. J. Am. Chem. Soc. 2008, 130, 12210– 12211,  DOI: 10.1021/ja802342m
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    Photochemistry of Adsorbed Nitrate
    Schuttlefield, Jennifer; Rubasinghege, Gayan; El-Maazawi, Mohamed; Bone, Jason; Grassian, Vicki H.
    Journal of the American Chemical Society (2008), 130 (37), 12210-12211CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    In the atm., gas-phase nitrogen oxides, including nitric acid, react with particle surfaces (e.g., mineral dust and sea salt aerosol) to yield adsorbed nitrate, yet little is known about the photochem. of nitrate on the surface of these particles. In this study, nitrate adsorbed on alumina surfaces, a surrogate for mineral dust aerosol, is irradiated with broadband light (λ >300 nm) in the absence and presence of coadsorbed water, at <1% and 45 ±2% relative humidity (%RH), resp., and mol. oxygen. Upon irradn., the nitrate ion readily undergoes photolysis to yield nitrogen-contg. gas-phase products, NO2, NO, and N2O. Although NO2, NO, and N2O form under the different conditions investigated, both coadsorbed water and mol. oxygen change the gas-phase product distribution, with NO being the major product under dry and humid conditions in the absence of mol. oxygen and NO2 the major product in the presence of mol. oxygen. To the best of our knowledge, this is the first study to investigate the role of solvation by coadsorbed water in the photochem. of adsorbates at solid interfaces and the roles that mol. oxygen, adsorbed water, and relative humidity may have in photochem. processes on aerosol surfaces that have the potential to alter the chem. balance of the atm.
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  87. 87
    Scharko, N. K.; Berke, A. E.; Raff, J. D. Release of Nitrous Acid and Nitrogen Dioxide from Nitrate Photolysis in Acidic Aqueous Solutions. Environ. Sci. Technol. 2014, 48, 11991– 12001,  DOI: 10.1021/es503088x
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    87
    Release of Nitrous Acid and Nitrogen Dioxide from Nitrate Photolysis in Acidic Aqueous Solutions
    Scharko, Nicole K.; Berke, Andrew E.; Raff, Jonathan D.
    Environmental Science & Technology (2014), 48 (20), 11991-12001CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Nitrate (NO3‾) is an abundant component of aerosols, boundary layer surface films, and surface water. Photolysis of NO3‾ leads to NO2 and HONO, both of which play important roles in tropospheric ozone and OH prodn. Field and lab. studies suggest that NO3‾ photochem. is a more important source of HONO than once thought, although a mechanistic understanding of the variables controlling this process is lacking. We present results of cavity-enhanced absorption spectroscopy measurements of NO2 and HONO emitted during photodegrdn. of aq. NO3‾ under acidic conditions. Nitrous acid is formed in higher quantities at pH 2-4 than expected based on consideration of primary photochem. channels alone. Both exptl. and modeled results indicate that the addnl. HONO is not due to enhanced NO3‾ absorption cross sections or effective quantum yields, but rather to secondary reactions of NO2 in soln. We find that NO2 is more efficiently hydrolyzed in soln. when it is generated in situ during NO3‾ photolysis than for the heterogeneous system where mass transfer of gaseous NO2 into bulk soln. is prohibitively slow. The presence of nonchromophoric OH scavengers that are naturally present in the environment increases HONO prodn. 4-fold, and therefore play an important role in enhancing daytime HONO formation from NO3‾ photochem.
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  88. 88
    Dubowski, Y.; Colussi, A. J.; Hoffmann, M. R. Nitrogen dioxide release in the 302 nm band photolysis of spray-frozen aqueous nitrate solutions. Atmospheric implications. J. Phys. Chem. A 2001, 105, 4928– 4932,  DOI: 10.1021/jp0042009
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    88
    Nitrogen dioxide release in the 302 nm band photolysis of spray-frozen aqueous nitrate solutions. Atmospheric implications
    Dubowski, Yael; Colussi, A. J.; Hoffmann, M. R.
    Journal of Physical Chemistry A (2001), 105 (20), 4928-4932CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    To est. input of NO2 to the atm. from illuminated snowpacks, we quantify the NO2 fluxes released into the gas phase during the continuous λ ∼ 300 nm photolysis of NO3- in submillimeter ice layers produced by freezing aq. KNO3 sprays on cold surfaces. Fluxes, FNO2, increase weakly with [NO3-] between 5 ≤ [NO3-]/mM ≤ 50 and increase markedly with temp. in the range of 268 ≥ T/K ≥ 248. We found that FNO2, the photostationary concn. of NO2- (another primary photoproduct), and the quantum yield of 2-nitrobenzaldehyde in situ photoisomerization are nearly independent of ice layer thickness d within 80 ≤ d/μm ≤ 400. We infer that radiation is uniformly absorbed over the depth of the ice layers, where NO3- is photodecomposed into NO2 (+ OH) and NO2- (+ O), but that only the NO2 produced on the uppermost region is able to escape into the gas phase. The remainder is trapped and further photolyzed into NO. We obtain φNO2- ∼ 4.8 × 10-3 at 263 K, i.e., about the quantum yield of nitrite formation in neutral NO3- aq. solns., and an apparent quantum yield of NO2 release φ'NO2 ∼ 1.3 × 10-3 that is about a factor of 5 smaller than soln. φOH data extrapolated to 263 K. These results suggest that NO3- photolysis in ice takes place in a liquidlike environment and that actual φ'NO2 values may depend on the morphol. of ice deposits. Present φ'NO2 data, in conjunction with snow albedo and absorptivity data, lead to FNO2 values in essential agreement with recent measurements in Antarctic snow under solar illumination.
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  89. 89
    Richards, N. K.; Wingen, L. M.; Callahan, K. M.; Nishino, N.; Kleinman, M. T.; Tobias, D. J.; Finlayson-Pitts, B. J. Nitrate Ion Photolysis in Thin Water Films in the Presence of Bromide Ions. J. Phys. Chem. A 2011, 115, 5810– 5821,  DOI: 10.1021/jp109560j
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    89
    Nitrate Ion Photolysis in Thin Water Films in the Presence of Bromide Ions
    Richards, Nicole K.; Wingen, Lisa M.; Callahan, Karen M.; Nishino, Noriko; Kleinman, Michael T.; Tobias, Douglas J.; Finlayson-Pitts, Barbara J.
    Journal of Physical Chemistry A (2011), 115 (23), 5810-5821CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    Nitrate ions commonly coexist with halide ions in aged sea salt particles, as well as in the Arctic snowpack, where NO3- photochem. is believed to be an important source of NOy (NO + NO2 + HONO + ...). The effects of bromide ions on nitrate ion photochem. were investigated at 298 ± 2 K in air using 311 nm photolysis lamps. Reactions were carried out using NaBr/NaNO3 and KBr/KNO3 deposited on the walls of a Teflon chamber. Gas phase halogen products and NO2 were measured as a function of photolysis time using long path FTIR, NOy chemiluminescence and atm. pressure ionization mass spectrometry (API-MS). Irradiated NaBr/NaNO3 mixts. show an enhancement in the rates of prodn. of NO2 and Br2 as the bromide mole fraction (χNaBr) increased. However, this was not the case for KBr/KNO3 mixts. where the rates of prodn. of NO2 and Br2 remained const. over all values of χKBr. Mol. dynamics (MD) simulations show that the presence of bromide in the NaBr solns. pulls sodium toward the soln. surface, which in turn attracts nitrate to the interfacial region, allowing for more efficient escape of NO2 than in the absence of halides. However, in the case of KBr/KNO3, bromide ions do not appreciably affect the distribution of nitrate ions at the interface. Clustering of Br- with NO3- and H2O predicted by MD simulations for sodium salts may facilitate a direct intermol. reaction, which could also contribute to higher rates of NO2 prodn. Enhanced photochem. in the presence of halide ions may be important for oxides of nitrogen prodn. in field studies such as in polar snowpacks where the use of quantum yields from lab. studies in the absence of halide ions would lead to a significant underestimate of the photolysis rates of nitrate ions.
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  90. 90
    Richards-Henderson, N. K.; Callahan, K. M.; Nissenson, P.; Nishino, N.; Tobias, D. J.; Finlayson-Pitts, B. J. Production of gas phase NO2 and halogens from the photolysis of thin water films containing nitrate, chloride and bromide ions at room temperature. Phys. Chem. Chem. Phys. 2013, 15, 17636– 17646,  DOI: 10.1039/c3cp52956h
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    90
    Production of gas phase NO2 and halogens from the photolysis of thin water films containing nitrate, chloride and bromide ions at room temperature
    Richards-Henderson, Nicole K.; Callahan, Karen M.; Nissenson, Paul; Nishino, Noriko; Tobias, Douglas J.; Finlayson-Pitts, Barbara J.
    Physical Chemistry Chemical Physics (2013), 15 (40), 17636-17646CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Nitrate and halide ions coexist in particles generated in marine regions, around alk. dry lakes, and in the Arctic snowpack. Although the photochem. of nitrate ions in bulk aq. soln. is well known, there is recent evidence that it may be more efficient at liq.-gas interfaces, and that the presence of other ions in soln. may enhance interfacial reactivity. This study examines the 311 nm photolysis of thin aq. films of ternary halide-nitrate salt mixts. (NaCl-NaBr-NaNO3) deposited on the walls of a Teflon chamber at 298 K. The films were generated by nebulizing aq. 0.25 M NaNO3 solns. which had NaCl and NaBr added to vary the mole fraction of halide ions. Molar ratios of chloride to bromide ions were chosen to be 0.25, 1.0, or 4.0. The subsequent generation of gas phase NO2 and reactive halogen gases (Br2, BrCl and Cl2) were monitored with time. The rate of gas phase NO2 formation was shown to be enhanced by the addn. of the halide ions to thin films contg. only aq. NaNO3. At [Cl-]/[Br-] ≤ 1.0, the NO2 enhancement was similar to that obsd. for binary NaBr-NaNO3 mixts., while with excess chloride NO2 enhancement was similar to that obsd. for binary NaCl-NaNO3 mixts. Mol. dynamics simulations predict that the halide ions draw nitrate ions closer to the interface where a less complete solvent shell allows more efficient escape of NO2 to the gas phase, and that bromide ions are more effective in bringing nitrate ions closer to the surface. The combination of theory and expts. suggests that under atm. conditions where nitrate ion photochem. plays a role, the impact of other species such as halide ions should be taken into account in predicting the impacts of nitrate ion photochem.
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  91. 91
    Yu, Y.; Ezell, M. J.; Zelenyuk, A.; Imre, D.; Alexander, L.; Ortega, J.; Thomas, J. L.; Gogna, K.; Tobias, D. J.; D’Anna, B.; Harmon, C. W.; Johnson, S. N.; Finlayson-Pitts, B. J. Nitrate ion photochemistry at interfaces: a new mechanism for oxidation of alpha-pinene. Phys. Chem. Chem. Phys. 2008, 10, 3063– 3071,  DOI: 10.1039/b719495a
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    91
    Nitrate ion photochemistry at interfaces: a new mechanism for oxidation of α-pinene
    Yu, Yong; Ezell, Michael J.; Zelenyuk, Alla; Imre, Dan; Alexander, Liz; Ortega, John; Thomas, Jennie L.; Gogna, Karun; Tobias, Douglas J.; D'Anna, Barbara; Harmon, Chris W.; Johnson, Stanley N.; Finlayson-Pitts, Barbara J.
    Physical Chemistry Chemical Physics (2008), 10 (21), 3063-3071CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The photooxidn. of 0.6-0.9 ppm α-pinene in the presence of a deliquesced thin film of NaNO3, and for comparison increasing concns. of NO2, was studied in a 100-L Teflon chamber at relative humidities 72-88% and temps. 296-304 K. The loss of α-pinene and the formation of gaseous products were followed with time using proton transfer mass spectrometry. The yields of gas phase products were smaller in the NaNO3 expts. than in NO2 expts. Pinonic acid, pinic acid, trans-sobrerol and other unidentified products were detected in the exts. of the wall washings only for the NaNO3 photolysis. These data indicate enhanced loss of α-pinene at the NaNO3 thin film during photolysis. Supporting the exptl. results are mol. dynamics simulations which predict that α-pinene has an affinity for the surface of the deliquesced nitrate thin film, enhancing the opportunity for oxidn. of the impinging org. gas during the nitrate photolysis. This new mechanism of oxidn. of orgs. may be partially responsible for the correlation between nitrate and the org. component of particles obsd. in many field studies, and may also contribute to the missing source of SOA needed to reconcile model predictions and field measurements. Photolysis of nitrate on surfaces in the boundary layer may lead to the oxidn. of co-adsorbed orgs.
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  92. 92
    Wingen, L. M.; Moskun, A. C.; Johnson, S. N.; Thomas, J. L.; Roeselova, M.; Tobias, D. J.; Kleinman, M. T.; Finlayson-Pitts, B. J. Enhanced surface photochemistry in chloride-nitrate ion mixtures. Phys. Chem. Chem. Phys. 2008, 10, 5668– 5677,  DOI: 10.1039/b806613b
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    Enhanced surface photochemistry in chloride-nitrate ion mixtures
    Wingen, Lisa M.; Moskun, Amy C.; Johnson, Stanley N.; Thomas, Jennie L.; Roeselova, Martina; Tobias, Douglas J.; Kleinman, Michael T.; Finlayson-Pitts, Barbara J.
    Physical Chemistry Chemical Physics (2008), 10 (37), 5668-5677CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Heterogeneous reactions of sea salt aerosol with various oxides of nitrogen lead to replacement of chloride ion by nitrate ion. Studies of the photochem. of a model system were carried out using deliquesced mixts. of NaCl and NaNO3 on a Teflon substrate. Varying molar ratios of NaCl to NaNO3 (1:9 Cl-:NO3-, 1:1 Cl-:NO3-, 3:1 Cl-:NO3-, 9:1 Cl-:NO3-) and NaNO3 at the same total concn. were irradiated in air at 299 ± 3 K and at a relative humidity of 75 ± 8% using broadband UVB light (270-380 nm). Gaseous NO2 prodn. was measured as a function of time using a chemiluminescence NOy detector. Surprisingly, an enhanced yield of NO2 was obsd. as the chloride to nitrate ratio increased. Mol. dynamics (MD) simulations show that as the Cl-:NO3- ratio increases, the nitrate ions are drawn closer to the interface due to the existence of a double layer of interfacial Cl- and subsurface Na+. This leads to a decreased solvent cage effect when the nitrate ion photodissociates to NO2 + O·-, increasing the effective quantum yield and hence the prodn. of gaseous NO2. The implications of enhanced NO2 and likely OH prodn. as sea salt aerosols become processed in the atm. are discussed.
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    Richards-Henderson, N. K.; Anderson, C.; Anastasio, C.; Finlayson-Pitts, B. J. The effect of cations on NO2 production from the photolysis of aqueous thin water films of nitrate salts. Phys. Chem. Chem. Phys. 2015, 17, 32211– 32218,  DOI: 10.1039/C5CP05325K
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    93
    The effect of cations on NO2 production from the photolysis of aqueous thin water films of nitrate salts
    Richards-Henderson, Nicole K.; Anderson, Crisand; Anastasio, Cort; Finlayson-Pitts, Barbara J.
    Physical Chemistry Chemical Physics (2015), 17 (48), 32211-32218CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The photochem. of nitrate ions in bulk aq. soln. is well known, yet recent evidence suggests that the photolysis of nitrate may be more efficient at the air-water interface. Whether and how this surface enhancement is altered by the presence of different cations is not known. In the present studies, thin aq. films of nitrate salts with different cations were deposited on the walls of a Teflon chamber and irradiated with 311 nm light at 298 K. The films were generated by nebulizing aq. 0.5 M solns. of the nitrate salts and the generation of gas-phase NO2 was monitored with time. The nitrate salts fall into three groups based on their obsd. rate of NO2 formation (RNO2): (1) RbNO3 and KNO3, which readily produce NO2 (RNO2 > 3 ppb min-1), (2) Ca(NO3)2, which produces NO2 more slowly (RNO2 < 1 ppb min-1), and (3) Mg(NO3)2 and NaNO3, which lie between the other two groups. Neither differences in the UV-visible spectra of the nitrate salt solns. nor the results of bulk-phase photolysis studies could explain the differences in the rates of NO2 prodn. between these three groups. These exptl. results, combined with some insights from previous mol. dynamic simulations and vibrational sum frequency generation studies, show that cations may impact the concn. of nitrate ions in the interface region, thereby directly impacting the effective quantum yields for nitrate ions.
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    Finlayson-Pitts, B. J. Introductory lecture: atmospheric chemistry in the Anthropocene. Faraday Discuss. 2017, 200, 11– 58,  DOI: 10.1039/C7FD00161D
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    Introductory lecture: atmospheric chemistry in the Anthropocene
    Finlayson-Pitts, Barbara J.
    Faraday Discussions (2017), 200 (Atomospheric Chemistry in the Anthropocene), 11-58CODEN: FDISE6; ISSN:1359-6640. (Royal Society of Chemistry)
    The term "Anthropocene" was coined by Professor Paul Crutzen in 2000 to describe an unprecedented era in which anthropogenic activities are impacting planet Earth on a global scale. Greatly increased emissions into the atm., reflecting the advent of the Industrial Revolution, have caused significant changes in both the lower and upper atm. Atm. reactions of the anthropogenic emissions and of those with biogenic compds. have significant impacts on human health, visibility, climate and weather. Two activities that have had particularly large impacts on the troposphere are fossil fuel combustion and agriculture, both assocd. with a burgeoning population. Emissions are also changing due to alterations in land use. This paper describes some of the tropospheric chem. assocd. with the Anthropocene, with emphasis on areas having large uncertainties. These include heterogeneous chem. such as those of oxides of nitrogen and the neonicotinoid pesticides, reactions at liq. interfaces, org. oxidns. and particle formation, the role of sulfur compds. in the Anthropocene and biogenic-anthropogenic interactions. A clear and quant. understanding of the connections between emissions, reactions, deposition and atm. compn. is central to developing appropriate cost-effective strategies for minimizing the impacts of anthropogenic activities. The evolving nature of emissions in the Anthropocene places atm. chem. at the fulcrum of detg. human health and welfare in the future.
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    Ye, C. X.; Gao, H. L.; Zhang, N.; Zhou, X. L. Photolysis of Nitric Acid and Nitrate on Natural and Artificial Surfaces. Environ. Sci. Technol. 2016, 50, 3530– 3536,  DOI: 10.1021/acs.est.5b05032
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    95
    Photolysis of Nitric Acid and Nitrate on Natural and Artificial Surfaces
    Ye, Chunxiang; Gao, Honglian; Zhang, Ning; Zhou, Xianliang
    Environmental Science & Technology (2016), 50 (7), 3530-3536CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Photolysis of nitric acid and nitrate (HNO3/nitrate) was investigated on the surfaces of natural and artificial materials, including plant leaves, metal sheets, and construction materials. The surfaces were conditioned in the outdoor air prior to expts. to receive natural depositions of ambient HNO3/nitrate and other atm. constituents. The photolysis rate const. (JHNO3(s)) of the surface HNO3/nitrate was measured based on the prodn. rates of nitrous acid (HONO) and nitrogen oxides (NOx). The JHNO3(s) values, from 6.0 × 10-6 s-1 to 3.7 × 10-4 s-1, are 1 to 3 orders of magnitude higher than that of gaseous HNO3. The HONO was the major product from photolysis of HNO3/nitrate on most plant leaves, whereas NOx was the major product on most artificial surfaces. The JHNO3(s) values decreased with HNO3/nitrate surface d. and could be described by a simple anal. equation. Within a typical range of HNO3/nitrate surface d. in the low-NOx forested areas, photolysis of HNO3/nitrate on the forest canopy can be a significant source for HONO and NOx for the overlying atm.
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    Ye, C. X.; Zhang, N.; Gao, H. L.; Zhou, X. L. Matrix effect on surface-catalyzed photolysis of nitric acid. Sci. Rep. 2019, 9, 10,  DOI: 10.1038/s41598-018-37973-x
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    Nissenson, P.; Knox, C. J. H.; Finlayson-Pitts, B. J.; Phillips, L. F.; Dabdub, D. Enhanced photolysis in aerosols: evidence for important surface effects. Phys. Chem. Chem. Phys. 2006, 8, 4700– 4710,  DOI: 10.1039/b609219e
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    Enhanced photolysis in aerosols: evidence for important surface effects
    Nissenson, Paul; Knox, Christopher J. H.; Finlayson-Pitts, Barbara J.; Phillips, Leon F.; Dabdub, Donald
    Physical Chemistry Chemical Physics (2006), 8 (40), 4700-4710CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    While there is increasing evidence for unique chem. reactions at interfaces, there are fewer data on photochem. at liq.-vapor junctions. This work compared photolysis of molybdenum hexacarbonyl, Mo(CO)6, in 1-decene as liq. droplets or in bulk-liq. solns. Mo(CO)6 photolysis was faster by at least 3 orders of magnitude in aerosols than bulk liqs. Two possible sources of this enhancement were considered: increased light intensity due to morphol.-dependent resonances (MDR) in spherical aerosol particles and/or to increased path lengths for light inside droplets due to refraction (phys. effects); and interface effects, e.g., incomplete solvent-cage at the gas-liq. boundary and/or enhanced interfacial Mo(CO)6 concns. (chem. effects). Quant. calcns. of the first possibility were made in which light intensity distribution in droplets averaged >215-360 nm was obtained for 1-decene droplets. Calcns. showed the av. light intensity increase over the entire droplet was 106%, with an av. increase of 51% at the interface. These increases were much smaller than obsd. increases in the apparent photolysis rate of droplets vs. bulk liq.; thus, chem. effects, i.e., a decreased solvent-cage effect at the interface and/or enhanced surface concn. of Mo(CO)6, were most likely responsible for the dramatic increase in photolysis rate. Similar calcns. were also made for broadband (290-600 nm) solar irradn. of water droplets, relevant to atm. conditions. These calcns. showed that, in agreement with previous calcns. by B. Mayer and S. Madronich (2004), MDR produced only a moderate av. intensity enhancement relative to corresponding bulk-liq. slabs when averaged over a range of wavelengths characteristic of solar radiation at the earth surface. However, as for Mo(CO)6 in 1-decene, chem. effects may play a role in enhanced photochem. at the aerosol-air interface for airborne particles.
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  98. 98
    Yang, H. S.; Finlayson-Pitts, B. J. Infrared spectroscopic studies of binary solutions of nitric acid and water and ternary solutions of nitric acid, sulfuric acid, and water at room temperature: Evidence for molecular nitric acid at the surface. J. Phys. Chem. A 2001, 105, 1890– 1896,  DOI: 10.1021/jp004224f
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    98
    Infrared Spectroscopic Studies of Binary Solutions of Nitric Acid and Water and Ternary Solutions of Nitric Acid, Sulfuric Acid, and Water at Room Temperature: Evidence for Molecular Nitric Acid at the Surface
    Yang, Husheng; Finlayson-Pitts, Barbara J.
    Journal of Physical Chemistry A (2001), 105 (10), 1890-1896CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    Evidence from surface tension, sum frequency, and mol. scattering measurements in other labs. suggested that mol. HNO3 is present at the surface of concd. binary HNO3-H2O and ternary HNO3-H2SO4-H2O solns. The authors report here direct IR spectroscopic evidence for the presence of HNO3 at the air interface with both types of solns. at room temp. Both attenuated total reflectance (ATR) and single reflectance SR-FTIR were applied to HNO3-H2O solns. ranging from 0.9 to 40 mol % (0.5-15.7 M) and to ternary solns. contg. a const. amt. of H2SO4 (25 mol %) and increasing concns. of HNO3 from 0 to 25 mol % and assocd. decreasing amts. of H2O from 75 to 50 mol %. ATR spectra are analogous to transmission spectra, while SR spectra are more sensitive to species at or near the air interface where reflection occurs. Comparison of the ATR and SR spectra of the most concd. solns. clearly show mol. HNO3 at or near the surface. also, the spectra suggest that for these more concd. solns., the major form of HNO3 in the HNO3-H2O soln. is the monohydrate, HNO3·H2O, while in the H2SO4-HNO3-H2O soln., it is the anhyd. form. The potential atm. implications are discussed.
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  99. 99
    Shamay, E. S.; Buch, V.; Parrinello, M.; Richmond, G. L. At the water’s edge: Nitric acid as a weak acid. J. Am. Chem. Soc. 2007, 129, 12910– 12911,  DOI: 10.1021/ja074811f
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    99
    At the water's edge: nitric acid as a weak acid
    Shamay, Eric S.; Buch, Victoria; Parrinello, Michele; Richmond, Geraldine L.
    Journal of the American Chemical Society (2007), 129 (43), 12910-12911CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Nitric acid plays a role in many important chem. processes that happen in our environment, often at surfaces where less is known about its reactive behavior. Recent studies have shown that undissociated nitric acid is present on the surface of a nitric acid soln. Using ab initio mol. dynamics simulations, the authors showed that a nitric acid mol. present on an aq. soln. surface structures and orients in a way that significantly reduces its ability to be the strong dissocg. acid that it is in aq. soln. Hydrogen bonding to surface solvating water mols. plays a key role in this altered mol. behavior.
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  100. 100
    Bianco, R.; Wang, S. Z.; Hynes, J. T. Theoretical study of the dissociation of nitric acid at a model aqueous surface. J. Phys. Chem. A 2007, 111, 11033– 11042,  DOI: 10.1021/jp075054a
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    100
    Theoretical Study of the Dissociation of Nitric Acid at a Model Aqueous Surface
    Bianco, Roberto; Wang, Shuzhi; Hynes, James T.
    Journal of Physical Chemistry A (2007), 111 (43), 11033-11042CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    The issue of acid dissocn. of nitric acid at an aq. surface is relevant in various portions of the atm. in connection with ozone depletion. This proton-transfer reaction is studied here via electronic structure calcns. at the HF/SBK+(d) level of theory on the HNO3·(H2O)3 model reaction system embedded in clusters comprising 33, 40, 45, and 50 classical, polarizable waters with an increasing degree of solvation of the nitrate group. Free energy ests. for all the cases examd. favor undissociated, mol. nitric acid over the 0-300 K temp. range, including that relevant for the upper troposphere, where it is connected to the issue of the mechanism of nitric acid uptake by water ice aerosols. The presence of mol. HNO3 at 300 K at the surface is further supported by vibrational band assignments in good agreement with a very recent surface-sensitive vibrational spectroscopy study of dild. HNO3/H2O solns.
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  101. 101
    Hirokawa, J.; Kato, T.; Mafune, F. Uptake of Gas-Phase Nitrous Acid by pH-Controlled Aqueous Solution Studied by a Wetted Wall Flow Tube. J. Phys. Chem. A 2008, 112, 12143– 12150,  DOI: 10.1021/jp8051483
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    101
    Uptake of Gas-Phase Nitrous Acid by pH-Controlled Aqueous Solution Studied by a Wetted Wall Flow Tube
    Hirokawa, Jun; Kato, Takehiro; Mafune, Fumitaka
    Journal of Physical Chemistry A (2008), 112 (47), 12143-12150CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    Uptake kinetics of gas phase nitrous acid (HONO) by a pH-controlled aq. soln. was investigated by using a wetted wall flow tube. The gas phase concn. of HONO after exposure to the aq. soln. was measured selectively by the chem. ionization mass spectrometer in a high sensitive manner. The uptake rate of the gaseous HONO was found to depend on the pH of the soln. For the uptake by neutral and alk. solns., the gas phase concn. was obsd. to decay exponentially, suggesting that the uptake was fully limited by the gas phase diffusion. On the other hand, the uptake by the acidic soln. was found to be detd. by both the gas phase diffusion and the liq. phase processes such as phys. absorption and reversible acid dissocn. reaction. The decay was analyzed by the rate equations using the time dependent uptake coeff. involving the satn. of the liq. surface. While the uptake processes by the soln. at pH = 2-3 were well described by those calcd. using the phys. and chem. parameters reported for the bulk, the uptake rates by the soln. at 4 < pH < 7 deviate from the calcd. ones. The present result can suggest that the pH at the liq. surface is lower than that in the bulk liq., which is responsible for the addnl. resistance of mass transfer from the gas to the liq. phase.
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  102. 102
    Miller, Y.; Thomas, J. L.; Kemp, D. D.; Finlayson-Pitts, B. J.; Gordon, M. S.; Tobias, D. J.; Gerber, R. B. Structure of Large Nitrate-Water Clusters at Ambient Temperatures: Simulations with Effective Fragment Potentials and Force Fields with Implications for Atmospheric Chemistry. J. Phys. Chem. A 2009, 113, 12805– 12814,  DOI: 10.1021/jp9070339
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    102
    Structure of Large Nitrate-Water Clusters at Ambient Temperatures: Simulations with Effective Fragment Potentials and Force Fields with Implications for Atmospheric Chemistry
    Miller, Yifat; Thomas, Jennie L.; Kemp, Daniel D.; Finlayson-Pitts, Barbara J.; Gordon, Mark S.; Tobias, Douglas J.; Gerber, R. Benny
    Journal of Physical Chemistry A (2009), 113 (46), 12805-12814CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    Structural properties of large NO3-·(H2O)n (n = 15-500) clusters are studied by Monte Carlo simulations using effective fragment potentials (EFPs) and by classical mol. dynamics simulations using a polarizable empirical force field. The simulation results are analyzed with a focus on the description of hydrogen bonding and solvation in the clusters. In addn., a comparison between the electronic structure based EFP and the classical force field description of the 32 water cluster system is presented. The EFP simulations, which focused on the cases of n = 15 and 32, show an internal, fully solvated structure and a "surface adsorbed" structure for the 32 water cluster at 300 K, with the latter configuration being more probable. The internal solvated structure and the "surface adsorbed" structure differ considerably in their hydrogen bonding coordination nos. The force field based simulations agree qual. with these results, and the local geometry of NO3- and solvation at the surface-adsorbed site in the force field simulations are similar to those predicted using EFPs. Differences and similarities between the description of hydrogen bonding of the anion in the two approaches are discussed. Extensive classical force field based simulations at 250 K predict that long time scale stability of "internal" NO3-, which is characteristic of extended bulk aq. interfaces, emerges only for n > 300. Ab initio Moller-Plesset perturbation theory is used to test the geometries of selected surface and interior anions for n = 32, and the results are compared to the EFP and MD simulations. Qual., all approaches agree that surface structures are preferred over the interior structures for clusters of this size. The relatively large aq. clusters of NO3- studied here are of comparable size to clusters that lead to new particle formation in air. Nitrate ions on the surface of such clusters may have significantly different photochem. than the internal species. The possible implications of surface-adsorbed nitrate ions for atm. chem. are discussed.
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  103. 103
    Wang, S.; Bianco, R.; Hynes, J. T. Dissociation of nitric acid at an aqueous surface: Large amplitude motions in the contact ion pair to solvent-separated ion pair conversion. Phys. Chem. Chem. Phys. 2010, 12, 8241– 8249,  DOI: 10.1039/c002299n
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    103
    Dissociation of nitric acid at an aqueous surface: Large amplitude motions in the contact ion pair to solvent-separated ion pair conversion
    Wang, Shuzhi; Bianco, Roberto; Hynes, James T.
    Physical Chemistry Chemical Physics (2010), 12 (29), 8241-8249CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Beyond its fundamental interest, the acid dissocn. of nitric acid (HNO3) at an aq. interface is of importance in a wide variety of atm. contexts. Here we present a Car-Parrinello mol. dynamics (CPMD) study of the second step of this process, the formation, via proton transfer (PT), of a solvent-sepd. ion pair (SSIP) from a contact ion pair (CIP) of the hydronium (H3O+) and the nitrate (NO-3) ions. This reaction represents an extension of our earlier CPMD study of the first PT step to produce the CIP from mol. HNO3 at various locations at and below the aq. surface (S. Wang, R. Bianco and J. T. Hynes, J. Phys. Chem. A, 2009, 113, 1295); it is important in establishing the ionic distribution in the aq. interfacial region, with potential consequences for heterogeneous reactions occurring in that region. We focus on the large amplitude, microscopic level motions-such as the hydrogen-bonding coordination no. changes around the proton-donating and -accepting species-which are key for the CIP SSIP PT conversion.
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  104. 104
    Lewis, T.; Winter, B.; Stern, A. C.; Baer, M. D.; Mundy, C. J.; Tobias, D. J.; Hemminger, J. C. Does Nitric Acid Dissociate at the Aqueous Solution Surface?. J. Phys. Chem. C 2011, 115, 21183– 21190,  DOI: 10.1021/jp205842w
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    104
    Does Nitric Acid Dissociate at the Aqueous Solution Surface?
    Lewis, Tanza; Winter, Bernd; Stern, Abraham C.; Baer, Marcel D.; Mundy, Christopher J.; Tobias, Douglas J.; Hemminger, John C.
    Journal of Physical Chemistry C (2011), 115 (43), 21183-21190CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Nitric acid is a prevalent component of atm. aerosols, and the extent of nitric acid dissocn. at aq. interfaces is relevant to its role in heterogeneous atm. chem. Several exptl. and theor. studies have suggested that the extent of dissocn. of nitric acid near aq. interfaces is less than that in bulk soln. Here dissocn. of HNO3 at the surface of aq. soln. is quantified using XPS of the nitrogen local electronic structure. The relative amts. of undissociated HNO3(aq) and dissocd. NO3-(aq) are identified by the distinguishable N1s core-level photoelectron spectra of the two species, and we det. the degree of dissocn., αint, in the interface (approx. the first three layers of soln.) as a function of HNO3 concn. Our measurements show that dissocn. is decreased by ∼20% near the soln. interface compared with bulk soln. and furthermore that dissocn. occurs in the topmost soln. layer. The exptl. results are supported by first-principles MD simulations, which show that hydrogen bonds between HNO3 and water mols. at the soln. surface stabilize the mol. form even at low concn. by analogy to the stabilization of mol. HNO3 that occurs in bulk soln. at high concn.
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  105. 105
    Moussa, S. G.; Stern, A. C.; Raff, J. D.; Dilbeck, C. W.; Tobias, D. J.; Finlayson-Pitts, B. J. Experimental and theoretical studies of the interaction of gas phase nitric acid and water with a self-assembled monolayer. Phys. Chem. Chem. Phys. 2013, 15, 448– 458,  DOI: 10.1039/C2CP42405C
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    105
    Experimental and theoretical studies of the interaction of gas phase nitric acid and water with a self-assembled monolayer
    Moussa, S. G.; Stern, A. C.; Raff, J. D.; Dilbeck, C. W.; Tobias, D. J.; Finlayson-Pitts, B. J.
    Physical Chemistry Chemical Physics (2013), 15 (2), 448-458CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Nitric acid in air is formed by atm. reactions of oxides of nitrogen and is removed primarily through deposition to surfaces, either as the gas or after conversion to particulate nitrate. Many of the surfaces and particles have org. coatings, but relatively little is known about the interaction of nitric acid with org. films. We report here studies of the interaction of gaseous HNO3 with a self-assembled monolayer (SAM) formed by reacting 7-octenyltrichlorosilane [H2C:CH(CH2)6SiCl3] with the surface of a germanium IR-transmitting attenuated total reflectance (ATR) crystal that was coated with a thin layer of silicon oxide (SiOx). The SAM was exposed at 298 ± 2 K to dry HNO3 in a flow of N2, followed by HNO3 in humid N2 at a controlled relative humidity (RH) between 20-90%. For comparison, similar studies were carried out using a similar crystal without the SAM coating. Changes in the surface were followed using Fourier transform IR spectroscopy (FTIR). In the case of the SAM-coated crystal, mol. HNO3 and smaller amts. of NO3- ions were obsd. on the surface upon exposure to dry HNO3. Addn. of water vapor led to less mol. HNO3 and more H3O+ and NO3- complexed to water, but surprisingly, mol. HNO3 was still evident in the spectra up to 70% RH. This suggests that part of the HNO3 obsd. was initially trapped in pockets within the SAM and shielded from water vapor. After increasing the RH to 90% and then exposing the film to a flow of dry N2, mol. nitric acid was regenerated, as expected from recombination of protons and nitrate ions as water evapd. The nitric acid ultimately evapd. from the film. On the other hand, exposure of the SAM to HNO3 and H2O simultaneously gave only hydronium and nitrate ions. Mol. dynamics simulations of defective SAMs in the presence of HNO3 and water predict that nitric acid intercalates in defects as a complex with a single water mol. that is protected by alkyl chains from interacting with addnl. water mols. These studies are consistent with the recently proposed hydrophobic nature of HNO3. Under atm. conditions, if HNO3 is formed in org. layers on surfaces in the boundary layer, e.g. through NO3 or N2O5 reactions, it may exist to a significant extent in its mol. form rather than fully dissocd. to nitrate ions.
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    Kido Soule, M. C.; Blower, P. G.; Richmond, G. L. Nonlinear Vibrational Spectroscopic Studies of the Adsorption and Speciation of Nitric Acid at the Vapor/Acid Solution Interface. J. Phys. Chem. A 2007, 111, 3349– 3357,  DOI: 10.1021/jp0686994
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    Nonlinear Vibrational Spectroscopic Studies of the Adsorption and Speciation of Nitric Acid at the Vapor/Acid Solution Interface
    Kido Soule, Melissa C.; Blower, Patrick G.; Richmond, Geraldine L.
    Journal of Physical Chemistry A (2007), 111 (17), 3349-3357CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    Nitric acid plays an important role in the heterogeneous chem. of the atm. Reactions involving HNO3 at aq. interfaces in the stratosphere and troposphere depend on the state of nitric acid at these surfaces. The vapor/liq. interface of HNO3-H2O binary solns. and HNO3-H2SO4-H2O ternary solns. are examd. here using vibrational sum frequency spectroscopy (VSFS). Spectra of the NO2 group at different HNO3 mole fractions and under different polarization combinations are used to develop a detailed picture of these atmospherically important systems. Consistent with surface tension and spectroscopic measurements from other labs., mol. nitric acid is identified at the surface of concd. solns. However, the data here reveal the adsorption of two different hydrogen-bonded species of undissociated HNO3 in the interfacial region that differ in their degree of solvation of the nitro group. The adsorption of these undissociated nitric acid species is shown to be sensitive to the H2O:HNO3 ratio as well as to the concn. of sulfuric acid.
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    Stockwell, W. R.; Calvert, J. G. The mechanism of the HO-SO2 reaction. Atmos. Environ. 1983, 17, 2231– 2235,  DOI: 10.1016/0004-6981(83)90220-2
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    The mechanism of the hydroxyl-sulfur dioxide reaction
    Stockwell, William R.; Calvert, Jack G.
    Atmospheric Environment (1967-1989) (1983), 17 (11), 2231-5CODEN: ATENBP; ISSN:0004-6981.
    Kinetic studies of the HO-SO2 reaction were made by using long-path Fourier transform IR spectroscopy to follow the chem. changes in irradiated HONO, CO, SO2, NOx, O/N gaseous mixts. The HO concn., as monitored by the rate of CO2 generation from the reaction HO + CO(+O2) → HO2 + CO2 is insensitive to the concn. of SO2 in the mixt. (0-172 ppm). SO2 termination of HO-HO2-chain reactions is relatively unimportant. The often-employed, simplified overall reaction, HO + SO2 → H2SO4, should be replaced with the more nearly correct overall reaction, HO + SO2 (+ O2, H2O) → H2SO4 + HO2 in atm. models used to simulate SO2 transformation and transport rates.
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    Jayne, J. T.; Pöschl, U.; Chen, Y.-m.; Dai, D.; Molina, L. T.; Worsnop, D. R.; Kolb, C. E.; Molina, M. J. Pressure and temperature dependence of the gas-phase reaction of SO3 with H2O and the heterogeneous reaction of SO3 with H2O/H2SO4 surfaces. J. Phys. Chem. A 1997, 101, 10000– 10011,  DOI: 10.1021/jp972549z
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    Pressure and temperature dependence of the gas-phase reaction of SO3 with H2O and the heterogeneous reaction of SO3 with H2O/H2SO4 surfaces
    Jayne, John T.; Poeschl, Ulrich; Chen, Yu-min; Dai, David; Molina, Luisa T.; Worsnop, Douglas R.; Kolb, Charles E.; Molina, Mario J.
    Journal of Physical Chemistry A (1997), 101 (51), 10000-10011CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    The gas-phase reaction of SO3 with H2O and the heterogeneous reaction of SO3 with H2O-H2SO4 surfaces have been studied in a fast flow reactor coupled to a chem. ionization mass spectrometer (CIMS) for species detection. The gas-phase reaction was studied under turbulent flow conditions over the pressure range from 100 to 760 Torr N2 and the temp. range from 283 to 370 K. The loss rate of SO3 was measured under pseudo-first-order conditions; it exhibits a second-order dependence on water vapor concn. and has a strong neg. temp. dependence. The first-order rate coeff. for the SO3 loss by gas-phase reaction shows no significant pressure dependence and can be expressed as kI(s-1) = 3.90 × 10-41 exp(6830.6/T)[H2O]2 where [H2O] is in units of mol. cm-3 and T is in Kelvin. The overall uncertainty of our exptl. detd. rate coeffs. is estd. to be ±20%. At sufficiently low SO3 concns. (<1012 mol. cm-3) the rate coeff. is independent of the initial SO3 level, as expected for a gas-phase reaction mechanism involving one SO3 and two H2O mols. However, at higher concns. and lower temps., increased rate coeffs. were obsd., indicating a fast heterogeneous reaction after the onset of binary homogeneous nucleation of acid hydrate clusters leading to particle formation, which was verified by light-scattering expts. The heterogeneous loss of SO3 to the reactor walls has also been investigated under low pressure (1.1-12.5 Torr) laminar flow conditions. The loss rate is highly dependent on the humidity of the surface. In the presence of excess water the reactive sticking coeff. approaches unity and the wall loss rate is gas diffusion limited; under dry conditions it approaches zero, as expected. The atm. implications of the homogeneous and heterogeneous SO3-water reaction are discussed.
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    Morokuma, K.; Muguruma, C. Ab initio Molecular orbital Study of the Mechanism of the Gas Phase Reaction SO3 + H2O: Importance of the secons water molecule. J. Am. Chem. Soc. 1994, 116, 10316– 10317,  DOI: 10.1021/ja00101a068
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    Ab initio Molecular Orbital Study of the Mechanism of the Gas Phase Reaction SO3 + H2O: Importance of the Second Water Molecule
    Morokuma, Keiji; Muguruma, Chizuru
    Journal of the American Chemical Society (1994), 116 (22), 10316-17CODEN: JACSAT; ISSN:0002-7863.
    An ab initio MO method was used to calc. the structures and energetics of complexes and transition states for the reaction of nH2O + SO3 → (n-1)H2SO4 for n = 0 and 1. While the barrier for the 1:1 reaction is very high, two water mols. can react with SO3 very easily to convert it to sulfuric acid. The transition state for this easy reaction is six-centered, with transfer of two protons taking place simultaneously.
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    Loerting, T.; Liedl, K. R. Toward elimination of discrepancies between theory and experiment: The rate constant of the atmospheric conversion of SO3 to H2SO4. Proc. Natl. Acad. Sci. U. S. A. 2000, 97, 8874– 8878,  DOI: 10.1073/pnas.97.16.8874
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    Toward elimination of discrepancies between theory and experiment: the rate constant of the atmospheric conversion of SO3 to H2SO4
    Loerting, Thomas; Liedl, Klaus R.
    Proceedings of the National Academy of Sciences of the United States of America (2000), 97 (16), 8874-8878CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    The hydration rate const. of SO3 to H2SO4 was shown to depend sensitively on water vapor pressure. In a 1:1 SO3-H2O complex, the rate was predicted to be slower by ∼25 orders of magnitude vs. lab. results. This discrepancy was removed mostly by allowing a second and third water mol. to participate. An asynchronous water-mediated double proton transfer concerted with the nucleophilic attack and a double proton transfer accompanied by a transient H3O+ rotation were predicted to be the fastest reaction mechanisms. Comparing predicted neg. apparent activation energy with the exptl. observations indicated that in the tested atm., different reaction paths involving 2 and 3 water mols. are followed in the process of forming SO42- aerosols and consequently acid rain.
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    Seinfeld, J. H.; Pandis, S. N. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, 2nd ed.; Wiley, 2006.
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    Mirabel, P.; Clavelin, J. L. Application of nucleation to the study of the gas-phase photooxidation of sulfur-dioxide. J. Chem. Phys. 1979, 70, 5767– 5772,  DOI: 10.1063/1.437405
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    Application of nucleation to the study of the gas phase photooxidation of sulfur dioxide
    Mirabel, P.; Clavelin, J. L.
    Journal of Chemical Physics (1979), 70 (12), 5767-72CODEN: JCPSA6; ISSN:0021-9606.
    The photooxidn. of SO2 by disproportionation was investigated by excitation in the singlet absorption band (λ = 305 nm). The expts. were conducted in a diffusion cloud chamber in which H2O, used as the working fluid, was maintained at a max. activity of 2.65. The rate of SO3 prodn. is directly related to the obsd. rate of nucleation through a previous study on binary nucleation in H2SO4-H2O mixts. The results lead to a rate const. of SO2 disproportionation of (8.4 ± 2) × 109 L mol-1s-1, a value which agrees with that of the recent study by D. C. Marvin and H. Reiss (1977) if one corrects their results to take into account the pressure satn. quenching effect, but is 200 times larger than the previous values detd. in the absence of H2O vapor.
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    Gurjar, B. R.; Jain, A.; Sharma, A.; Agarwal, A.; Gupta, P.; Nagpure, A. S.; Lelieveld, J. Human health risks in megacities due to air pollution. Atmos. Environ. 2010, 44, 4606– 4613,  DOI: 10.1016/j.atmosenv.2010.08.011
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    Human health risks in megacities due to air pollution
    Gurjar, B. R.; Jain, A.; Sharma, A.; Agarwal, A.; Gupta, P.; Nagpure, A. S.; Lelieveld, J.
    Atmospheric Environment (2010), 44 (36), 4606-4613CODEN: AENVEQ; ISSN:1352-2310. (Elsevier Ltd.)
    This study evaluates the health risks in megacities in terms of mortality and morbidity due to air pollution. A new spreadsheet model, Risk of Mortality/Morbidity due to Air Pollution (Ri-MAP), is used to est. the excess nos. of deaths and illnesses. By adopting the World Health Organization (WHO) guideline concns. for the air pollutants SO2, NO2 and total suspended particles (TSP), concn.-response relationships and a population attributable-risk proportion concept are employed. Results suggest that some megacities like Los Angeles, New York, Osaka Kobe, Sao Paulo and Tokyo have very low excess cases in total mortality from these pollutants. In contrast, the approx. nos. of cases is highest in Karachi (15,000/yr) characterized by a very high concn. of total TSP (∼670 μg m-3). Dhaka (7000/yr), Beijing (5500/yr), Karachi (5200/yr), Cairo (5000/yr) and Delhi (3500/yr) rank highest with cardiovascular mortality. The morbidity (hospital admissions) due to Chronic Obstructive Pulmonary Disease (COPD) follows the tendency of cardiovascular mortality. Dhaka and Karachi lead the rankings, having about 2100/yr excess cases, while Osaka-Kobe (∼20/yr) and Sao Paulo (∼50/yr) are at the low end of all megacities considered. Since air pollution is increasing in many megacities, and our database of measured pollutants is limited to the period up to 2000 and does not include all relevant components (e.g. O3), these nos. should be interpreted as lower limits. South Asian megacities most urgently need improvement of air quality to prevent excess mortality and morbidity due to exceptionally high levels of air pollution. The risk ests. obtained from Ri-MAP present a realistic baseline evaluation for the consequences of ambient air pollution in comparison to simple air quality indexes, and can be expanded and improved in parallel with the development of air pollution monitoring networks.
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  114. 114
    Donaldson, D. J.; Kroll, J. A.; Vaida, V. Gas-phase hydrolysis of triplet SO2: A possible direct route to atmospheric acid formation. Sci. Rep. 2016, 6, 30000,  DOI: 10.1038/srep30000
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    114
    Gas-phase hydrolysis of triplet SO2: A possible direct route to atmospheric acid formation
    Donaldson, D. James; Kroll, Jay A.; Vaida, Veronica
    Scientific Reports (2016), 6 (), 30000CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
    Sulfur chem. is of great interest to the atm. chem. of several planets. In the presence of water, oxidized sulfur can lead to new particle formation, influencing climate in significant ways. Observations of sulfur compds. in planetary atmospheres when compared with model results suggest that there are missing chem. mechanisms. Here we propose a novel mechanism for the formation of sulfurous acid, which may act as a seed for new particle formation. In this proposed mechanism, the lowest triplet state of SO2 (3B1), which may be accessed by near-UV solar excitation of SO2 to its excited 1B1 state followed by rapid intersystem crossing, reacts directly with water to form H2SO3 in the gas phase. For ground state SO2, this reaction is endothermic and has a very high activation barrier; our quantum chem. calcns. point to a facile reaction being possible in the triplet state of SO2. This hygroscopic H2SO3 mol. may act as a condensation nucleus for water, giving rise to facile new particle formation (NPF).
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  115. 115
    Kroll, J. A.; Frandsen, B. N.; Kjaergaard, H. G.; Vaida, V. Atmospheric Hydroxyl Radical Source: Reaction of Triplet SO2 and Water. J. Phys. Chem. A 2018, 122, 4465– 4469,  DOI: 10.1021/acs.jpca.8b03524
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    115
    Atmospheric Hydroxyl Radical Source: Reaction of Triplet SO2 and Water
    Kroll, Jay A.; Frandsen, Benjamin N.; Kjaergaard, Henrik G.; Vaida, Veronica
    Journal of Physical Chemistry A (2018), 122 (18), 4465-4469CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    The reaction of electronically excited triplet state sulfur dioxide (3SO2) with water was investigated both theor. and exptl. The quantum chem. calcns. find that the reaction leads to the formation of hydroxyl radical (OH) and hydroxysulfinyl radical (HOSO) via a low energy barrier pathway. Exptl. the formation of OH was monitored via its reaction with methane, which itself is relatively unreactive with 3SO2, making it a suitable probe of OH prodn. from the reaction of 3SO2 and water. This reaction has implications for the formation of OH in environments that are assumed to be depleted in OH, such as volcanic plumes. This reaction also provides a mechanism for the formation of OH in planetary atmospheres with little or no oxygen (O2) or ozone (O3) present.
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  116. 116
    Anglada, J. M.; Martins-Costa, M. T. C.; Francisco, J. S.; Ruiz-López, M. F. Triplet State Promoted Reaction of SO2 with H2O by Competition Between Proton Coupled Electron Transfer (pcet) and Hydrogen Atom Transfer (hat) Processes. Phys. Chem. Chem. Phys. 2019, 21, 9779– 9784,  DOI: 10.1039/C9CP01105F
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    116
    Triplet state promoted reaction of SO2 with H2O by competition between proton coupled electron transfer (pcet) and hydrogen atom transfer (hat) processes
    Anglada, Josep M.; Martins-Costa, Marilia T. C.; Francisco, Joseph S.; Ruiz-Lopez, Manuel F.
    Physical Chemistry Chemical Physics (2019), 21 (19), 9779-9784CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The SO2 + H2O reaction is proposed to be the starting process for the oxidn. of sulfur dioxide to sulfate in liq. water, although the thermal reaction displays a high activation barrier. Recent studies have suggested that the reaction can be promoted by light absorption in the near UV. We report ab initio calcns. showing that the SO2 excited triplet state is unstable in water, as it immediately reacts with H2O through a water-assisted proton coupled electron transfer mechanism forming OH and HOSO radicals. The work provides new insights for a general class of excited-state promoted reactions of related YXY compds. with water, where Y is a chalcogen atom and X is either an atom or a functional group, which opens up interesting chem. perspectives in technol. applications of photoinduced H-transfer.
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  117. 117
    Olivella, S.; Anglada, J. M.; Sole, A.; Bofill, J. M. Mechanism of the hydrogen transfer from the OH group to oxygen-centered radicals: Proton-coupled electron-transfer versus radical hydrogen abstraction. Chem. - Eur. J. 2004, 10, 3404– 3410,  DOI: 10.1002/chem.200305714
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    117
    Mechanism of the hydrogen transfer from the OH group to oxygen-centered radicals: Proton-coupled electron-transfer versus radical hydrogen abstraction
    Olivella, Santiago; Anglada, Josep M.; Sole, Albert; Bofill, Josep M.
    Chemistry - A European Journal (2004), 10 (14), 3404-3410CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    High-level ab initio electronic structure calcns. were carried out with respect to the inter-mol. hydrogen-transfer reaction HCOOH.+OH→HCOO.+H2O and the intramol. hydrogen-transfer reaction .OOCH2OH→HOOCH2O. In both cases the hydrogen atom transfer can take place via two different transition structures. The lowest energy transition structure involves a proton transfer coupled to an electron transfer from the ROH species to the radical, whereas the higher energy transition structure corresponds to the conventional radical hydrogen atom abstraction. An anal. of the at. spin population, computed within the framework of the topol. theory of atoms in mols., suggests that the triplet repulsion between the unpaired electrons located on the oxygen atoms that undergo hydrogen exchange must be much higher in the transition structure for the radical hydrogen abstraction than that for the proton-coupled electron-transfer mechanism. Probably in the gas phase, hydrogen atom transfer from the OH group to oxygen-centered radicals occurs by the proton-coupled electron-transfer mechanism when this pathway is accessible.
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  118. 118
    Anglada, J. M. Complex mechanism of the gas phase reaction between formic acid and hydroxyl radical. Proton coupled electron transfer versus radical hydrogen abstraction mechanisms. J. Am. Chem. Soc. 2004, 126, 9809– 9820,  DOI: 10.1021/ja0481169
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    118
    Complex Mechanism of the Gas Phase Reaction between Formic Acid and Hydroxyl Radical. Proton Coupled Electron Transfer versus Radical Hydrogen Abstraction Mechanisms
    Anglada, Josep M.
    Journal of the American Chemical Society (2004), 126 (31), 9809-9820CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The gas phase reaction between formic acid and hydroxyl radical has been investigated with high level quantum mech. calcns. using DFT-B3LYP, MP2, CASSCF, QCISD, and CCSD(T) theor. approaches in connection with the 6-311+G(2df,2p) and aug-cc-pVTZ basis sets. The reaction has a very complex mechanism involving several elementary processes, which begin with the formation of a reactant complex before the hydrogen abstraction by hydroxyl radical. The results obtained in this investigation explain the unexpected exptl. fact that hydroxyl radical exts. predominantly the acidic hydrogen of formic acid. This is due to a mechanism involving a proton coupled electron-transfer process. The calcns. show also that the abstraction of formyl hydrogen has an increased contribution at higher temps., which is due to a conventional hydrogen abstraction radical type mechanism. The overall rate const. computed at 298 K is 6.24 × 10-13 cm3 mols.-1 s-1, and compares quite well with the range from 3.2 ± 1 to 4.9 ± 1.2 × 10-13 cm3 mols.-1 s-1, reported exptl.
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  119. 119
    Anglada, J. M.; Olivella, S.; Sole, A. Hydrogen transfer between sulfuric acid and hydroxyl radical in the gas phase: Competition among hydrogen atom transfer, proton-coupled electron-transfer, and double proton transfer. J. Phys. Chem. A 2006, 110, 1982– 1990,  DOI: 10.1021/jp056155g
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    119
    Hydrogen Transfer between Sulfuric Acid and Hydroxyl Radical in the Gas Phase: Competition among Hydrogen Atom Transfer, Proton-Coupled Electron-Transfer, and Double Proton Transfer
    Anglada, Josep M.; Olivella, Santiago; Sole, Albert
    Journal of Physical Chemistry A (2006), 110 (5), 1982-1990CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    In an attempt to assess the potential role of the hydroxyl radical in the atm. degrdn. of sulfuric acid, the hydrogen transfer between H2SO4 and HO• in the gas phase has been investigated by means of DFT and quantum-mech. electronic-structure calcns., as well as classical transition state theory computations. The first step of the H2SO4 + HO• reaction is the barrierless formation of a prereactive hydrogen-bonded complex (Cr1) lying 8.1 kcal mol-1 below the sum of the (298 K) enthalpies of the reactants. After forming Cr1, a single hydrogen transfer from H2SO4 to HO• and a degenerate double hydrogen-exchange between H2SO4 and HO• may occur. The single hydrogen transfer, yielding HSO4• and H2O, can take place through three different transition structures, the two lowest energy ones (TS1 and TS2) corresponding to a proton-coupled electron-transfer mechanism, whereas the higher energy one (TS3) is assocd. with a hydrogen atom transfer mechanism. The double hydrogen-exchange, affording products identical to reactants, takes place through a transition structure (TS4) involving a double proton-transfer mechanism and is predicted to be the dominant pathway. A rate const. of 1.50 × 10-14 cm3 mol.-1 s-1 at 298 K is obtained for the overall reaction H2SO4 + HO•. The single hydrogen transfer through TS1, TS2, and TS3 contributes to the overall rate const. at 298 K with a 43.4%. It is concluded that the single hydrogen transfer from H2SO4 to HO• yielding HSO4• and H2O might well be a significant sink for gaseous sulfuric acid in the atm.
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  120. 120
    Gonzalez, J.; Anglada, J. M. Gas Phase Reaction of Nitric Acid with Hydroxyl Radical without and with Water. A Theoretical Investigation. J. Phys. Chem. A 2010, 114, 9151– 9162,  DOI: 10.1021/jp102935d
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    120
    Gas Phase Reaction of Nitric Acid with Hydroxyl Radical without and with Water. A Theoretical Investigation
    Gonzalez, Javier; Anglada, Josep M.
    Journal of Physical Chemistry A (2010), 114 (34), 9151-9162CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    The gas phase reaction between nitric acid and hydroxyl radical, without and with a single water mol., has been investigated theor. using the DFT-B3LYP, MP2, QCISD, and CCSD(T) theor. approaches with the 6-311+G(2df,2p) and aug-cc-pVTZ basis sets. The reaction without water begins with the formation of a prereactive hydrogen-bonded complex and has several elementary reactions processes. They include proton coupled electron transfer, hydrogen atom transfer, and proton transfer mechanisms, and our kinetic study shows a quite good agreement of the behavior of the rate const. with respect to the temp. and to the pressure with the exptl. results from the literature. The addn. of a single water mol. results in a much more complex potential energy surface although the different elementary reactions found have the same electronic features that the naked reaction. Two transition states are stabilized by the effect of a hydrogen bond interaction originated by the water mol., and in the prereactive hydrogen bond region there is a geometrical rearrangement necessary to prep. the HO and HNO3 moieties to react to each other. This step contributes the reaction to be slower than the reaction without water and explains the exptl. finding, pointing out that there is no dependence for the HNO3 + HO reaction on water vapor.
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  121. 121
    Anglada, J. M.; Gonzalez, J. Different Catalytic Effects of a Single Water Molecule: The Gas-Phase Reaction of Formic Acid with Hydroxyl Radical in Water Vapor. ChemPhysChem 2009, 10, 3034– 3045,  DOI: 10.1002/cphc.200900387
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    121
    Different Catalytic Effects of a Single Water Molecule on the Gas-Phase Reaction of Formic Acid with Hydroxyl Radical in Water Vapor
    Anglada, Josep M.; Gonzalez, Javier
    ChemPhysChem (2009), 10 (17), 3034-3045CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The effect of a single water mol. on the reaction mechanism of the gas-phase reaction between formic acid and the hydroxyl radical was investigated with high-level quantum mech. calcns. using DFT-B3LYP, MP2 and CCSD(T) theor. approaches in concert with the 6-311+G(2df,2p) and aug-cc-pVTZ basis sets. The reaction between HCOOH and HO has a very complex mechanism involving a proton-coupled electron transfer process (pcet), two hydrogen-atom transfer reactions (hat) and a double proton transfer process (dpt). The hydroxyl radical predominantly abstrs. the acidic hydrogen of formic acid through a PCETt mechanism. A single water mol. affects each one of these reaction mechanisms in different ways, depending on the way the water interacts. Very interesting is also the fact that our calcns. predict that the participation of a single water mol. results in the abstraction of the formyl hydrogen of formic acid through a hydrogen atom transfer process (hat).
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  122. 122
    Jorgensen, S.; Jensen, C.; Kjaergaard, H. G.; Anglada, J. M. The gas-phase reaction of methane sulfonic acid with the hydroxyl radical without and with water vapor. Phys. Chem. Chem. Phys. 2013, 15, 5140– 5150,  DOI: 10.1039/c3cp44034f
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    122
    The gas-phase reaction of methane sulfonic acid with the hydroxyl radical without and with water vapor
    Jorgensen, Solvejg; Jensen, Camilla; Kjaergaard, Henrik G.; Anglada, Josep M.
    Physical Chemistry Chemical Physics (2013), 15 (14), 5140-5150CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The gas phase reaction between methanesulfonic acid (CH3SO3H; MSA) and the hydroxyl radical (HO), without and with a water mol., was investigated with DFT-B3LYP and CCSD(T)-F12 methods. For the bare reaction we have found two reaction mechanisms, involving proton coupled electron transfer and hydrogen atom transfer processes that produce CH3SO3 and H2O. We also found a third reaction mechanism involving the double proton transfer process, where the products and reactants are identical. The computed rate const. for the oxidn. process is 8.3 × 10-15 cm3 s-1 mol.-1. CH3SO3H forms two very stable complexes with water with computed binding energies of about 10 kcal mol-1. The presence of a single water mol. makes the reaction between CH3SO3H and HO much more complex, introducing a new reaction that consists in the interchange of H2O between HO and CH3SO3H. Our kinetic calcns. show that 99.5% of the reaction involves this interchange of the water mol. and, consequently, water vapor does not play any role in the oxidn. reaction of methane sulfonic acid by the hydroxyl radical.
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  123. 123
    Anglada, J. M.; Olivella, S.; Solé, A. Unexpected Reactivity of Amidogen Radical in the Gas Phase Degradation of Nitric Acid. J. Am. Chem. Soc. 2014, 136, 6834– 6837,  DOI: 10.1021/ja501967x
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    123
    Unexpected Reactivity of Amidogen Radical in the Gas Phase Degradation of Nitric Acid
    Anglada, Josep M.; Olivella, Santiago; Sole, Albert
    Journal of the American Chemical Society (2014), 136 (19), 6834-6837CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The gas phase reaction between nitric acid and amidogen radical has been investigated employing high level quantum-mech. electronic structure methods and variational transition state theory kinetic calcns. Our results show that the reaction proceeds through a proton coupled electron transfer mechanism with a rate const. of 1.81 × 10-13 cm3·mol.-1·s-1 at 298 K. This value is similar to the rate consts. for the reactions of hydroxyl radical with either ammonia or nitric acid. An anal. of these data in the context of the chem. of the atm. suggests that the amidogen radical, formed in the oxidn. of ammonia by hydroxyl radical, reacts with nitric acid regenerating ammonia. On the basis of these findings, we propose a potential new catalytic-like cycle which couples the oxidn. of ammonia by hydroxyl radical and the reaction of nitric acid with amidogen radical in the Earth's atm.
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  124. 124
    Anglada, J. M.; Olivella, S.; Sole, A. Atmospheric formation of the NO3 radical from gas-phase reaction of HNO3 acid with the NH2 radical: proton-coupled electron-transfer versus hydrogen atom transfer mechanisms. Phys. Chem. Chem. Phys. 2014, 16, 19437– 19445,  DOI: 10.1039/C4CP02792B
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    124
    Atmospheric formation of the NO3 radical from gas-phase reaction of HNO3 acid with the NH2 radical: proton-coupled electron-transfer versus hydrogen atom transfer mechanisms
    Anglada, Josep M.; Olivella, Santiago; Sole, Albert
    Physical Chemistry Chemical Physics (2014), 16 (36), 19437-19445CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The gas-phase reaction of nitric acid with the amidogen radical under atm. conditions has been investigated using quantum mech. (QCISD and CCSD(T)) and DFT (B3LYP, BH&HLYP, M05, M05-2X, and M06-2X) calcns. with the 6-311+G(2df,2p), aug-cc-pVTZ, aug-cc-pVQZ and extrapolation to the CBS basis sets. The reaction begins with the barrierless formation of a hydrogen-bonded complex, which can undergo two different reaction pathways, in addn. to the decompn. back to the reactants. The lowest energy barrier pathway involves a proton-coupled electron-transfer mechanism, whereas the highest energy barrier pathway takes place through a hydrogen atom transfer mechanism. The performance of the different DFT functionals in predicting both the geometries and relative energies of the stationary points investigated has been analyzed.
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  125. 125
    Anglada, J. M.; Solé, A. The Atmospheric Oxidation of HONO by OH, Cl, and ClO Radicals. J. Phys. Chem. A 2017, 121, 9698– 9707,  DOI: 10.1021/acs.jpca.7b10715
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    125
    The Atmospheric Oxidation of HONO by OH, Cl, and ClO Radicals
    Anglada, Josep M.; Sole, Albert
    Journal of Physical Chemistry A (2017), 121 (51), 9698-9707CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    The atm. oxidn. of nitrous acid by hydroxyl radical, chlorine atom, and chlorine monoxide radical was investigated with high-level theor. methods. Nitrous acid has two conformers (cis and trans), and we found a reaction path for the oxidn. of each of these conformers with the radicals considered. In all cases, the oxidn. of the cis conformer is much more favorable than the oxidn. of the trans conformer. Interestingly all transition states in these oxidn. processes follow a proton-coupled electron-transfer mechanism. Our computed rate const. at 298 K for the reaction of cis-HONO + ·OH is 4.83 × 10-12 cm3 mol.-1 s-1, in excellent agreement with their exptl. values (4.85 × 10-12 and 6.48 × 10-12 cm3 mol.-1 s-1). For the trans-HONO + ·OH reaction our calcd. rate const. at 298 K is 9.05 × 10-18 cm3 mol.-1 s-1, and we computed an effective rate const. for the oxidn. of the whole nitrous acid by hydroxyl radical of 1.81 × 10-12 cm3 mol.-1 s-1. For the oxidn. of nitrous acid by chlorine atom we predict greater rate consts. (7.38 × 10-11, 3.33 × 10-15, and 2.76 × 10-11 cm3 mol.-1 s-1, for the cis and trans conformers and for the whole HONO), these results suggesting that this reaction should contribute to the tropospheric oxidn. of nitrous acid, esp. in marine boundary areas, and to the formation of tropospheric ozone. For the oxidn. of nitrous acid by chlorine monoxide we predict rate consts. roughly 6 orders of magnitude smaller than the oxidn. by chlorine atom, and therefore we consider that this process should play a minor role in the troposphere.
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  126. 126
    Anglada, J. M.; Crehuet, R.; Sole, A. The gas phase oxidation of HCOOH by Cl and NH2 radicals. Proton coupled electron transfer versus hydrogen atom transfer. Mol. Phys. 2019, 117, 1430– 1441,  DOI: 10.1080/00268976.2018.1554829
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    126
    The gas phase oxidation of HCOOH by Cl and NH2 radicals. Proton coupled electron transfer versus hydrogen atom transfer*
    Anglada, Josep M.; Crehuet, Ramon; Sole, Albert
    Molecular Physics (2019), 117 (9-12), 1430-1441CODEN: MOPHAM; ISSN:0026-8976. (Taylor & Francis Ltd.)
    The reaction of formic acid (HCOOH) with chlorine atom and amidogen radical (NH2) have been investigated using high level theor. methods such BH&HLYP, MP2, QCISD, and CCSD(T) with the 6-311 + G(2df,2p), aug-cc-pVTZ, aug-cc-pVQZ and extrapolation to CBS basis sets. The abstraction of the acidic and formyl hydrogen atoms of the acid by the two radicals has been considered, and the different reactions proceed either by a proton coupled electron transfer (pcet) and hydrogen atom transfer (hat) mechanisms. Our calcd. rate const. at 298 K for the reaction with Cl is 1.14 x 10-13 cm3 mol.-1 s-1 in good agreement with the exptl. value 1.8 ± 0.12/2.0 x 10-13 cm3 mol.-1 s-1 and the reaction proceeds exclusively by abstraction of the formyl hydrogen atom, via hat mechanism, producing HOCO+ClH. The calcd. rate const., at 298 K, for the reaction with NH2 is 1.71 x 10-15 cm3 mol.-1 s-1, and the reaction goes through the abstraction of the acidic hydrogen atom, via a pcet mechanism, leading to the formation of HCOO+NH3.
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  127. 127
    Anglada, J. M.; Crehuet, R.; Adhikari, S.; Francisco, J. S.; Xia, Y. Reactivity of hydropersulfides toward the hydroxyl radical unraveled: disulfide bond cleavage, hydrogen atom transfer, and proton-coupled electron transfer. Phys. Chem. Chem. Phys. 2018, 20, 4793– 4804,  DOI: 10.1039/C7CP07570G
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    127
    Reactivity of hydropersulfides toward the hydroxyl radical unraveled: disulfide bond cleavage, hydrogen atom transfer, and proton-coupled electron transfer
    Anglada, Josep M.; Crehuet, Ramon; Adhikari, Sarju; Francisco, Joseph S.; Xia, Yu
    Physical Chemistry Chemical Physics (2018), 20 (7), 4793-4804CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Hydropersulfides (RSSH) are highly reactive as nucleophiles and hydrogen atom transfer reagents. These chem. properties are believed to be key for them to act as antioxidants in cells. The reaction involving the radical species and the disulfide bond (S-S) in RSSH, a known redox-active group, however, has been scarcely studied, resulting in an incomplete understanding of the chem. nature of RSSH. We have performed a high-level theor. investigation on the reactions of the hydroxyl radical ( OH) toward a set of RSSH (R = -H, -CH3, -NH2, -C(O)OH, -CN, and -NO2). The results show that S-S cleavage and H-atom abstraction are the two competing channels. The electron inductive effect of R induces selective OH substitution at one sulfur atom upon S-S cleavage, forming RSOH and SH for the electron donating groups (EDGs), whereas producing HSOH and SR for the electron withdrawing groups (EWGs). The H-Atom abstraction by OH follows a classical hydrogen atom transfer (hat) mechanism, producing RSS and H2O. Surprisingly, a proton-coupled electron transfer (pcet) process also occurs for R being an EDG. Although for RSSH having EWGs hat is the leading channel, S-S cleavage can be competitive or even dominant for the EDGs. The overall reactivity of RSSH toward OH attack is greatly enhanced with the presence of an EDG, with CH3SSH being the most reactive species found in this study (overall rate const.: 4.55 x 1012 M-1 s-1). Our results highlight the complexity in RSSH reaction chem., the extent of which is closely modulated by the inductive effect of the substituents in the case of the oxidn. by hydroxyl radicals.
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  128. 128
    Zhong, J.; Zhu, C.; Li, L.; Richmond, G. L.; Francisco, J. S.; Zeng, X. C. Interaction of SO2 with the Surface of a Water Nanodroplet. J. Am. Chem. Soc. 2017, 139, 17168– 17174,  DOI: 10.1021/jacs.7b09900
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    128
    Interaction of SO2 with the Surface of a Water Nanodroplet
    Zhong, Jie; Zhu, Chongqin; Li, Lei; Richmond, Geraldine L.; Francisco, Joseph S.; Zeng, Xiao Cheng
    Journal of the American Chemical Society (2017), 139 (47), 17168-17174CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    We present a comprehensive computational study of interaction of a SO2 with water mols. in the gas phase and with the surface of various sized water nanodroplets to study the solvation behavior of SO2 in different atm. environments. Born-Oppenheimer mol. dynamics (BOMD) simulation shows that, in the gas phase and at a temp. of 300 K, the dominant interaction between SO2 and H2O is (SO2)S···O(H2O), consistent with previous d.-functional theory (DFT) computation at 0 K. However, at the surface of a water nanodroplet, BOMD simulation shows that the hydrogen-bonding interaction of (SO2)O···H(H2O) becomes increasingly important with the increase of droplet size, reflecting a marked effect of the water surface on the SO2 solvation. This conclusion is in good accordance with spectroscopy evidence obtained previously (J. Am. Chem. Soc.2005, 127, 16806; J. Am. Chem. Soc.2006, 128, 3256). The prevailing interaction (SO2)O···H(H2O) on a large droplet is mainly due to favorable exposure of H atoms of H2O at the air-water interface. The conversion of the dominant interaction in the gas phase (SO2)S···O(H2O) to the dominant interaction on the water nanodroplet (SO2)O···H(H2O) may incur effects on the SO2 chem. in atm. aerosols because the solvation of SO2 at the water surface can affect the reactive sites and electrophilicity of SO2. Hence, the solvation of SO2 on the aerosol surface may have new implications when studying SO2 chem. in the aerosol-contg. troposphere.
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  129. 129
    Jayne, J.; Davidovits, P.; Worsnop, D.; Zahniser, M.; Kolb, C. Uptake of sulfur dioxide (g) by aqueous surfaces as a function of pH: the effect of chemical reaction at the interface. J. Phys. Chem. 1990, 94, 6041– 6048,  DOI: 10.1021/j100378a076
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    129
    Uptake of sulfur dioxide(G) by aqueous surfaces as a function of pH: the effect of chemical reaction at the interface
    Jayne, J. T.; Davidovits, P.; Worsnop, D. R.; Zahniser, M. S.; Kolb, C. E.
    Journal of Physical Chemistry (1990), 94 (15), 6041-8CODEN: JPCHAX; ISSN:0022-3654.
    The uptake of SO2(g) by fast-moving water droplets was measured as a function of pH and surface-gas contact time in the range 0.5-10 ms. In the high pH range (>5), a parameter governing the uptake of SO2(g) by water is the rate for the reaction of SO2 with H2O to form HSO3-. The exptl. obsd. uptake is significantly greater than predicted by the rate measured for this reaction in bulk liq. water. Likewise at low pH, where uptake is limited by Henry's law soly., the uptake is significantly greater than predicted. These observations together with the observation of uptake as a function of time suggest that at the gas-liq. interface the SO2-H2O reaction is facile, forming a HSO3--H+ surface complex which is in equil. with the gas-phase SO2. The species enters the bulk water as HSO3- via this complex. The equil. ratio of densities of the surface complex (cm-2) and gas-phase SO2 (cm-3) is 0.13 cm-1 at 10°. Kinetic and thermodn. parameters governing surface interactions are derived and discussed.
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  130. 130
    Donaldson, D.; Guest, J. A.; Goh, M. C. Evidence for adsorbed SO2 at the aqueous-air interface. J. Phys. Chem. 1995, 99, 9313– 9315,  DOI: 10.1021/j100023a002
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    Evidence for Adsorbed SO2 at the Aqueous-Air Interface
    Donaldson, D. J.; Guest, Joyce A.; Goh, M. Cynthia
    Journal of Physical Chemistry (1995), 99 (23), 9313-15CODEN: JPCHAX; ISSN:0022-3654. (American Chemical Society)
    The authors present direct exptl. evidence for the existence of a surface-bound state of partially hydrated SO2 on H2O. Surface 2nd harmonic generation (SHG) and static surface tension measurements are used to examine the SO2 surface coverage as a function of its bulk aq. concn. The results indicate a Langmuir-type adsorption of SO2 at the air-water interface. These expts. represent the 1st report of the application of surface SHG techniques to detect gas adsorption on a liq. surface.
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    Yang, H.; Wright, N. J.; Gagnon, A. M.; Gerber, R. B.; Finlayson-Pitts, B. J. An upper limit to the concentration of an SO2 complex at the air–water interface at 298 K: infrared experiments and ab initio calculations. Phys. Chem. Chem. Phys. 2002, 4, 1832– 1838,  DOI: 10.1039/b108907b
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    131
    An upper limit to the concentration of an SO2 complex at the air-water interface at 298 K: infrared experiments and ab initio calculations
    Yang, Husheng; Wright, Nicholas J.; Gagnon, Aaron M.; Benny Gerber, R.; Finlayson-Pitts, Barbara J.
    Physical Chemistry Chemical Physics (2002), 4 (10), 1832-1838CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Unique reactions occurring at the interface between air and aq. solns. are increasingly recognized to be of potential importance in atm. processes. Sulfur dioxide was one of the first species for which exptl. evidence for the existence of a surface complex was obtained by several different groups, based on the kinetics of SO2 uptake into aq. solns., large decreases in surface tension and second harmonic generation spectroscopic studies. The uptake has been proposed to involve an uncharged surface complex which subsequently converts into ionic species. We report here the results of a search for an uncharged SO2 complex at or near the surface using attenuated total reflectance Fourier transform IR spectrometry (ATR-FTIR) at 298 K guided by ab initio calcns. of a 1:1 SO2-H2O complex. No IR absorption bands attributable to such a complex of SO2 were obsd. exptl. in the expected region, giving an upper bound of 4 × 1014 SO2 cm-2 to the concn. of neutral SO2 mols. weakly sorbed to the surface in equil. with ∼1 atm SO2(g). The implications for the nature of the surface species and previous observations are discussed.
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    Shamay, E. S.; Johnson, K. E.; Richmond, G. L. Dancing on Water: The Choreography of Sulfur Dioxide Adsorption to Aqueous Surfaces. J. Phys. Chem. C 2011, 115, 25304– 25314,  DOI: 10.1021/jp2064326
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    132
    Dancing on Water: The Choreography of Sulfur Dioxide Adsorption to Aqueous Surfaces
    Shamay, Eric S.; Johnson, Kevin E.; Richmond, Geraldine L.
    Journal of Physical Chemistry C (2011), 115 (51), 25304-25314CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    One might expect the high surface tension of water to be a barrier to absorption of a gas into the liq. phase, but we know that gaseous adsorption onto and subsequent absorption into a water surface is a common phenomenon on this planet. What is not commonly known is how an atm. gas such as SO2 and mols. at the water surface can overcome the barrier created by strong water-water surface bonding interactions. What this interplay looks like, the distances from the water surface at which these attractive interactions begin, and how they influence the orientational nature of both SO2 and surface water mols. is the focus of this computational study. The results fill a void in the information about this system existing from previous exptl. studies by providing information about the dimensional nature of the gas-surface interactions, and the details of how the two species twist and turn orientationally with increased surface interactions. Classical mol. dynamics have been employed in both equil. and steered mol. dynamics (SMD) simulations for SO2 at a neat-water surface and at a surface with high interfacial SO2 concns. The results provide new mol. insights for understanding the interaction of this prevalent gas on aerosols and other aq. surfaces in the environment.
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    Martins-Costa, M. T. C.; Anglada, J. M.; Francisco, J. S.; Ruiz-López, M. F. Photochemistry of SO2 at the Air–Water Interface: A Source of OH and HOSO Radicals. J. Am. Chem. Soc. 2018, 140, 12341– 12344,  DOI: 10.1021/jacs.8b07845
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    133
    Photochemistry of SO2 at the Air-Water Interface: A Source of OH and HOSO Radicals
    Martins-Costa, Marilia T. C.; Anglada, Josep M.; Francisco, Joseph S.; Ruiz-Lopez, Manuel F.
    Journal of the American Chemical Society (2018), 140 (39), 12341-12344CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The photochem. of CO2 in the near UV-vis energy range was assessed in aq. environments. The combination of previously reported exptl. measurements and accurate quantum chem. calcns. from this work revealed the process represents an important source of tropospheric OH- radicals. It implicates the reaction of the lowest triplet excited state of SO2 with a water mol. When the process occurs in the gas-phase, photochem. OH- prodn. is only significant under high humidity/high SO2 concn. conditions as those measured in polluted urban areas; however, the OH- prodn. rate increased several orders of magnitude when the process occurred at the water droplet surface. Results indicated the atm. importance of SO2 goes beyond its well-known role as acid rain and aerosol formation precursor.
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    Ruiz-Lopez, M. F.; Martins-Costa, M. T. C.; Anglada, J. M.; Francisco, J. S. A New Mechanism of Acid Rain Generation from HOSO at the Air-Water Interface. J. Am. Chem. Soc. 2019, 141, 16564– 16568,  DOI: 10.1021/jacs.9b07912
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    134
    A New Mechanism of Acid Rain Generation from HOSO at the Air-Water Interface
    Ruiz-Lopez, Manuel F.; Martins-Costa, Marilia T. C.; Anglada, Josep M.; Francisco, Joseph S.
    Journal of the American Chemical Society (2019), 141 (42), 16564-16568CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The photochem. of SO2 at the air-water interface of water droplets leads to the formation of HOSO radicals. Using first-principles simulations, we show that HOSO displays an unforeseen strong acidity (pKa = -1) comparable with that of nitric acid and is fully dissocd. at the air-water interface. Accordingly, this radical might play an important role in acid rain formation. Potential implications are discussed.
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    Ciuraru, R.; Fine, L.; van Pinxteren, M.; D’Anna, B.; Herrmann, H.; George, C. Photosensitized production of functionalized and unsaturated organic compounds at the air-sea interface. Sci. Rep. 2015, 5, 12741  DOI: 10.1038/srep12741
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    135
    Photosensitized production of functionalized and unsaturated organic compounds at the air-sea interface
    Ciuraru, Raluca; Fine, Ludovic; van Pinxteren, Manuela; D'Anna, Barbara; Herrmann, Hartmut; George, Christian
    Scientific Reports (2015), 5 (), 12741CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
    The sea-surface microlayer (SML) has different phys., chem. and biol. properties compared to the subsurface water, with an enrichment of org. matter i.e., dissolved org. matter including UV absorbing humic substances, fatty acids and many others. Here we present exptl. evidence that dissolved org. matter, such as humic acids, when exposed to sunlight, can photosensitize the chem. conversion of linear satd. fatty acids at the air-water interface into unsatd. functionalized gas phase products (i.e. satd. and unsatd. aldehydes and acids, alkenes and dienes,...) which are known precursors of secondary org. aerosols. These functionalized mols. have previously been thought to be of biol. origin, but here we demonstrate that abiotic interfacial photochem. has the potential to produce such mols. As the ocean is widely covered by the SML, this new understanding will impact on our ability to describe atm. chem. in the marine environment.
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  136. 136
    Ciuraru, R.; Fine, L.; van Pinxteren, M.; D’Anna, B.; Herrmann, H.; George, C. Unravelling New Processes at Interfaces: Photochemical Isoprene Production at the Sea Surface. Environ. Sci. Technol. 2015, 49, 13199– 13205,  DOI: 10.1021/acs.est.5b02388
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    136
    Unraveling New Processes at Interfaces: Photochemical Isoprene Production at the Sea Surface
    Ciuraru, Raluca; Fine, Ludovic; Pinxteren, Manuela van; D'Anna, Barbara; Herrmann, Hartmut; George, Christian
    Environmental Science & Technology (2015), 49 (22), 13199-13205CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Isoprene is an important reactive gas that is produced mainly in terrestrial ecosystems but is also produced in marine ecosystems. In the marine environment, isoprene is produced in the seawater by various biol. processes. We show that photosensitized reactions involving the sea-surface microlayer lead to the prodn. of significant amts. of isoprene. It is suggested that H-abstraction processes are initiated by photochem. excited dissolved org. matter which will the degrade fatty acids acting as surfactants. This chem. interfacial processing may represent a significant abiotic source of isoprene in the marine boundary layer.
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    Brüggemann, M.; Hayeck, N.; Bonnineau, C.; Pesce, S.; Alpert, P. A.; Perrier, S.; Zuth, C.; Hoffmann, T.; Chen, J.; George, C. Interfacial photochemistry of biogenic surfactants: a major source of abiotic volatile organic compounds. Faraday Discuss. 2017, 200, 59– 74,  DOI: 10.1039/C7FD00022G
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    Interfacial photochemistry of biogenic surfactants: a major source of abiotic volatile organic compounds
    Bruggemann, Martin; Hayeck, Nathalie; Bonnineau, Chloe; Pesce, Stephane; Alpert, Peter A.; Perrier, Sebastien; Zuth, Christoph; Hoffmann, Thorsten; Chen, Jianmin; George, Christian
    Faraday Discussions (2017), 200 (Atomospheric Chemistry in the Anthropocene), 59-74CODEN: FDISE6; ISSN:1359-6640. (Royal Society of Chemistry)
    Films of biogenic compds. exposed to the atm. are ubiquitously found on the surfaces of cloud droplets, aerosol particles, buildings, plants, soils and the ocean. These air/water interfaces host countless amphiphilic compds. concd. there with respect to in bulk water, leading to a unique chem. environment. Here, photochem. processes at the air/water interface of biofilm-contg. solns. were studied, demonstrating abiotic VOC prodn. from authentic biogenic surfactants under ambient conditions. Using a combination of online-APCI-HRMS and PTR-ToF-MS, unsatd. and functionalized VOCs were identified and quantified, giving emission fluxes comparable to previous field and lab. observations. Interestingly, VOC fluxes increased with the decay of microbial cells in the samples, indicating that cell lysis due to cell death was the main source for surfactants and VOC prodn. In particular, irradn. of samples contg. solely biofilm cells without matrix components exhibited the strongest VOC prodn. upon irradn. In agreement with previous studies, LC-MS measurements of the liq. phase suggested the presence of fatty acids and known photosensitizers, possibly inducing the obsd. VOC prodn. via peroxy radical chem. Up to now, such VOC emissions were directly accounted to high biol. activity in surface waters. However, the results obtained suggest that abiotic photochem. can lead to similar emissions into the atm., esp. in less biol.-active regions. Furthermore, chamber expts. suggest that oxidn. (O3/OH radicals) of the photochem.-produced VOCs leads to aerosol formation and growth, possibly affecting atm. chem. and climate-related processes, such as cloud formation or the Earth's radiation budget.
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    Shrestha, M.; Luo, M.; Li, Y.; Xiang, B.; Xiong, W.; Grassian, V. H. Let there be light: stability of palmitic acid monolayers at the air/salt water interface in the presence and absence of simulated solar light and a photosensitizer. Chem. Sci. 2018, 9, 5716– 5723,  DOI: 10.1039/C8SC01957F
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    Let there be light: stability of palmitic acid monolayers at the air/salt water interface in the presence and absence of simulated solar light and a photosensitizer
    Shrestha, Mona; Luo, Man; Li, Yingmin; Xiang, Bo; Xiong, Wei; Grassian, Vicki H.
    Chemical Science (2018), 9 (26), 5716-5723CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    Long-chain fatty acid monolayers are known surfactants present at air/water interfaces. However, little is known about the stability of these long-chain fatty acid monolayers in the presence of solar radiation. Here we have investigated, for the first time, the stability of palmitic acid monolayers on salt water interfaces in the presence and absence of simulated solar light with and without a photosensitizer in the underlying salt subphase. Using surface sensitive probes to measure changes in the properties of these monolayers upon irradn., we found that the monolayers become less stable in the presence of light and a photosensitizer, in this case humic acid, in the salt soln. The presence of the photosensitizer is essential in significantly reducing the stability of the monolayer upon irradn. The mechanism for this loss of stability is due to interfacial photochem. involving electronically excited humic acid and mol. oxygen reacting with palmitic acid at the interface to form more oxygenated and less surface-active species. These oxygenated species can then more readily partition into the underlying soln.
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    Rapf, R. J.; Vaida, V. Sunlight as an energetic driver in the synthesis of molecules necessary for life. Phys. Chem. Chem. Phys. 2016, 18, 20067– 20084,  DOI: 10.1039/C6CP00980H
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    Sunlight as an energetic driver in the synthesis of molecules necessary for life
    Rapf, Rebecca J.; Vaida, Veronica
    Physical Chemistry Chemical Physics (2016), 18 (30), 20067-20084CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    A review. Solar radiation was overwhelmingly the largest source of energy on the early Earth. Energy provided by the Sun has the potential to access different chemistries than energy provided by other sources, such as hydrothermal vents, because of the unique characteristics of photochem. that differentiate it from conventional thermal chem. This review considers how sunlight-driven reactions can abiotically generate prebiotic mols. necessary for the evolution of life. We discuss briefly the characteristics of the early Sun and the likely environmental conditions on the early Earth because photochem. is both environment- and mol.-specific. An overview of the fundamental principles of photophysics and photochem. is followed by discussion of a selection of prebiotically-relevant examples of photochem. reactions, focusing on syntheses that lead to the prodn. of cellular components (e.g. sugars, lipids, and biopolymer precursors). The role of photostability as an evolutionary driving force is also considered. These examples highlight the ability of simple org. mols. to harness solar energy and convert it into high-energy chem. bonds, generating mol. complexity.
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    Grosjean, D.; Williams, E. L.; Grosjean, E. Atmospheric chemistry of isoprene and of its carbonyl products. Environ. Sci. Technol. 1993, 27, 830– 840,  DOI: 10.1021/es00042a004
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    Atmospheric chemistry of isoprene and of its carbonyl products
    Grosjean, Daniel; Williams, Edwin L., II; Grosjean, Eric
    Environmental Science and Technology (1993), 27 (5), 830-40CODEN: ESTHAG; ISSN:0013-936X.
    The carbonyl products of isoprene, methacrolein (I), Me vinyl ketone (II), hydroxyacetaldehyde, and hydroxyacetone were identified and their concns. measured in expts. involving sunlight irradiations of 1 ppm org. and 200 ppb NO in purified air. The I/II yield ratio was 1.4 for isoprene. The hydroxycarbonyl/methylglyoxal yield ratio was 4.3 for I and 1.9 for II. The peroxyacyl nitrates, PAN, (RC(O)OONO2, R = CH3-) and MPAN (R = CH2=C(CH3)-) were measured. MPAN/PAN concn. ratios were 0.65±0.04 for isoprene and 2.3±0.1 for I. A search for hydroxy-PAN (R = CHOH-) as a reaction product was not conclusive. The N-contg. products NO2, PAN, and MPAN accounted for 78-90% of the reacted NO. The reaction of O3 with isoprene, I, and II was studied with excess cyclohexane added as a scavenger for OH which is formed in the O3-org. reaction. The O3-org. reaction rate consts. were 1.02±0.05, 4.72±0.09, and 8.95±0.25 × 10-18 cm3/mol-s for I (18±2°), II (18±2°), and isoprene (20±2°), resp. Carbonyl products accounted for 70-92% of the reacted org. The I/II yield ratio from isoprene was 2.6. The results are discussed in terms of OH-org. and O3-org. atm. oxidn. pathways.
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    Kawamura, K.; Tachibana, E.; Okuzawa, K.; Aggarwal, S. G.; Kanaya, Y.; Wang, Z. F. High abundances of water-soluble dicarboxylic acids, ketocarboxylic acids and alpha-dicarbonyls in the mountaintop aerosols over the North China Plain during wheat burning season. Atmos. Chem. Phys. 2013, 13, 8285– 8302,  DOI: 10.5194/acp-13-8285-2013
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    High abundances of water-soluble dicarboxylic acids, ketocarboxylic acids and α-dicarbonyls in the mountaintop aerosols over the North China Plain during wheat burning season
    Kawamura, K.; Tachibana, E.; Okuzawa, K.; Aggarwal, S. G.; Kanaya, Y.; Wang, Z. F.
    Atmospheric Chemistry and Physics (2013), 13 (16), 8285-8302, 18 pp.CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)
    Aerosol (TSP) samples were collected at the summit of Mount Tai (elevation: 1534m a.s.l., 36.25° N, 117.10° E) located in the North China Plain using a high-vol. air sampler and pre-combusted quartz filters. Sampling was conducted on day/night or 3 h basis in the period from 29 May to 28 June 2006 during the field burning of wheat straw residue and the post-burning season. The filter samples were analyzed for low-mol.-wt. dicarboxylic acids, ketoacids and α-dicarbonyls using capillary gas chromatog. (GC) and GC-MS employing water extn. and Bu ester derivatization. Mol. distributions of dicarboxylic acids (C2-C11, 220-6070 ng m-3) were characterized by a predominance of oxalic (C2) acid (105-3920 ng m-3) followed by succinic (C4) or malonic (C3) acid. Unsatd. aliph. diacids, including maleic (M), isomaleic (iM) and fumaric (F) acids, were also detected together with arom. diacids (phthalic, isophthalic and terephthalic acids). ω-Oxocarboxylic acids (C2-C9, 24-610 ng m-3) were detected as the second most abundant compd. class with the predominance of glyoxylic acid (11-360 ng m-3), followed by α-ketoacid (pyruvic acid, 3-140 ng m-3) and α-dicarbonyls (glyoxal, 1-230 ng m-3 and methylglyoxal, 2-120 ng m-3). We found that these levels (>6000 ng m-3 for diacids) are several times higher than those reported in Chinese megacities at ground levels. The concns. of diacids increased from late May to early June, showing a max. on 7 June, and then significantly decreased during the period 8-11 June, when the wind direction shifted from southerly to northerly. Similar temporal trends were found for ketocarboxylic acids and α-dicarbonyls as well as total carbon (TC) and water-sol. org. carbon (WSOC). The temporal variations of watersol. orgs. were interpreted by the direct emission from the field burning of agricultural wastes (wheat straw) in the North China Plain and the subsequent photochem. oxidn. of volatile and semi-volatile org. precursors emitted from field burning as well as dark ozonolysis of volatile org. compds. and other orgs., accretion reactions and oxidn. of nonvolatile orgs. such as unsatd. fatty acids. This study demonstrates that the field burning of agricultural wastes in early summer strongly influenced the air quality of the free troposphere over the North China Plain.
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  142. 142
    Reed Harris, A. E.; Doussin, J.-F.; Carpenter, B. K.; Vaida, V. Gas-Phase Photolysis of Pyruvic Acid: The Effect of Pressure on Reaction Rates and Products. J. Phys. Chem. A 2016, 120, 10123– 10133,  DOI: 10.1021/acs.jpca.6b09058
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    142
    Gas-Phase Photolysis of Pyruvic Acid: The Effect of Pressure on Reaction Rates and Products
    Reed Harris, Allison E.; Doussin, Jean-Francois; Carpenter, Barry K.; Vaida, Veronica
    Journal of Physical Chemistry A (2016), 120 (51), 10123-10133CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    In this work, we investigate the impact of pressure and oxygen on the kinetics of and products from the gas-phase photolysis of pyruvic acid. The results reveal a decrease in the photolysis quantum yield as pressure of air or nitrogen is increased, a trend not yet documented in the literature. A Stern-Volmer anal. demonstrates this effect is due to deactivation of the singlet state of pyruvic acid when the photolysis is performed in nitrogen, and from quenching of both the singlet and triplet state in air. Consistent with previous studies, acetaldehyde and CO2 are obsd. as the major products; however, other products, most notably acetic acid, are also identified in this work. The yield of acetic acid increases with increasing pressure of buffer gas, an effect that is amplified by the presence of oxygen. At least two mechanisms are necessary to explain the acetic acid, including one that requires reaction of photolysis intermediates with O2. These findings extend the fundamental understanding of the gas-phase photochem. of pyruvic acid, highlighting the importance of pressure on the photolysis quantum yields and products.
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  143. 143
    Chang, X. P.; Fang, Q.; Cui, G. L. Mechanistic photodecarboxylation of pyruvic acid: Excited-state proton transfer and three-state intersection. J. Chem. Phys. 2014, 141, 154311  DOI: 10.1063/1.4898085
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    143
    Mechanistic photodecarboxylation of pyruvic acid: Excited-state proton transfer and three-state intersection
    Chang, Xue-Ping; Fang, Qiu; Cui, Ganglong
    Journal of Chemical Physics (2014), 141 (15), 154311/1-154311/8CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    Photodissocn. dynamics of pyruvic acid exptl. differs from that of commonly known ketones. We have employed the complete active space SCF and its multi-state second-order perturbation methods to study its photodissocn. mechanism in the S0, T1, and S1 states. We have uncovered four nonadiabatic photodecarboxylation paths. (i) The S1 system relaxes via an excited-state intramol. proton transfer (ESIPT) to a hydrogen-transferred tautomer, near which an S1/S0 conical intersection funnels the S1 to S0 state. Then, some trajectories continue completing the decarboxylation reaction in the S0 state; the remaining trajectories via a reverse hydrogen transfer return to the S0 min., from which a thermal decarboxylation reaction occurs. (ii) Due to a small S1 -T1 energy gap and a large S1/T1 spin-orbit coupling, an efficient S1 → T1 intersystem crossing process happens again near this S1/S0 conical intersection. When decaying to T1 state, a direct photodecarboxylation proceeds. (iii) Prior to ESIPT, the S1 system first decays to the T1 state via an S1 → T1 intersystem crossing; then, the T1 system evolves to a hydrogen-transferred tautomer. Therefrom, an adiabatic T1 decarboxylation takes place due to a small barrier of 7.7 kcal/mol. (iv) Besides the aforementioned T1 ESIPT process, there also exists a comparable Norrish type I reaction in the T1 state, which forms the ground-state products of CH3CO and COOH. Finally, we have found that ESIPT plays an important role. It closes the S1-T1 and S1-S0 energy gaps, effecting an S1/T1/S0 three-state intersection region, and mediating nonadiabatic photodecarboxylation reactions of pyruvic acid. (c) 2014 American Institute of Physics.
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  144. 144
    Reed Harris, A. E.; Cazaunau, M.; Gratien, A.; Pangui, E.; Doussin, J.-F.; Vaida, V. Atmospheric Simulation Chamber Studies of the Gas-Phase Photolysis of Pyruvic Acid. J. Phys. Chem. A 2017, 121, 8348– 8358,  DOI: 10.1021/acs.jpca.7b05139
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    144
    Atmospheric Simulation Chamber Studies of the Gas-Phase Photolysis of Pyruvic Acid
    Reed Harris, Allison E.; Cazaunau, Mathieu; Gratien, Aline; Pangui, Edouard; Doussin, Jean-Francois; Vaida, Veronica
    Journal of Physical Chemistry A (2017), 121 (44), 8348-8358CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    Pyruvic acid is an atmospherically abundant α-keto-acid which efficiently degrades in the troposphere by gas-phase photolysis. To examine relevant environmental conditions, 2-12 ppm pyruvic acid was irradiated by a solar simulator in the Exptl. Multiphasic Atm. Simulation Chamber. The combination of available in-chamber long path length and its low surface area:vol. ratio allowed the authors to quant. assess the quantum yield and photochem. products of pyruvic acid. Such details are new to the literature for the low initial pyruvic acid concns. used here. Photolysis quantum yields of .vphi.N2obs = 0.84 ± 0.1 in N, and .vphi.Airobs = 3.2 ± 0.5 in air, were higher than those reported by previous studies using higher pyruvic acid partial pressures. The quantum yield greater than unity in air was due to secondary chem., driven by O2, which emerged under the exptl. conditions. The low pyruvic acid concn. and resulting O effect also altered product distribution such that acetic acid, rather than acetaldehyde, was the primary product in air. Results indicated tropospheric pyruvic acid may degrade, in part, by photo-induced mechanisms which are different than previously expected.
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  145. 145
    Chiang, Y.; Kresge, A. J.; Pruszynski, P. Keto-enol equilibria in the pyruvic acid system: determination of the keto-enol equilibrium constants of pyruvic acid and pyruvate anion and the acidity constant of pyruvate enol in aqueous solution. J. Am. Chem. Soc. 1992, 114, 3103– 3107,  DOI: 10.1021/ja00034a053
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    145
    Keto-enol equilibria in the pyruvic acid system: determination of the keto-enol equilibrium constants of pyruvic acid and pyruvate anion and the acidity constant of pyruvate enol in aqueous solution
    Chiang, Y.; Kresge, A. J.; Pruszynski, P.
    Journal of the American Chemical Society (1992), 114 (8), 3103-7CODEN: JACSAT; ISSN:0002-7863.
    Keto-enol equil. consts. for the pyruvic acid system in aq. soln. at 25° were detd. by Meyer halogen titrn. and also by another method that evaluates these consts. as ratios of enolization to ketonization rate consts., KE = kE/kK. Measurements by each method were made in both acidic and basic soln., and enol required for the ketonization rate measurements was supplied by hydrolysis of a silyl deriv. and also by an equilibrated DMSO soln. in which the enol content is greater than it is in water. The various methods gave consistent results, which nevertheless differed between acidic and basic soln.,s in accord with the different states of ionization of pyruvic acid in the two media; the values obtained were pKE = 3.21 for pyruvic acid in the carboxylic acid form and pKE = 5.03 for the pyruvate ion. The latter gives a free energy change for the ketonization of pyruvate enol that is 47% of the free energy liberated by the hydrolysis of the high-energy mol., phosphoenolpyruvate; this shows that nearly half of the high energy content of this mol. resides in its masked enol function. An acidity const. for ionization of the enol hydroxyl group of pyruvate enol, pKaE = 11.55, was also detd., and this, when combined with pKE for this species, gives pKaK = 16.58 as the acidity const. of the pyruvate ion ionizing as a carbon acid.
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  146. 146
    Guzman, M. I.; Colussi, A. J.; Hoffmann, M. R. Photoinduced oligomerization of aqueous pyruvic acid. J. Phys. Chem. A 2006, 110, 3619– 3626,  DOI: 10.1021/jp056097z
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    146
    Photoinduced Oligomerization of Aqueous Pyruvic Acid
    Guzman, M. I.; Colussi, A. J.; Hoffmann, M. R.
    Journal of Physical Chemistry A (2006), 110 (10), 3619-3626CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    The 320 nm-band photodecarboxylation of aq. pyruvic acid (PA), a representative of the α-oxocarboxylic acids widely found in the atm. aerosol, yields 2,3-dimethyltartaric (A) and 2-(3-oxobutan-2-yloxy)-2-hydroxypropanoic (B) acids, rather than 3-hydroxy-2-oxobutanone as previously reported. A and B are identified by liq. chromatog. with UV and ESI-MS detection, complemented by collisionally induced dissocn. and 2H and 13C isotope labeling expts. The multifunctional ether B gives rise to characteristic δ ∼ 80 ppm 13C NMR resonances. Product quantum yields are proportional to [PA](a + [PA])-1 in the range [PA] = 5-100 mM. CO2(g) release rates are halved, while A and B are suppressed by the addn. of >1.5 mM TEMPO. A and B are only partially quenched in air-satd. solns. These observations are shown to be consistent with an oligomerization process initiated by a bimol. reaction between 3PA* and PA producing ketyl, CH3̇C(OH)C(O)OH, and acetyl, CH3C(O)·, radicals, rather than by the unimol. decompn. of 3PA* into 1-hydroxyethylidene, 3HO(CH3)C: (+CO2), or [CH3C(O)· + ·C(O)OH] pairs. A arises from the dimerization of ketyl radicals, while B ensues the facile decarboxylation of the C8β-ketoacid formed by assocn. of acetyl radicals with the ketyl radical adduct of PA. Since the radical precursors to A and B are scavenged by O2 with a low probability per encounter (ksc ∼ 1 × 106 M-1 s-1), PA is able to accrete into multifunctional polar species in aerated aq. media under solar illumination.
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  147. 147
    Griffith, E. C.; Carpenter, B. K.; Shoemaker, R. K.; Vaida, V. Photochemistry of aqueous pyruvic acid. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 11714– 11719,  DOI: 10.1073/pnas.1303206110
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    147
    Photochemistry of aqueous pyruvic acid
    Griffith, Elizabeth C.; Carpenter, Barry K.; Shoemaker, Richard K.; Vaida, Veronica
    Proceedings of the National Academy of Sciences of the United States of America (2013), 110 (29), 11714-11719,S11714/1-S11714/11CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    The study of org. chem. in atm. aerosols and cloud formation is of interest in predictions of air quality and climate change. It is now known that aq. phase chem. is important in the formation of secondary org. aerosols. Here, the photoreactivity of pyruvic acid (PA; CH3COCOOH) is investigated in aq. environments characteristic of atm. aerosols. PA is currently used as a proxy for α-dicarbonyls in atm. models and is abundant in both the gas phase and the aq. phase (atm. aerosols, fog, and clouds) in the atm. The photoreactivity of PA in these phases, however, is very different, thus prompting the need for a mechanistic understanding of its reactivity in different environments. Although the decarboxylation of aq. phase PA through UV excitation has been studied for many years, its mechanism and products remain controversial. In this work, photolysis of aq. PA is shown to produce acetoin (CH3CHOHCOCH3), lactic acid (CH3CHOHCOOH), acetic acid (CH3COOH), and oligomers, illustrating the progression from a three-carbon mol. to four-carbon and even six-carbon mols. through direct photolysis. These products are detected using vibrational and electronic spectroscopy, NMR, and MS, and a reaction mechanism is presented accounting for all products detected. The relevance of sunlight-initiated PA chem. in aq. environments is then discussed in the context of processes occurring on atm. aerosols.
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  148. 148
    Eugene, A. J.; Xia, S.-S.; Guzman, M. I. Negative production of acetoin in the photochemistry of aqueous pyruvic acid. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, E4274– E4275,  DOI: 10.1073/pnas.1313991110
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    148
    Negative production of acetoin in the photochemistry of aqueous pyruvic acid
    Eugene, Alexis J.; Xia, Sha-Sha; Guzman, Marcelo I.
    Proceedings of the National Academy of Sciences of the United States of America (2013), 110 (46), E4274-E4275CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    A polemic to E. Griffith, et al. (Proc. Natl. Acad. Sci. U S A, 110(29), 11714-11719). A chromatograph of photoproduct mixt. from photolysis of aq. pyruvic acid does not show presence of acetoin.
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  149. 149
    Griffith, E. C.; Carpenter, B. K.; Shoemaker, R. K.; Vaida, V. Reply to Eugene et al.: Photochemistry of aqueous pyruvic acid. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, E4276– E4276,  DOI: 10.1073/pnas.1316367110
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    149
    Reply to Eugene et al.: photochemistry of aqueous pyruvic acid
    Griffith, Elizabeth C.; Carpenter, Barry K.; Shoemaker, Richard K.; Vaida, Veronica
    Proceedings of the National Academy of Sciences of the United States of America (2013), 110 (46), E4276CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    A polemic in response to A. Eugene et al. (Proc. Natl. Acad. Sci. USA, 110:E4274-E4275).
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    Eugene, A. J.; Guzman, M. I. Reactivity of Ketyl and Acetyl Radicals from Direct Solar Actinic Photolysis of Aqueous Pyruvic Acid. J. Phys. Chem. A 2017, 121, 2924– 2935,  DOI: 10.1021/acs.jpca.6b11916
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    150
    Reactivity of Ketyl and Acetyl Radicals from Direct Solar Actinic Photolysis of Aqueous Pyruvic Acid
    Eugene, Alexis J.; Guzman, Marcelo I.
    Journal of Physical Chemistry A (2017), 121 (15), 2924-2935CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    The photochem. of aq. PA (5-300 mM) continuously sparged with air is re-examd. in the lab. under comparable irradiance at 38° N at noon on a summer day. Several anal. methods are employed to monitor the time series of the reaction, including (1) the derivatization of carbonyl (C = O) functional groups with 2,4-dinitrophenylhydrazine (DNPH), (2) the sepn. of photoproducts by ultrahigh pressure liq. chromatog. (UHPLC) and ion chromatog. (IC) coupled to mass spectrometry (MS), (3) high resoln. MS, (4) the assignment of 1H NMR and 13C gCOSY spectroscopic features, and (5) quant. 1H NMR. The primary photoproducts are 2,3-dimethyltartaric acid and unstable 2-(1-carboxy-1-hydroxyethoxy)-2-methyl-3-oxobutanoic acid, a polyfunctional β-ketocarboxylic acid with eight carbons (C8) that quickly decarboxylates into 2-hydroxy-2-((3-oxobutan-2-yl)oxy)propanoic acid. Kinetic isotope effect studies performed for the first time for this system reveal the existence of tunneling during the initial loss of PA. Thus, the KIEs support a mechanism initiated by photoinduced proton coupled electron transfer (PCET). Measured reaction rates at variable initial [PA]0 were used to calc. the sum of the quantum yields for the products, which displays a hyperbolic dependence: .sum.Φproduct = 1.99 [PA]0/(113.2 + [PA]0). The fast photochem. loss of aq. PA with an estd. lifetime of 21.7 min is interpreted as a significant atm. sink for this species. The complexity of these aq. phase pathways indicates that the solar photochem. of an abundant α-ketocarboxylic acid can activate chem. processes for SOA formation.
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  151. 151
    Vaida, V.; Reed Harris, A. E.; Rapf, R. J.; Perkins, R. J.; Carpenter, B. K. Comment on “Reactivity of Ketyl and Acetyl Radicals from Direct Solar Actinic Photolysis of Aqueous Pyruvic Acid”. J. Phys. Chem. A 2017, 121, 8738– 8740,  DOI: 10.1021/acs.jpca.7b06018
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    151
    Comment on "Reactivity of Ketyl and Acetyl Radicals from Direct Solar Actinic Photolysis of Aqueous Pyruvic Acid"
    Vaida, Veronica; Reed Harris, Allison E.; Rapf, Rebecca J.; Perkins, Russell J.; Carpenter, Barry K.
    Journal of Physical Chemistry A (2017), 121 (45), 8738-8740CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    A polemic in response to Eugene, A. J. and Guzman, M. I. (ibid., 2017, 121, 2924).
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  152. 152
    Eugene, A. J.; Guzman, M. I. Reply to “Comment on ‘Reactivity of Ketyl and Acetyl Radicals from Direct Solar Actinic Photolysis of Aqueous Pyruvic Acid’”. J. Phys. Chem. A 2017, 121, 8741– 8744,  DOI: 10.1021/acs.jpca.7b08273
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    152
    Reply to "Comment on 'Reactivity of Ketyl and Acetyl Radicals from Direct Solar Actinic Photolysis of Aqueous Pyruvic Acid'"
    Eugene, Alexis J.; Guzman, Marcelo I.
    Journal of Physical Chemistry A (2017), 121 (45), 8741-8744CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    Reply to Comment on "Reactivity of Ketyl and Acetyl Radicals from Direct Solar Actinic Photolysis of Aq. Pyruvic Acid".
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    Rapf, R. J.; Perkins, R. J.; Carpenter, B. K.; Vaida, V. Mechanistic Description of Photochemical Oligomer Formation from Aqueous Pyruvic Acid. J. Phys. Chem. A 2017, 121, 4272– 4282,  DOI: 10.1021/acs.jpca.7b03310
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    153
    Mechanistic Description of Photochemical Oligomer Formation from Aqueous Pyruvic Acid
    Rapf, Rebecca J.; Perkins, Russell J.; Carpenter, Barry K.; Vaida, Veronica
    Journal of Physical Chemistry A (2017), 121 (22), 4272-4282CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    The aq. phase photochem. of pyruvic acid, an important oxidn. product of isoprene, is known to generate larger oligomeric species that may contribute to the formation of secondary org. aerosol in the atm. Using high resoln. neg. mode electrospray ionization mass spectrometry, the aq. photochem. of dil. solns. of pyruvic acid (10, 1, and 0.5 mM) under anaerobic conditions was investigated. Even at the lowest concn., covalently bonded dimers and trimers of pyruvic acid were obsd. as photochem. products. We calc. that it is energetically possible to photochem. generate parapyruvic acid, a dimer of pyruvic acid that is known to form via dark oligomerization processes. Subsequent photochem. reactions of parapyruvic acid with pyruvic acid form larger oligomeric products, such as 2,4-dihydroxy-2-methyl-5-oxohexanoic acid. A robust and relatively simple photochem. mechanism is discussed that explains both the conditional dependence and wide array of products that are obsd.
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    Xia, S. S.; Eugene, A. J.; Guzman, M. I. Cross Photoreaction of Glyoxylic and Pyruvic Acids in Model Aqueous Aerosol. J. Phys. Chem. A 2018, 122, 6457– 6466,  DOI: 10.1021/acs.jpca.8b05724
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    154
    Cross Photoreaction of Glyoxylic and Pyruvic Acids in Model Aqueous Aerosol
    Xia, Sha-Sha; Eugene, Alexis J.; Guzman, Marcelo I.
    Journal of Physical Chemistry A (2018), 122 (31), 6457-6466CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    Aerosols of variable compn., size, and shape are assocd. with public health concerns as well as with light-particle interactions that play a role in the energy balance of the atm. Photochem. reactions of 2-oxocarboxylic acids in the aq. phase are now known to contribute to the total secondary org. aerosol (SOA) budget. This work explores the cross reaction of glyoxylic acid (GA) and pyruvic acid (PA) in water, the two most abundant 2-oxocarboxylic acids in the atm., under solar irradn. and dark thermal aging steps. During irradn., PA and GA are excited and initiate proton-coupled electron transfer or hydrogen abstraction and α-cleavage reactions, resp. The time series of photoproducts is studied by ion chromatog. (IC) with cond. and electrospray ionization (ESI) mass spectrometry (MS) detection, direct ESI-MS anal. in the neg. ion mode, and NMR spectroscopy (NMR). The use of one-dimensional (1H and 13C NMR) and two-dimensional NMR techniques includes gradient correlation spectroscopy (gCOSY) and heteronuclear single quantum correlation (HSQC). The aging of photoproducts in the dark is monitored by UV-visible spectroscopy. The periodicity in the time domain of the optical properties is explained in terms of chromophores that undergo alternating thermochromism and photobleaching between nighttime and daytime cycles, resp. A reaction mechanism for the cross reaction of GA and PA explaining the generation of trimers with general formulas C5H8O5 (148 Da), C6H10O5 (162 Da), and C5H8O6 (164 Da) is provided based on all exptl. observations.
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    Leermakers, P. A.; Vesley, G. F. Photochemistry of alpha-keto acids and alpha-keto esters 0.1. Photolysis of pyruvic acid and benzoylformic acid. J. Am. Chem. Soc. 1963, 85, 3776– 3779,  DOI: 10.1021/ja00906a013
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    155
    The photochemistry of α-keto acids and α-keto esters. I. Photolysis of pyruvic acid and benzoylformic acid
    Leermakers, Peter A.; Vesley, George F.
    Journal of the American Chemical Society (1963), 85 (23), 3776-9CODEN: JACSAT; ISSN:0002-7863.
    The dicarbonyl systems of pyruvic acid and benzoylformic acid have been found to be highly sensitive to ultraviolet irradiation. Aq. solns. of pyruvic acid and benzoylformic acid upon irradiation rapidly evolve considerable quantities of CO2. The main org. products remaining in soln. are acetoin and benzaldehyde, resp. In various org. solvents pyruvic acid is readily photoreduced to give dimethyltartaric acid and, at least in MeOH, a one-to-one solvent adduct. In the gas phase pyruvic acid undergoes decarboxylation to yield acetaldehyde.
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  156. 156
    Closs, G. L.; Miller, R. J. Photo-reduction and photodecarboxylation of pyruvic-acid. Applications of CIDNP to mechanistic photochemistry. J. Am. Chem. Soc. 1978, 100, 3483– 3494,  DOI: 10.1021/ja00479a033
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    Photoreduction and photodecarboxylation of pyruvic acid. Applications of CIDNP to mechanistic photochemistry
    Closs, G. L.; Miller, R. J.
    Journal of the American Chemical Society (1978), 100 (11), 3483-94CODEN: JACSAT; ISSN:0002-7863.
    Detailed mechanisms for the redn. of n,π* triplet MeCOCO2H (I) by EtOH, Me2CHOH and MeCHO in MeCN were elucidated by ClDNP. Various geminate combination and disproportionation reactions, including the general formation of CH2:C(OH)CO2H, are obsd. The significance of escape reactions involving H exchange between ketyl radicals and ground-state I is demonstrated. With moderate concns. of EtOH, competitive abstraction from product MeCHO becomes important. Photodecarboxylation of n,π* triplet I in H2O and other nonreducing polar solvents is initiated via unimol. scission of the carbonyl-carboxy bond. Rapid redn. of ground-state I by carboxyl radicals followed by radical coupling yields MeCOCMe(OH)CO2H as the primary photoproduct in all solvents. The rate coeff. for α-cleavage in MeCN is estd. as 1 × 106 s-1. All observations are explained with the radical-pair theory of CIDNP.
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    Eugene, A. J.; Guzman, M. I. Production of Singlet Oxygen (1O2) during the Photochemistry of Aqueous Pyruvic Acid: The Effects of pH and Photon Flux under Steady-State O2(aq) Concentration. Environ. Sci. Technol. 2019, 53, 12425– 12432,  DOI: 10.1021/acs.est.9b03742
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    157
    Production of Singlet Oxygen (1O2) during the Photochemistry of Aqueous Pyruvic Acid: The Effects of pH and Photon Flux under Steady-State O2(aq) Concentration
    Eugene, Alexis J.; Guzman, Marcelo I.
    Environmental Science & Technology (2019), 53 (21), 12425-12432CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    The photochem. of pyruvic acid (PA) in aq. atm. particles produces reactive ketyl (K•) and acetyl (Y•) radicals which contribute to secondary org. aerosol prodn. This work assessed the photolysis (λ ≥305 nm) of 5-100 mM PA at steady-state [O2(aq)] = 260 μM at 1.0 ≤ pH ≤ 4.5 and equiv. tropospheric photon fluxes of 1-10 suns. Quickly diffusing radicals heterogeneously react at the micro-bubble water/air interface with dissolved O2 to produce the atm. oxidant, singlet mol. oxygen (1O2*). Furfuryl alc. (FFA), an abundant mol. from biomass burning, traps and bracket a steady-state prodn. of 2 × 10-12 ≤ [1O2*] ≤ 1 × 10-11 M. Ion chromatog./mass spectrometry showed 2,3-dimethyltartaric acid (DMTA), 2-(3-oxobutan-2-yloxy)-2-hydroxypropanoic acid (oxo-C7 product), and 2-(1-carboxy-1-hydroxyethoxy)-2-methyl-3-oxobutanoic acid (oxo-C8 product) were formed under all studied conditions. The rates of product formation and reactant decay at variable pH perfectly overlapped with the predicted sigmoid curve for the PA undissociated fraction, with matching inflection points at pH = pKa, PA = 2.39 (except for acetic acid). The DMTA and oxocarboxylic acid products (oxo-C7 + oxo-C8) scale linearly for an increasing photon flux. However, the products branching ratio shifted away from the simple bimol. radical recombination which favored DMTA formation toward multi-step radical chem. and formation of the more complex oxocarboxylic acid products. This large steady-state prodn. of 1O2 indicated that PA in aerosols can be a significant source of atm. oxidants on par with natural org. matter. This work is of major relevance to understand the chem. of pyruvic acid under realistic tropospheric conditions.
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    Rapf, R. J.; Dooley, M. R.; Kappes, K.; Perkins, R. J.; Vaida, V. pH Dependence of the Aqueous Photochemistry of α-Keto Acids. J. Phys. Chem. A 2017, 121, 8368– 8379,  DOI: 10.1021/acs.jpca.7b08192
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    pH Dependence of the Aqueous Photochemistry of α-Keto Acids
    Rapf, Rebecca J.; Dooley, Michael R.; Kappes, Keaten; Perkins, Russell J.; Vaida, Veronica
    Journal of Physical Chemistry A (2017), 121 (44), 8368-8379CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    α-Keto acids are important, atmospherically relevant species, and their photochem. has been considered in the formation and processing of aerosols. Despite their atm. relevance, the photochem. of these species has primarily been studied under extremely low pH conditions. Using a variety of anal. techniques, we characterize the extent of hydration and deprotonation for solns. of two α-keto acids, pyruvic acid and 2-oxooctanoic acid, as a function of pH. We find that changes in the initial soln. compn. govern the accessibility of different photochem. pathways, resulting in slowed photolysis under high pH conditions and a shift in photoproducts that can be predicted mechanistically.
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    Rapf, R. J.; Perkins, R. J.; Dooley, M. R.; Kroll, J. A.; Carpenter, B. K.; Vaida, V. Environmental Processing of Lipids Driven by Aqueous Photochemistry of alpha-Keto Acids. ACS Cent. Sci. 2018, 4, 624– 630,  DOI: 10.1021/acscentsci.8b00124
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    Environmental Processing of Lipids Driven by Aqueous Photochemistry of α-Keto Acids
    Rapf, Rebecca J.; Perkins, Russell J.; Dooley, Michael R.; Kroll, Jay A.; Carpenter, Barry K.; Vaida, Veronica
    ACS Central Science (2018), 4 (5), 624-630CODEN: ACSCII; ISSN:2374-7951. (American Chemical Society)
    Sunlight can initiate photochem. reactions of org. mols. though direct photolysis, photosensitization, and indirect processes, often leading to complex radical chem. that can increase mol. complexity in the environment. α-Keto acids act as photoinitiators for org. species that are not themselves photoactive. Here, we demonstrate this capability through the reaction of two α-keto acids, pyruvic acid and 2-oxooctanoic acid, with a series of fatty acids and fatty alcs. We show for five different cases that a cross-product between the photoinitiated α-keto acid and non-photoactive species is formed during photolysis in aq. soln. Fatty acids and alcs. are relatively unreactive species, which suggests that α-keto acids are able to act as radical initiators for many atmospherically relevant mols. found in the sea surface microlayer and on atm. aerosol particles.
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    Grgić, I.; Nieto-Gligorovski, L. I.; Net, S.; Temime-Roussel, B.; Gligorovski, S.; Wortham, H. Light induced multiphase chemistry of gas-phase ozone on aqueous pyruvic and oxalic acids. Phys. Chem. Chem. Phys. 2010, 12, 698– 707,  DOI: 10.1039/B914377G
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    Light induced multiphase chemistry of gas-phase ozone on aqueous pyruvic and oxalic acids
    Grgic, I.; Nieto-Gligorovski, L. I.; Net, S.; Temime-Roussel, B.; Gligorovski, S.; Wortham, H.
    Physical Chemistry Chemical Physics (2010), 12 (3), 698-707CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    In this study for the first time it has been shown that pyruvic acid can affect the atm. multiphase reactions of ozone with oxalic acid due to its properties as a photosensitizer. To this end, the photochem. batch multiphase reactions of a mixt. of pyruvic acid/oxalic acid (PA/OA) and gas-phase ozone under simulated sunlight were studied as a function of time using high pressure liq. chromatog. equipped with a UV detector (HPLC-UV) and electrospray ionization mass spectrometry (ESI-MS) to investigate product formation. Following the simultaneous ozone and light irradn. the first peak for pyruvic and oxalic acids (retention time = 3.68 min) decreased to 67% of the initial intensity after a 12 h reaction while a broad and not well defined peak appeared at longer retention times. After prolonged exposure times this broad peak shifted to shorter retention times: from 14 min at 2 h reaction to 8 min at 12 h. The HPLC-UV anal. of the reaction mixt. simultaneously exposed to ozone and irradiated by simulated sunlight for 6-12 h revealed the presence of high wt. mol. mass products and formation at longer times of highly non-polar products. The results obtained from ESI-MS have clearly demonstrated that the distribution of high mol. wt. products is consistent with an oligomer system. No evidence of oligomer formation was found after the sample (PA/OA) was exposed only to either ozone or irradiated with UV/Vis light using the same instrumental conditions.
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    Gordon, B. P.; Moore, F. G.; Scatena, L. F.; Richmond, G. L. On the Rise: Experimental and Computational Vibrational Sum Frequency Spectroscopy Studies of Pyruvic Acid and Its Surface Active Oligomer Species at the Air-Water Interface. J. Phys. Chem. A 2019, 123, 10609– 10619,  DOI: 10.1021/acs.jpca.9b08854
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    On the Rise: Experimental and Computational Vibrational Sum Frequency Spectroscopy Studies of Pyruvic Acid and Its Surface-Active Oligomer Species at the Air-Water Interface
    Gordon, Brittany P.; Moore, Frederick G.; Scatena, Lawrence F.; Richmond, Geraldine L.
    Journal of Physical Chemistry A (2019), 123 (49), 10609-10619CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    It is known that atm. aerosol play important roles in the environment. However, there is still much to learn about the processes that form aerosols, particularly aq. secondary org. aerosols (aqSOA). While pyruvic acid (PA) is often better known for its biol. significance, it is also an abundant atm. secondary org. In bulk aq. environments, PA exists in equil. between unhydrated α-keto carboxylic acid (PYA) and singly hydrated geminal diol carboxylic acid (PYT), favoring the diol. These studies have also identified oligomer products in the bulk, including zymonic acid (ZYA) and parapyruvic acid (PPA). The surface behavior of these oligomers was not studied and their contributions (if any) to the interface are unknown. Here, the authors address this knowledge gap by examg. the mol. species present at the interface of aq. PA systems using vibrational sum-frequency spectroscopy (VSFS), a surface sensitive technique. VSFS provides information about interfacial mol. populations, orientations and behaviors. Computational studies using classical mol. dynamics (MD) and quantum mech. d. functional theory (DFT) are employed in combination to afford further insights into these systems. Studies indicate populations of at least 2 intensely surface active oligomeric species at the interface. Computational results demonstrate that along with PYA and PYT, both PPA and ZYA are surface active with strong VSF responses that can account for features in the exptl. spectra.
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    Eugene, A. J.; Pillar, E. A.; Colussi, A. J.; Guzman, M. I. Enhanced Acidity of Acetic and Pyruvic Acids on the Surface of Water. Langmuir 2018, 34, 9307– 9313,  DOI: 10.1021/acs.langmuir.8b01606
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    Enhanced Acidity of Acetic and Pyruvic Acids on the Surface of Water
    Eugene, Alexis J.; Pillar-Little, Elizabeth A.; Colussi, Agustin J.; Guzman, Marcelo I.
    Langmuir (2018), 34 (31), 9307-9313CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Understanding the acid-base behavior of carboxylic acids on aq. interfaces is a fundamental issue in nature. Surface processes involving carboxylic acids such as acetic and pyruvic acids play roles in (1) the transport of nutrients through cell membranes, (2) the cycling of metabolites relevant to the origin of life, and (3) the photooxidative processing of biogenic and anthropogenic emissions in aerosols and atm. waters. Here, we report that 50% of gaseous acetic acid and pyruvic acid mols. transfer a proton to the surface of water at pH 2.8 and 1.8 units lower than their resp. acidity consts. pKa = 4.6 and 2.4 in bulk water. These findings provide key insights into the relative Bronsted acidities of common carboxylic acids vs. interfacial water. In addn., the work ests. the reactive uptake coeff. of gaseous pyruvic acid by water to be γPA = 0.06. This work is useful to interpret the interfacial behavior of pyruvic acid under low water activity conditions, typically found in haze aerosols, clouds, and fog waters.
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    Fu, Y.; Zhang, Y.; Zhang, F.; Chen, J.; Zhu, Z.; Yu, X.-Y. Does interfacial photochemistry play a role in the photolysis of pyruvic acid in water?. Atmos. Environ. 2018, 191, 36– 45,  DOI: 10.1016/j.atmosenv.2018.07.061
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    Does interfacial photochemistry play a role in the photolysis of pyruvic acid in water
    Fu, Yao; Zhang, Yanyan; Zhang, Fei; Chen, Jianmin; Zhu, Zihua; Yu, Xiao-Ying
    Atmospheric Environment (2018), 191 (), 36-45CODEN: AENVEQ; ISSN:1352-2310. (Elsevier Ltd.)
    Pyruvic acid (PA) exists in fogs, aerosols and clouds. The photochem.-driven reaction pathways of PA in the aq. phase are more elusive than the gas phase. The PA photochem. process may occur in the bulk liq. phase and at the air-liq. interface in ambient conditions. We conducted two sample prepn. methods to simulate two possible scenarios: the air-liq. interface and the bulk liq. phase under photolysis. Time-of-flight secondary ion mass spectrometer (ToF-SIMS) was used to analyze samples because of its high sensitivity and mass accuracy in surface anal. Both neg. and pos. ion mode mass spectra provide complementary information of the products under different reaction conditions. Spectral principal component anal. (PCA) is used to det. similarities and differences among various samples. The air-liq. interface facilitates more radical reactions and form higher mol. wt. compds. (HMWCs) more quickly than the bulk liq. phase, which mainly has non-radical reactions such as anhydride reactions and decarboxylation reactions. Our results show that interfacial chem. plays an important role in atm. scenarios. Moreover, different types of secondary org. aerosols (SOAs) are formed, suggesting the strong influence of interfacial photochem. has on the earth atm.
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    Fu, H. B.; Ciuraru, R.; Dupart, Y.; Passananti, M.; Tinel, L.; Rossignol, S.; Perrier, S.; Donaldson, D. J.; Chen, J. M.; George, C. Photosensitized Production of Atmospherically Reactive Organic Compounds at the Air/Aqueous Interface. J. Am. Chem. Soc. 2015, 137, 8348– 8351,  DOI: 10.1021/jacs.5b04051
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    Photosensitized Production of Atmospherically Reactive Organic Compounds at the Air/Aqueous Interface
    Fu, Hongbo; Ciuraru, Raluca; Dupart, Yoan; Passananti, Monica; Tinel, Liselotte; Rossignol, Stephanie; Perrier, Sebastien; Donaldson, D. James; Chen, Jianmin; George, Christian
    Journal of the American Chemical Society (2015), 137 (26), 8348-8351CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    We report on expts. that probe photosensitized chem. at the air/water interface, a region that does not just connect the two phases but displays its own specific chem. Here, we follow reactions of octanol, a proxy for environmentally relevant sol. surfactants, initiated by an attack by triplet-state carbonyl compds., which are themselves concd. at the interface by the presence of this surfactant. Gas-phase products are detd. using PTR-ToF-MS, and those remaining in the org. layer are detd. by ATR-FTIR spectroscopy and HPLC-HRMS. We observe the photosensitized prodn. of carboxylic acids as well as unsatd. and branched-chain oxygenated products, compds. that act as org. aerosol precursors and had been thought to be produced solely by biol. activity. A mechanism that is consistent with the observations is detailed here, and the energetics of several key reactions are calcd. using quantum chem. methods. The results suggest that the concg. nature of the interface leads to its being a favorable venue for radical reactions yielding complex and functionalized products that themselves could initiate further secondary chem. and new particle formation in the atm. environment.
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    Tinel, L.; Rossignol, S.; Bianco, A.; Passananti, M.; Perrier, S.; Wang, X.; Brigante, M.; Donaldson, D. J.; George, C. Mechanistic Insights on the Photosensitized Chemistry of a Fatty Acid at the Air/Water Interface. Environ. Sci. Technol. 2016, 50, 11041– 11048,  DOI: 10.1021/acs.est.6b03165
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    Mechanistic Insights on the Photosensitized Chemistry of a Fatty Acid at the Air/Water Interface
    Tinel, Liselotte; Rossignol, Stephanie; Bianco, Angelica; Passananti, Monica; Perrier, Sebastien; Wang, Xinming; Brigante, Marcello; Donaldson, D. James; George, Christian
    Environmental Science & Technology (2016), 50 (20), 11041-11048CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Interfaces are ubiquitous in the environment and many atm. key processes, such as gas deposition, aerosol, and cloud formation are, at one stage or another, strongly impacted by phys. and chem. processes occurring at interfaces. Here, the photoinduced chem. of an air/water interface coated with nonanoic acid, a fatty acid surfactant we use as a proxy for chem. complex natural aq. surface microlayers, was studied as a source of volatile and semivolatile reactive org. species. The carboxylic acid coating significantly increased the propensity of photosensitizers, chosen to mimic those obsd. in real environmental waters, to partition to the interface and enhance reactivity there. Photochem. formation of functionalized and unsatd. compds. was systematically obsd. upon irradn. of these coated surfaces. The role of a coated interface appears to be crit. in providing a concd. medium allowing radical-radical reactions to occur in parallel with mol. O addns. Mechanistic insights are provided from extensive anal. of products obsd. in both gas and aq. phases by online switchable reagent ion-time of flight-mass spectrometry and by off-line uPLC coupled to a Q Exactive high resoln. mass spectrometer through heated electrospray ionization, resp.
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    Bernard, F.; Ciuraru, R.; Boreave, A.; George, C. Photosensitized Formation of Secondary Organic Aerosols above the Air/Water Interface. Environ. Sci. Technol. 2016, 50, 8678– 8686,  DOI: 10.1021/acs.est.6b03520
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    Photosensitized Formation of Secondary Organic Aerosols above the Air/Water Interface
    Bernard, F.; Ciuraru, R.; Boreave, A.; George, C.
    Environmental Science & Technology (2016), 50 (16), 8678-8686CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    This work evaluated photo-sensitized chem. at the air/sea interface as a source of secondary org. aerosols (SOA). Results showed that in addn. to biogenic emissions, abiotic processes could also be important in the marine boundary layer. Photo-sensitized prodn. of marine secondary org. aerosol was examd. in a custom-built, multi-phase atm. simulation chamber. This exptl. chamber contained water, humic acid (1-10 mg/L) as a dissolved org. matter proxy, and nonanoic acid (0.1-10 mM), a fatty acid proxy, which formed an org. film at the air/water interface. Dark secondary reactions with O3 after illumination resulted in SOA particle concns. >1000/cm3, illustrating prodn. of unsatd. compds. by chem. reactions at the air/water interface. SOA nos. by photo-sensitization alone and in the absence of O3 did not exceed background levels. From these results, a dependence of SOA nos. on nonanoic acid surface coverage and dissolved org. matter concn. was derived. The potential role of the air/sea interface in producing atm. org. aerosols from photo-sensitized origin is discussed.
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    Rossignol, S.; Tinel, L.; Bianco, A.; Passananti, M.; Brigante, M.; Donaldson, D. J.; George, C. Atmospheric photochemistry at a fatty acid-coated air-water interface. Science 2016, 353, 699– 702,  DOI: 10.1126/science.aaf3617
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    Atmospheric photochemistry at a fatty acid-coated air-water interface
    Rossignol, Stephanie; Tinel, Liselotte; Bianco, Angelica; Passananti, Monica; Brigante, Marcello; Donaldson, D. James; George, Christian
    Science (Washington, DC, United States) (2016), 353 (6300), 699-702CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Although fatty acids are believed to be photochem. inert in the actinic region, complex volatile org. compds. are produced during illumination of an air/water interface coated solely with a carboxylic acid monolayer. When aq. solns. contg. nonanoic acid (NA) in bulk concns. which give rise to just over a monolayer of NA coverage were illuminated with actinic radiation, satd. and unsatd. aldehydes were obsd. seen in the gas phase and more highly oxygenated products appeared in the aq. phase. This chem. was probably initiated by triplet-state NA mols. excited by direct absorption of actinic light at the water surface. Since fatty acid-covered interfaces are ubiquitous in the environment, such photochem. processing will have a substantial effect on local O3 and particle formation.
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    Donaldson, D. J.; Vaida, V. The Influence of Organic Films at the Air-Aqueous Boundary on Atmospheric Processes. Chem. Rev. 2006, 106, 1445– 1461,  DOI: 10.1021/cr040367c
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    The Influence of Organic Films at the Air-Aqueous Boundary on Atmospheric Processes
    Donaldson, D. J.; Vaida, Veronica
    Chemical Reviews (Washington, DC, United States) (2006), 106 (4), 1445-1461CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review concerning org. films at the air-aq. boundary (seawater surface and aq. atm. aerosols) effect on atm. processes is given. Topics discussed include: nature of coated interface (general principles, insol. surfactant films, sol. surfactants, effect of sub-phase compn.); lab. techniques; field measurements (seawater surface micro-layer, aerosol measurements); physicochem. consequences of interfacial films (non-reactive interactions, chem. reaction at the surface); optical and photochem. effects on surface films; possible role of water-air interfaces in pre-biotic chem.; and summary and outlook for future work.
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    Andreae, M. O.; Crutzen, P. J. Atmospheric aerosols: Biogeochemical sources and role in atmospheric chemistry. Science 1997, 276, 1052– 1058,  DOI: 10.1126/science.276.5315.1052
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    Atmospheric aerosols: biogeochemical sources and role in atmospheric chemistry
    Andreae, Meinrat O.; Crutzen, Paul J.
    Science (Washington, D. C.) (1997), 276 (5315), 1052-1058CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Atm. aerosols play important roles in climate and atm. chem.; they scatter sunlight, provide condensation nuclei for cloud droplets, and participate in heterogeneous chem. reactions. Two important aerosol species, sulfate and org. particles, have large natural biogenic sources that depend in a highly complex fashion on environmental and ecol. parameters and therefore are prone to influence by global change. Reactions in and on sea-salt aerosol particles may have a strong influence on oxidn. processes in the marine boundary layer through the prodn. of halogen radicals, and reactions on mineral aerosols may significantly affect the cycles of nitrogen, sulfur, and atm. oxidants.
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    Bruggemann, M.; Hayeck, N.; George, C. Interfacial photochemistry at the ocean surface is a global source of organic vapors and aerosols. Nat. Commun. 2018, 9, 3222  DOI: 10.1038/s41467-018-05687-3
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    Author Correction: Interfacial photochemistry at the ocean surface is a global source of organic vapors and aerosols
    Bruggemann Martin; Hayeck Nathalie; George Christian; Bruggemann Martin
    Nature communications (2018), 9 (1), 3222 ISSN:.
    The authors became aware of a mistake in the data displayed in the original version of the paper. Specifically, for the calculation of the total emission estimates (i.e., from an average molecular weight and summed laboratory production values for all VOCs), the authors mistakenly added seasonal estimates to the annual estimates because both values are stored in the same variable of the code. Eventually, this additional sum resulted in a doubling of emission estimates.As a result of this, the following changes have been made to the originally published version of this Article:The fifth sentence of the abstract originally read "Our results indicate global emissions of 46.4-184 Tg C yr(-1) of organic vapors from the oceans into the marine atmosphere and a potential contribution to organic aerosol mass of more than 60% over the remote ocean." In the corrected version "46.4-184 Tg C yr(-1)" is replaced by "23.2-91.9 Tg C yr(-1)"The seventh sentence of the second paragraph of the Introduction originally read "We infer global emissions of 65.0-257 Tg yr(-1) (46.4-184 Tg C yr(-1)) of organic vapors from the oceans into the marine atmosphere." In the corrected version, "65.0-257 Tg yr(-1) (46.4-184 Tg C yr(-1))" is replaced by "32.5-129 Tg C yr(-1) (23.2-91.9 Tg C yr(-1))".The last sentence of the first paragraph of the Results subheading "Marine isoprene emissions from interfacial photochemistry" originally read "In the same way, we infer total emissions of organic vapors from abiotic interfacial photochemistry in the range of 65.0-257 Tg yr(-1) (46.4-184 Tg C yr(-1)), hence, contributing significantly to marine VOC emissions." In the corrected version, "65.0-257 Tg yr(-1) (46.4-184 Tg C yr(-1))" is replaced by "32.5-129 Tg C yr(-1) (23.2-91.9 Tg C yr(-1))".This has been corrected in both the PDF and the HTML versions of the Article. While the new estimates are lower than previously reported this error does not affect the original discussion or conclusions of the Article. The authors apologize for the confusion caused by this mistake.
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    Martins-Costa, M. T. C.; Anglada, J. M.; Francisco, J. S.; Ruiz-Lopez, M. Reactivity of Atmospherically Relevant Small Radicals at the Air–Water Interface. Angew. Chem., Int. Ed. 2012, 51, 5413– 5417,  DOI: 10.1002/anie.201200656
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    171
    Reactivity of Atmospherically Relevant Small Radicals at the Air-Water Interface
    Martins-Costa, Marilia T. C.; Anglada, Josep M.; Francisco, Joseph S.; Ruiz-Lopez, Manuel F.
    Angewandte Chemie, International Edition (2012), 51 (22), 5413-5417CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Chem. at the air-water interface with HO2 is different from bulk and also different from the gas phase. This has significant ramifications for HO2 chem. or aerosol and cloud chem. At the interface, 2 main effects influencing the reactivity can be expected: (1) a decrease of ionic dissocn. const. by about 1-2 units and increase of HO2 concn. with respect to the bulk; and (2) a change of the O2/O2- redox potential by about -0.3 V.
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    Wang, H. F.; Borguet, E.; Eisenthal, K. B. Generalized interface polarity scale based on second harmonic spectroscopy. J. Phys. Chem. B 1998, 102, 4927– 4932,  DOI: 10.1021/jp9806563
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    Generalized Interface Polarity Scale Based on Second Harmonic Spectroscopy
    Wang, Hongfei; Borguet, E.; Eisenthal, K. B.
    Journal of Physical Chemistry B (1998), 102 (25), 4927-4932CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
    Quant. polarity scales have been of great value in advancing our understanding of chem. and phys. processes in bulk solvents. However, an understanding of the polarity at liq. interfaces has been more elusive and no scale of interface polarity currently exists. Following demonstrations that second harmonic spectroscopy can be used to measure interface polarity, the polarity of several liq./liq. and vapor/liq. interfaces has been detd. The polarities of the water/1,2-dichloroethane and water/chlorobenzene interfaces have been investigated using the polarity indicator mol. N,N-diethyl-p-nitroaniline (DEPNA). The betaine dye 1-(2,4,6-triphenylpyridinium)-2,6-diphenylphenoxide, (ET(30)), was used to probe the polarity of the air/water interface. The intramol. charge transfer (CT) absorption band positions of both DEPNA and ET(30) are measured and used to define the interface solvent polarity. An important finding is that the polarity of the liq. interfaces is simply related to the polarity of the bulk phases. The interface polarity is found to be equal to the arithmetic av. of the polarity of the adjoining bulk phases. This surprisingly simple result suggests the possible dominance of the long-range solute-solvent interactions, not the local interface interactions, in detg. the difference in the excited- and ground-state solvation energies of the interface adsorbed mols.
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    Sen, S.; Yamaguchi, S.; Tahara, T. Different Molecules Experience Different Polarities at the Air/Water Interface. Angew. Chem., Int. Ed. 2009, 48, 6439– 6442,  DOI: 10.1002/anie.200901094
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    173
    Different Molecules Experience Different Polarities at the Air/Water Interface
    Sen, Sobhan; Yamaguchi, Shoichi; Tahara, Tahei
    Angewandte Chemie, International Edition (2009), 48 (35), 6439-6442, S6439/1-S6439/26CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    We report a systematic study on solvatochromism at the air-water interface by using electronic sum frequency generation spectroscopy. This work shows that the electronic spectra of a series of mols. exhibit significantly different solvatochromic shifts at the same air-water interface, thus demonstrating that different mols. experience different polarity at the air-water interface.
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    Martins-Costa, M. C.; Ruiz-Lopez, M. Solvation effects on electronic polarization and reactivity indices at the air–water interface: insights from a theoretical study of cyanophenols. Theor. Chem. Acc. 2015, 134, 17  DOI: 10.1007/s00214-014-1609-z
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    Hub, J. S.; Caleman, C.; van der Spoel, D. Organic molecules on the surface of water droplets - an energetic perspective. Phys. Chem. Chem. Phys. 2012, 14, 9537– 9545,  DOI: 10.1039/c2cp40483d
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    Organic molecules on the surface of water droplets - an energetic perspective
    Hub, Jochen S.; Caleman, Carl; van der Spoel, David
    Physical Chemistry Chemical Physics (2012), 14 (27), 9537-9545CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The soly. of org. mols. is a well established property, founded on decades of measurements, the results of which have been tabulated in handbooks. Under atm. conditions water droplets may form contg. small amts. of other mols. Such droplets typically have a very large area to vol. ratio, which may shift the solvation equil. towards mols. residing on the droplet surface. The presence of org. mols. on droplet surfaces is extremely important for reactivity - it is well established that certain chem. reactions are more prevalent under atm. conditions than in bulk. Here we present a thermodn. rationalization of the surface solvation properties of methanol, ethanol, propanoic acid, n-butylamine, di-Et ether, and neopentane based on potential of mean force (PMF) calcns. - we have previously demonstrated that an energetic description is a very powerful means of disentangling the factors governing solvation (Caleman et al., Proc. Natl. Acad. Sci. U. S. A., 2011, 108, 6838-6842). All org. mols. investigated here are preferentially solvated on the surface of the droplets rather than in the inside, yet the magnitude of surface preference may differ by orders of magnitude. In order to dissect the energetic contributions that govern surface preference, we decomp. the PMF into enthalpic and entropic components, and, in a second step, into contributions from water-water and solute-water interactions. The anal. demonstrates that surface preference is primarily an enthalpic effect, but the magnitude of surface preference of solutes contg. large apolar groups is enhanced due to entropy. We introduce an anal. of the droplet PMFs that allows one to extrapolate the results to larger droplets. From this we can est. the soly. of the solutes in water droplets, demonstrating that the soly. in droplets can be orders of magnitude larger than in bulk water. Our findings have implications for understanding the process of electrospray ionization, an important technique in biol. mass spectrometry, since our work strongly suggests that in equil. biomols. would be adsorbed on the droplet surface as well.
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    Caleman, C.; Hub, J. S.; van Maaren, P. J.; van der Spoel, D. Atomistic simulation of ion solvation in water explains surface preference of halides. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 6838– 6842,  DOI: 10.1073/pnas.1017903108
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    Atomistic simulation of ion solvation in water explains surface preference of halides
    Caleman, Carl; Hub, Jochen S.; van Maaren, Paul J.; van der Spoel, David
    Proceedings of the National Academy of Sciences of the United States of America (2011), 108 (17), 6838-6842, S6838/1-S6838/10CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    Water is a demanding partner. It strongly attracts ions, yet some halide anions - chloride, bromide, and iodide - are expelled to the air/water interface. This has important implications for chem. in the atm., including the ozone cycle. We present a quant. anal. of the energetics of ion solvation based on mol. simulations of all stable alkali and halide ions in water droplets. The potentials of mean force for Cl-, Br-, and I- have shallow min. near the surface. We demonstrate that these min. derive from more favorable water-water interaction energy when the ions are partially desolvated. Alkali cations are on the inside because of the favorable ion-water energy, whereas F- is driven inside by entropy. Models attempting to explain the surface preference based on one or more ion properties such as polarizability or size are shown to lead to qual. and quant. errors, prompting a paradigm shift in chem. away from such simplifications.
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    Jungwirth, P.; Tobias, D. J. Specific ion effects at the air/water interface. Chem. Rev. 2006, 106, 1259– 1281,  DOI: 10.1021/cr0403741
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    Specific Ion Effects at the Air/Water Interface
    Jungwirth, Pavel; Tobias, Douglas J.
    Chemical Reviews (Washington, DC, United States) (2006), 106 (4), 1259-1281CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review. Here we present the effect of specific ions at the air-water interface. The aim is not only expands the scope of the previous summaries but also brings an up-to-date account of this thriving field of research at a point when it is reaching a new level of maturity. At the same time, we aim at providing a unified mol. picture of the air-soln. interface of aq. inorg. salts, acids, and bases.
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    Petersen, P. B.; Saykally, R. J. On the nature of ions at the liquid water surface. Annu. Rev. Phys. Chem. 2006, 57, 333– 364,  DOI: 10.1146/annurev.physchem.57.032905.104609
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    On the nature of ions at the liquid water surface
    Petersen, Poul B.; Saykally, Richard J.
    Annual Review of Physical Chemistry (2006), 57 (), 333-364CODEN: ARPLAP; ISSN:0066-426X. (Annual Reviews Inc.)
    A review. A qual. new understanding of the nature of ions at the liq. water surface is emerging. Traditionally, the characterization of liq. surfaces was limited to macroscopic exptl. techniques such as surface tension and electrostatic potential measurements, wherein the microscopic picture then has to be inferred by applying theor. models. Because the surface tension of electrolyte solns. generally increases with ion concn., all inorg. ions were thought to be repelled from the air-water interface, leaving the outermost surface layer essentially devoid of ions. This oversimplified picture has recently been challenged: first by chem. kinetics measurements, then by theor. mol. dynamics simulations using polarizable models, and most recently by new surface-sensitive exptl. observations. Here an overview of the nature of the interfacial structure of electrolyte solns. is presented and a detailed description of the new picture is given.
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    Jungwirth, P.; Winter, B. Ions at aqueous interfaces: From water surface to hydrated proteins. Annu. Rev. Phys. Chem. 2008, 59, 343– 366,  DOI: 10.1146/annurev.physchem.59.032607.093749
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    Ions at aqueous interfaces: from water surface to hydrated proteins
    Jungwirth, Pavel; Winter, Bernd
    Annual Review of Physical Chemistry (2008), 59 (), 343-366CODEN: ARPLAP; ISSN:0066-426X. (Annual Reviews Inc.)
    A review. The surfaces of aq. solns. are traditionally viewed as devoid of inorg. ions. Mol. simulations and surface-selective spectroscopic techniques show, however, that large polarizable anions and hydronium cations can be found (and even enhanced) at the surface and are involved in chem. at the air/water interface. Here, recent studies of ions at the air/water interface are reviewed and are compared from this perspective water with other polar solvents. For water, it is focussed in particular on the surface behavior of its ionic product (i.e., hydronium and hydroxide ions). Also the feasibility of dielec. models for the description of the protein/water interface, in analogy to the air/water interface was investigated. Little correlation was found between these two interfaces in terms of ion segregation. Therefore, a local model of pairing of ions from the soln. with charged and polar groups at the protein surface is suggested. Also corresponding results of exptl. studies on aq. model systems are described.
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    Levin, Y.; dos Santos, A. P.; Diehl, A. Ions at the Air-Water Interface: An End to a Hundred-Year-Old Mystery?. Phys. Rev. Lett. 2009, 103, 257802  DOI: 10.1103/PhysRevLett.103.257802
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    Ions at the Air-Water Interface: An End to a Hundred-Year-Old Mystery?
    Levin, Yan; dos Santos, Alexandre P.; Diehl, Alexandre
    Physical Review Letters (2009), 103 (25), 257802/1-257802/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    Availability of highly reactive halogen ions at the surface of aerosols has tremendous implications for the atm. chem. Yet neither simulations, expts., nor existing theories are able to provide a fully consistent description of the electrolyte-air interface. In this Letter a new theory is proposed which allows us to explicitly calc. the ionic d. profiles, the surface tension, and the electrostatic p.d. across the soln.-air interface. Predictions of the theory are compared to expts. and are found to be in excellent agreement. The theory also sheds new light on one of the oldest puzzles of phys. chem.-the Hofmeister effect.
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    Otten, D. E.; Shaffer, P. R.; Geissler, P. L.; Saykally, R. J. Elucidating the mechanism of selective ion adsorption to the liquid water surface. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 701– 705,  DOI: 10.1073/pnas.1116169109
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    Elucidating the mechanism of selective ion adsorption to the liquid water surface
    Otten, Dale E.; Shaffer, Patrick R.; Geissler, Phillip L.; Saykally, Richard J.
    Proceedings of the National Academy of Sciences of the United States of America (2012), 109 (3), 701-705, S701/1-S701/5CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    Adsorption of aq. thiocyanate ions from bulk soln. to the liq./vapor interface was measured as a function of temp. by resonant UV second harmonic generation spectroscopy. The resulting adsorption enthalpy and entropy changes of this prototypical chaotrope were both detd. to be neg. This surprising result is supported by mol. simulations, which clarify the microscopic origins of obsd. thermodn. changes. Calcns. reveal energetic influences of adsorbed ions on their surroundings to be remarkably local. Neg. adsorption enthalpies thus reflect a simple repartitioning of solvent d. among surface, bulk, and coordination regions. A different, and much less spatially local, mechanism underlies the concomitant loss of entropy. Simulations indicate that ions at the interface can significantly bias surface height fluctuations even several mol. diams. away, imposing restrictions consistent with the scale of measured and computed adsorption entropies. Based on these results, we expect an ion's position in the Hofmeister lyotropic series to be detd. by a combination of driving forces assocd. with the pinning of capillary waves and with a competition between ion hydration energy and the neat liq.'s surface tension.
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    Tobias, D. J.; Stern, A. C.; Baer, M. D.; Levin, Y.; Mundy, C. J. Simulation and Theory of Ions at Atmospherically Relevant Aqueous Liquid-Air Interfaces. Annu. Rev. Phys. Chem. 2013, 64, 339– 359,  DOI: 10.1146/annurev-physchem-040412-110049
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    Simulation and theory of ions at atmospherically relevant aqueous liquid-air interfaces
    Tobias, Douglas J.; Stern, Abraham C.; Baer, Marcel D.; Levin, Yan; Mundy, Christopher J.
    Annual Review of Physical Chemistry (2013), 64 (), 339-359CODEN: ARPLAP; ISSN:0066-426X. (Annual Reviews Inc.)
    A review. Chem. occurring at or near the surface of aq. droplets and thin films in the atm. influences air quality and climate. Mol. dynamics simulations are becoming increasingly useful for gaining at.-scale insight into the structure and reactivity of aq. interfaces in the atm. Here we review simulation studies of atmospherically relevant aq. liq.-air interfaces, with an emphasis on ions that play important roles in the chem. of atm. aerosols. In addn. to surveying results from simulation studies, we discuss challenges to the refinement and exptl. validation of the methodol. for simulating ion adsorption to the air-water interface and recent advances in elucidating the driving forces for adsorption. We also review the recent development of a dielec. continuum theory capable of reproducing simulation and exptl. data on ion behavior at aq. interfaces.
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    Levin, Y.; dos Santos, A. P. Ions at hydrophobic interfaces. J. Phys.: Condens. Matter 2014, 26, 203101,  DOI: 10.1088/0953-8984/26/20/203101
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    Ions at hydrophobic interfaces
    Levin, Yan; dos Santos, Alexandre P.
    Journal of Physics: Condensed Matter (2014), 26 (20), 203101/1-203101/11, 11 pp.CODEN: JCOMEL; ISSN:0953-8984. (IOP Publishing Ltd.)
    Review is the present understanding of the behavior of ions at the air-water and oil-water interfaces. While the alkali metal cations remain strongly hydrated and are repelled from the hydrophobic surfaces, the anions must be classified into kosmotropes and chaotropes. The kosmotropes remain strongly hydrated in the vicinity of a hydrophobic surface, while the chaotropes lose their hydration shell and can become adsorbed to the interface. The mechanism of adsorption is still a subject of debate. There are two driving forces for anionic adsorption: the hydrophobic cavitational energy and the interfacial electrostatic surface potential of water. While the cavitational contribution to ionic adsorption is now well accepted, the role of the electrostatic surface potential is much less clear. The difficulty is that even the sign of this potential is a subject of debate, with the ab initio and the classical force field simulations predicting electrostatic surface potentials of opposite sign. The strong anionic adsorption found in the polarizable force field simulations is the result of the artificial electrostatic surface potential present in the classical water models. If the adsorption of anions were as large as predicted by the polarizable force field simulations, the excess surface tension of the NaI soln. would be strongly neg., contrary to the exptl. measurements. While the large polarizability of heavy halides is a fundamental property and must be included in realistic modeling of the electrolyte solns., it is argued that the point charge water models, studied so far, are incompatible with the polarizable ionic force fields when the translational symmetry is broken. The goal for the future should be the development of water models with very low electrostatic surface potential. Such water models will be compatible with the polarizable force fields, which can then be used to study the interaction of ions with hydrophobic surfaces and proteins.
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    Sun, L.; Li, X.; Tu, Y. Q.; Agren, H. Origin of ion selectivity at the air/water interface. Phys. Chem. Chem. Phys. 2015, 17, 4311– 4318,  DOI: 10.1039/C4CP03338H
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    Origin of ion selectivity at the air/water interface
    Sun, Lu; Li, Xin; Tu, Yaoquan; Aagren, Hans
    Physical Chemistry Chemical Physics (2015), 17 (6), 4311-4318CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Among many characteristics of ions, their capability to accumulate at air/water interfaces is a particular issue that has been the subject of much research attention. For example, the accumulation of halide anions (Cl-, Br-, I-) at the water surface is of great importance to heterogeneous reactions that are of environmental concern. However, the actual mechanism that drives anions towards the air/water interface remains unclear. In this work, the authors have performed atomistic simulations using polarizable models to mimic ionic behavior under atm. conditions. Larger anions are abundant at the water surface and the cations are pulled closer to the surface by the counterions. The authors propose that polarization effects stabilize the anions with large radii when approaching the surface. This energetically more favorable situation is caused by the fact that the more polarized anions at the surface attract water mols. more strongly. Of relevance is also the ordering of the surface water mols. with their hydrogen atoms pointing outwards which induce an external electronic field that leads to a different surface behavior of anions and cations. The water-water interaction is weakened by the distinct water-ion attraction, a point contradicting the proposition that F- is a kosmotrope. The simulation results thus allow one to obtain a more holistic understanding of the interfacial properties of ionic solns. and atm. aerosols.
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    Tong, Y.; Zhang, I. Y.; Campen, R. K. Experimentally quantifying anion polarizability at the air/water interface. Nat. Commun. 2018, 9, 1313,  DOI: 10.1038/s41467-018-03598-x
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    Experimentally quantifying anion polarizability at the air/water interface
    Tong Yujin; Zhang Igor Ying; Campen R Kramer; Zhang Igor Ying
    Nature communications (2018), 9 (1), 1313 ISSN:.
    The adsorption of large, polarizable anions from aqueous solution on the air/water interface controls important atmospheric chemistry and is thought to resemble anion adsorption at hydrophobic interfaces generally. While the favourability of adsorption of such ions is clear, quantifying adsorption thermodynamics has proven challenging because it requires accurate description of the structure of the anion and its solvation shell at the interface. In principle anion polarizability offers a structural window, but to the best of our knowledge there has so far been no experimental technique that allowed its characterization with interfacial specificity. Here, we meet this challenge using interface-specific vibrational spectroscopy of Cl-O vibrations of the [Formula: see text] anion at the air/water interface and report that the interface breaks the symmetry of the anion, the anisotropy of [Formula: see text]'s polarizability tensor is more than two times larger than in bulk water and concentration dependent, and concentration-dependent polarizability changes are consistent with correlated changes in surface tension.
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    Wise, P. K.; Ben-Amotz, D. Interfacial Adsorption of Neutral and Ionic Solutes in a Water Droplet. J. Phys. Chem. B 2018, 122, 3447– 3453,  DOI: 10.1021/acs.jpcb.7b10488
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    Interfacial Adsorption of Neutral and Ionic Solutes in a Water Droplet
    Wise, Patrick K.; Ben-Amotz, Dor
    Journal of Physical Chemistry B (2018), 122 (13), 3447-3453CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
    Direct (solute-water) and indirect (water-water) contributions to adsorption at an air-water interface are identified using the Widom potential distribution theorem and quantified using mol. dynamics simulations of a liq. water droplet contg. either neopentane or iodide-like solutes with charges of 0 or ±1. The results are used to quant. compare direct and indirect energetic and entropic contributions to adsorption, as well as to critically test surface capillary wave, linear response (LR), and mean field (MF) predictions. The neg. signs of the total adsorption energies and entropies of both the anionic and cationic solutes are found to result from indirect adsorption induced changes in water-water interactions, rather than from surface capillary wave perturbations, which are found to be asym. with respect to solute charge. The LR and MF approxns. both accurately describe the adsorption of neutral (hydrophobic) solutes, while for ionic solutes the MF approxn. is entirely inappropriate and LR predictions are qual. (but not quant.) accurate.
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    Agmon, N.; Bakker, H. J.; Campen, R. K.; Henchman, R. H.; Pohl, P.; Roke, S.; Thamer, M.; Hassanali, A. Protons and Hydroxide Ions in Aqueous Systems. Chem. Rev. 2016, 116, 7642– 7672,  DOI: 10.1021/acs.chemrev.5b00736
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    Protons and Hydroxide Ions in Aqueous Systems
    Agmon, Noam; Bakker, Huib J.; Campen, R. Kramer; Henchman, Richard H.; Pohl, Peter; Roke, Sylvie; Thamer, Martin; Hassanali, Ali
    Chemical Reviews (Washington, DC, United States) (2016), 116 (13), 7642-7672CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    Understanding the structure and dynamics of water's constituent ions, proton and hydroxide, has been a subject of numerous exptl. and theor. studies over the last century. Besides their obvious importance in acid-base chem., these ions play an important role in numerous applications ranging from enzyme catalysis to environmental chem. Despite a long history of research, many fundamental issues regarding their properties continue to be an active area of research. Here, we provide a review of the exptl. and theor. advances made in the last several decades in understanding the structure, dynamics, and transport of the proton and hydroxide ions in different aq. environments, ranging from water clusters to the bulk liq. and its interfaces with hydrophobic surfaces. The propensity of these ions to accumulate at hydrophobic surfaces has been a subject of intense debate, and we highlight the open issues and challenges in this area. Biol. applications reviewed include proton transport along the hydration layer of various membranes and through channel proteins, problems that are at the core of cellular bioenergetics.
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    Air/water interface: Two sides of the acid-base story
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    Nature Chemistry (2013), 5 (2), 82-84CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
    A polemic in response to H. Mishra et al., Proc. Natl Acad. Sci. USA 109, 18679 (2012).
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    Martins-Costa, M. T. C.; Anglada, J. M.; Francisco, J. S.; Ruiz-Lopez, M. F. Reactivity of Volatile Organic Compounds at the Surface of a Water Droplet. J. Am. Chem. Soc. 2012, 134, 11821– 11827,  DOI: 10.1021/ja304971e
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    Reactivity of Volatile Organic Compounds at the Surface of a Water Droplet
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    Journal of the American Chemical Society (2012), 134 (28), 11821-11827CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Knowledge of the role of water droplets and aerosols in atm. chem. is crucial to significantly improve the understanding of global warming and air quality. In particular, chem. at the air/water interface is poorly understood. There is a great need to understand how clouds and aerosols chem. process orgs. compds. prevalent in the atm. First computer simulations of a volatile org. compd. (VOC, formaldehyde) at the air/water interface, explicitly describing its ground and excited state electronic properties, using an elaborated technique which combined mol. dynamics simulations with a quantum/classical description of the formaldehyde/water system, are reported. Despite a large affinity for water, formaldehyde exhibited a preference for the air/water interface with respect to the bulk, by ∼1.5 kcal/mol. Another important simulation result was that frontier orbitals, HOMO and LUMO, undergo substantial stabilization at the interface due to surface water reorientation, which induces a local pos. electrostatic potential. Such a potential is significantly larger than the one estd. in bulk water, suggesting formaldehyde reactivity could change with respect to gas phase and bulk water. Conclusions are expected to help/guide future expts. assessing VOC chem. reactivity at the air/water interface.
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    Sitzmann, E. V.; Langan, J.; Eisenthal, K. B. Intermolecular effects on intersystem crossing studied on the picosecond time scale - the solvent polarity effect on the rate of singlet to triplet intersystem crossing of diphenylcarbene. J. Am. Chem. Soc. 1984, 106, 1868– 1869,  DOI: 10.1021/ja00318a069
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    Intermolecular effects on intersystem crossing studied on the picosecond timescale: the solvent polarity effect on the rate of singlet-to-triplet intersystem crossing of diphenylcarbene
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    Journal of the American Chemical Society (1984), 106 (6), 1868-9CODEN: JACSAT; ISSN:0002-7863.
    A novel solvent-polarity effect on the rate of intersystem crossing was obsd. The rate, kST, of the singlet-to-triplet transition of Ph2C: decreases with increasing solvent polarity. A linear correlation is found between log kST and the solvent-polarity parameter ET (30), which is related to the change in the energy gap sepg. the singlet and triplet states.
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    Intersystem crossing and internal conversion quantum yields of acridine in polar and nonpolar solvents
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    The S → T intersystem-crossing quantum yields of acridine in polar and nonpolar solvents were measured using the 3rd harmonic of a Nd-glass laser and detd. by comparing the triplet formation of acridine with that of anthracene in EtOH, used as a reference. The quantum yields of acridine in C6H6, Me3COH and H2O were 0.73, 0.61, and 0.39, resp. These data combined with the fluorescence yields show the existence of internal conversion from the 1st excited singlet state of acridine in the 3 solvents, and indicate a strong solvent effect on the rate consts. of the radiationless transitions. No significant D isotope effect on the quantum yields was obsd.
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    Munoz Losa, A.; Fdez. Galvan, I.; Sanchez, M. L.; Martin, M. E.; Aguilar, M. A. Solvent effects on internal conversions and intersystem crossings: The radiationless de-excitation of acrolein in water. J. Phys. Chem. B 2008, 112, 877– 884,  DOI: 10.1021/jp075706v
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    Sanchez-Rodriguez, J. A.; Mohamadzade, A.; Mai, S.; Ashwood, B.; Pollum, M.; Marquetand, P.; Gonzalez, L.; Crespo-Hernandez, C. E.; Ullrich, S. 2-Thiouracil intersystem crossing photodynamics studied by wavelength-dependent photoelectron and transient absorption spectroscopies. Phys. Chem. Chem. Phys. 2017, 19, 19756– 19766,  DOI: 10.1039/C7CP02258A
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    2-Thiouracil intersystem crossing photodynamics studied by wavelength-dependent photoelectron and transient absorption spectroscopies
    Sanchez-Rodriguez, Jose A.; Mohamadzade, Abed; Mai, Sebastian; Ashwood, Brennan; Pollum, Marvin; Marquetand, Philipp; Gonzalez, Leticia; Crespo-Hernandez, Carlos E.; Ullrich, Susanne
    Physical Chemistry Chemical Physics (2017), 19 (30), 19756-19766CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Single-atom substitution within a natural nucleobase - such as replacing oxygen by sulfur in uracil - can result in drastic changes in the relaxation dynamics after UV excitation. While the photodynamics of natural nucleobases like uracil are dominated by pathways along singlet excited states, the photodynamics of thiobases like 2-thiouracil populate the triplet manifold with near unity quantum yield. In the present study, a synergistic approach based on time-resolved photoelectron spectroscopy (TRPES), time-resolved absorption spectroscopy (TRAS), and ab initio computations has been particularly successful at unraveling the underlying photophys. principles and describing the dissimilarities between the natural and substituted nucleobases. Specifically, we find that varying the excitation wavelength leads to differences between gas-phase and condensed-phase exptl. results. Systematic trends are obsd. in the intersystem crossing time consts. with varying excitation wavelength, which can be readily interpreted in the context of ab initio calcns. performed both in vacuum and including solvent effects. Thus, the combination of TRPES and TRAS expts. with high-level computational techniques allows us to characterize the topol. of the potential energy surfaces defining the relaxation dynamics of 2-thiouracil in both gas and condensed phases, as well as investigate the accessibility of conical intersections and crossings, and potential energy barriers along the assocd. relaxation coordinates.
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    Toniolo, A.; Olsen, S.; Manohar, L.; Martinez, T. Conical intersection dynamics in solution: the chromophore of green fluorescent protein. Faraday Discuss. 2004, 127, 149– 163,  DOI: 10.1039/B401167H
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    Conical intersection dynamics in solution: The chromophore of Green Fluorescent Protein
    Toniolo, A.; Olsen, S.; Manohar, L.; Martinez, T. J.
    Faraday Discussions (2004), 127 (Non-Adiabatic Effects in Chemical Dynamics), 149-163CODEN: FDISE6; ISSN:1359-6640. (Royal Society of Chemistry)
    We use ab initio results to reparameterize a multi-ref. semiempirical method to reproduce the ground and excited state potential energy surfaces (PESs) for the chromophore of Green Fluorescent Protein (GFP). The validity of the new parameter set is tested, and the new method is combined with a quantum mech./mol. mech. (QM/MM) treatment so that it can be applied in the soln. phase. Solvent effects on the energetics of the relevant conical intersections are explored. We then combine this representation of the ground and excited state PESs with the full multiple spawning (FMS) nonadiabatic wavepacket dynamics method to simulate the photodynamics of the neutral GFP chromophore in both gas and soln. phases. In these calcns., the PESs and their nonadiabatic couplings are evaluated simultaneously with the nuclear dynamics, i.e. "on-the-fly". The effect of solvation is seen to be quite dramatic, resulting in an order of magnitude decrease in the excited state lifetime. We observe a correlated torsion about a double bond and its adjacent single bond in both gas and soln. phases. This is discussed in the context of previous proposals about minimal vol. isomerization mechanisms in protein environments.
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    Burghardt, I.; Cederbaum, L. S.; Hynes, J. T. Environmental effects on a conical intersection: A model study. Faraday Discuss. 2004, 127, 395– 411,  DOI: 10.1039/b315071b
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    Environmental effects on a conical intersection: A model study
    Burghardt, Irene; Cederbaum, Lorenz S.; Hynes, James T.
    Faraday Discussions (2004), 127 (Non-Adiabatic Effects in Chemical Dynamics), 395-411CODEN: FDISE6; ISSN:1359-6640. (Royal Society of Chemistry)
    Excited-state processes at conical intersections (CIs) involving charge transfer phenomena can depend sensitively on the influence of a polar and polarizable environment. We propose here a formulation to describe the chromophore-environment interaction for such situations. In a model study, we focus on an extension of the two-electron two-orbital model by V. Bonacic-Koutecky, J. Koutecky, and J. Michl [Angew. Chem., Int. Ed. Engl., 1987, 26, 170], which yields a diabatic model for the S1-S0 CI in protonated Schiff bases and related systems, and describes the charge properties and charge translocation phenomena assocd. with this CI. The electrostatic effects of the environment, which are expected to strongly affect the CI topol., are accounted for by a dielec. continuum model. This translates to the image of free energy surfaces for the coupled chromophore-environment system represented by mol. coordinates plus a solvent coordinate. The environment's impact on the location and character of the CI is investigated. The limiting situations of "frozen" and equil. solvation effects are examd.
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    Yamazaki, S.; Kato, S. Locating the lowest free-energy point on conical intersection in polar solvent: Reference interaction site model self-consistent field study of ethylene and CH2NH2+. J. Chem. Phys. 2005, 123, 114510  DOI: 10.1063/1.2038867
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    Locating the lowest free-energy point on conical intersection in polar solvent: Reference interaction site model self-consistent field study of ethylene and CH2NH+2
    Yamazaki, Shohei; Kato, Shigeki
    Journal of Chemical Physics (2005), 123 (11), 114510/1-114510/13CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    We present a theor. method for locating the lowest free-energy points on conical intersections (CIs) in soln. using the ref. interaction site model SCF (RISM-SCF) theory. Based on the linear-response theory, the nonequil. free energy is defined as a quadratic function of solvation coordinates, the parameters in which are directly obtained by ab initio RISM-SCF calcns. This free energy is easily incorporated into an efficient CI optimization procedure in gas phase. The present method is applied to the cis-trans photoisomerizations of ethylene and methaniminium cation (CH2NH2+) in polar solvents. We show that the geometries and energies of CIs are largely affected by the solute-solvent electrostatic interaction. In particular, the hydrogen migration of ethylene obsd. at CIs in the gas phase disappears in protic solvents due to the large stabilization of the zwitterionic state.
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    Spezia, R.; Burghardt, I.; Hynes, J. T. Conical intersections in solution: non-equilibrium versus equilibrium solvation. Mol. Phys. 2006, 104, 903– 914,  DOI: 10.1080/00268970500417895
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    Conical intersections in solution: non-equilibrium versus equilibrium solvation
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    Molecular Physics (2006), 104 (5-7), 903-914CODEN: MOPHAM; ISSN:0026-8976. (Taylor & Francis Ltd.)
    In this contribution, the authors discuss some dynamical properties of a recently proposed protonated Schiff base model to study the solvation effect on conical intersection (CI) dynamics [I. Burghardt, L. S. Cederbaum, and J. T. Hynes, Faraday Discuss. 127, 395 (2004)]. The equil. and non-equil. dynamical solvation regimes are considered. In the first regime, the solvent instantaneously equilibrates to the evolving solute charge distribution, an assumption of many schemes for quantum chem. calcns. in soln. In the second regime, appropriate for describing the actual dynamics, account is taken of the (inertial) dynamics of the solvent elec. polarization field. In this regime, both the excited electronic state dynamics and the non-adiabatic transitions in the CI region leading from the excited state to the ground electronic state, calcd. via a surface hopping method, are found to differ significantly from those which follow from an equil. solvation characterization. Nonetheless, an equil. solvation description is useful in locating the seam of CIs which results from the chromophore-solvent interaction.
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    Reaction dynamics at liquid interfaces
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    Annual Review of Physical Chemistry (2015), 66 (), 165-188CODEN: ARPLAP; ISSN:0066-426X. (Annual Reviews)
    The liq. interface is a narrow, highly anisotropic region, characterized by rapidly varying d., polarity, and mol. structure. Several aspects are reviewed of interfacial solvation and show how these affect reactivity at liq./liq. interfaces. Specifically consider is ion transfer, electron transfer, and SN2 reactions, showing that solvent effects on these reactions can be understood by examg. the unique structure and dynamics of the liq. interface region.
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    Marcus, R. A.
    Journal of Chemical Physics (1965), 43 (2), 679-701CODEN: JCPSA6; ISSN:0021-9606.
    cf. CA 58, 10771g. A unified theory of homogeneous and electrochem. electron-transfer rates is developed using statistical mechanics. The treatment is a generalization of earlier papers of this series and is concerned with seeking a fairly broad basis for the quant. correlations among chem. and electrochem. rate consts. predicted in these earlier papers. The at. motions inside the inner coordination shell of each reactant are treated as vibrations. The motions outside are treated by the "particle description," which emphasizes the functional dependence of potential energy and free energy on mol. properties and which avoids, thereby, some unnecessary assumptions about the mol. interactions.
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    Marcus, R. Reorganization free energy for electron transfers at liquid-liquid and dielectric semiconductor-liquid interfaces. J. Phys. Chem. 1990, 94, 1050– 1055,  DOI: 10.1021/j100366a005
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    Reorganization free energy for electron transfers at liquid-liquid and dielectric semiconductor-liquid interfaces
    Marcus, R. A.
    Journal of Physical Chemistry (1990), 94 (3), 1050-5CODEN: JPCHAX; ISSN:0022-3654.
    The reorganization free energy was calcd. for a reaction (1) between 2 reactants, each in its own dielec. medium, sepd. by an interface, and (2) between a reactant and some semiconductors. An expression is also given for the rate const. of an electron-transfer reaction at an interface between reactants in two immiscible phases. Under certain conditions it is shown that the reorganization energy for the 2 immiscible liq. system is the sum of the electrochem. reorganization energies of the 2 reactants, each in its own resp. solvent. The reorganization energy for a semiconductor-liq. system can differ considerably from the corresponding metal-liq. value, even a factor of 2.
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    Marcus, R. Theory of electron-transfer rates across liquid-liquid interfaces. J. Phys. Chem. 1990, 94, 4152– 4155,  DOI: 10.1021/j100373a051
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    Theory of electron-transfer rates across liquid-liquid interfaces
    Marcus, R. A.
    Journal of Physical Chemistry (1990), 94 (10), 4152-5CODEN: JPCHAX; ISSN:0022-3654.
    The theory developed in a previous paper for the geometry of the encounter complex, the reorganization energy, and the electron-transfer rate const. at a liq.-liq. interface is applied to existing data on the rate const. To treat cyclic voltammetric (CV) studies of electron transfer across the interface, the nature of the encounters is examd. and a bimol.-type rate treatment is used. When one redox pair is in large excess, it has been pointed out that a single-phase CV anal. for diffusion/reaction can be utilized. The assumption is avoided that the second ("concd.") phase is metallike. The exptl. result deduced in this way for the true exchange current electron-transfer rate const. at the interface is compared with that estd. from the present theory of the rate const., using metal-liq. electrochem. exchange rate consts. The type of agreement found is encouraging, considering the various approxns. involved, and further exptl. studies and tests would be of interest.
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    Marcus, R. Theory of electron-transfer rates across liquid-liquid interfaces. 2. Relationships and application. J. Phys. Chem. 1991, 95, 2010– 2013,  DOI: 10.1021/j100158a023
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    Theory of electron-transfer rates across liquid-liquid interfaces. 2. Relationships and application
    Marcus, R. A.
    Journal of Physical Chemistry (1991), 95 (5), 2010-13CODEN: JPCHAX; ISSN:0022-3654.
    In part 1 an expression was obtained for the exchange current rate const. k12ll for an electron transfer between a reactant in one liq. and a second reactant in a second immiscible liq., across an (assumed) sharp interfacial boundary. This expression is used to obtain a relation between k12ll and the self-exchange rate consts. k11 and k22 for electron transfer in homogeneous solns., each redox species being in its resp. liq. phase. The relation provides an extension of the "cross-relation" for one-phase electron transfer to this two-phase case. An expression is also given relating k12ll to the usual metal-liq. electrochem. exchange current rate consts. k1el and k2el, these kel's referring to each reactant in its resp. solvent phase. An alternative limiting model for the interfacial region is also considered, in which that region is now broad, instead of sharp. The results of both models are compared with the limited available kinetic data, and the desirability of further exptl. studies is noted.
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    Eugster, N.; Fermín, D. J.; Girault, H. H. Photoinduced electron transfer at liquid/liquid interfaces. Part VI. On the thermodynamic driving force dependence of the phenomenological electron-transfer rate constant. J. Phys. Chem. B 2002, 106, 3428– 3433,  DOI: 10.1021/jp015533o
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    Photoinduced Electron Transfer at Liquid/Liquid Interfaces. Part VI. On the Thermodynamic Driving Force Dependence of the Phenomenological Electron-Transfer Rate Constant
    Eugster, Nicolas; Fermin, David J.; Girault, Hubert H.
    Journal of Physical Chemistry B (2002), 106 (13), 3428-3433CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
    The dynamics of photoinduced heterogeneous electron transfer between a series of ferrocene derivs. and the heterodimer zinc meso-tetrakis(p-sulfonatophenyl)-porphyrin (ZnTPPS4-) and zinc meso-tetrakis(N-methylpyridyl)porphyrin (ZnTMPyP4+) were studied at the polarized water/1,2-dichloroethane interface. The photocurrent responses originating from the heterogeneous quenching of the heterodimer showed a well-defined dependence on the formal Gibbs energy of electron transfer (ΔG°'et). The use of various ferrocene derivs. with different redox potentials and potentiostatic control over the Galvani p.d. across the interface allowed modifying ΔG°'et over a range of 1 eV. The photocurrent as a function of ΔG°'et can be unambiguously described in terms of a Marcus-type behavior of the phenomenol. bimol. electron-transfer rate const. (ketII). The solvent reorganization energy was estd. to be 1.05 eV, from which an av. distance of 0.8 nm between the redox species can be evaluated within the framework of the Marcus model for sharp liq./liq. boundary. These studies also provided an est. of the activation-less limit of ketII of 3 × 10-19 cm4 s-1, which reflects a rather nonadiabatic behavior of the charge-transfer process. The origin of this nonadiabaticity is connected to the av. distance sepg. the redox species across the interface. Finally, the implications of the obsd. potential dependence of ketII on current debates about structure and potential distribution across the interface are briefly highlighted.
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    Eugster, N.; Fermín, D. J.; Girault, H. H. Photoinduced electron transfer at liquid| liquid interfaces: Dynamics of the heterogeneous photoreduction of quinones by self-assembled porphyrin ion pairs. J. Am. Chem. Soc. 2003, 125, 4862– 4869,  DOI: 10.1021/ja029589n
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    Photoinduced Electron Transfer at Liquid-Liquid Interfaces: Dynamics of the Heterogeneous Photoreduction of Quinones by Self-Assembled Porphyrin Ion Pairs
    Eugster, Nicolas; Fermin, David J.; Girault, Hubert H.
    Journal of the American Chemical Society (2003), 125 (16), 4862-4869CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The initial stages of the heterogeneous photoredn. of quinone species by self-assembled porphyrin ion pairs at the water-1,2-dichloroethane (DCE) interface have been studied by ultrafast time-resolved spectroscopy and dynamic photoelectrochem. measurements. Photoexcitation of the water-sol. ion pair formed by zinc meso-tetrakis(p-sulfonatophenyl)porphyrin (ZnTPPS4-) and zinc meso-tetrakis(N-methylpyridyl)porphyrin (ZnTMPyP4+) leads to a charge-sepd. state of the form ZnTPPS3--ZnTMPyP3+ within 40 ps. This charge-sepd. state is involved in the heterogeneous electron injection to acceptors in the org. phase in the microsecond time scale. The heterogeneous electron transfer manifests itself as photocurrent responses under potentiostatic conditions. In the case of electron acceptors such as 1,4-benzoquinone (BQ), 2,6-dichloro-1,4-benzoquinone (DCBQ), and tetrachloro-1,4-benzoquinone (TCBQ), the photocurrent responses exhibit a strong decay due to back electron transfer to the oxidized porphyrin ion pair. Interfacial protonation of the radical semiquinone also contributes to the photocurrent relaxation in the millisecond time scale. The photocurrent responses are modeled by a series of linear elementary steps, allowing estns. of the flux of heterogeneous electron injection to the acceptor species. The rate of electron transfer was studied as a function of the thermodn. driving force, confirming that the activation energy is controlled by the solvent reorganization energy. This anal. also suggests that the effective redox potential of BQ at the liq.|liq. boundary is shifted by 0.6 V toward pos. potentials with respect to the value in bulk DCE. The change of the redox potential of BQ is assocd. with the formation of hydrogen bonds at the liq.|liq. boundary. The relevance of this approach toward modeling the initial processes in natural photosynthetic reaction centers is briefly discussed.
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    McArthur, E. A.; Eisenthal, K. B. Ultrafast excited-state electron transfer at an organic liquid/aqueous interface. J. Am. Chem. Soc. 2006, 128, 1068– 1069,  DOI: 10.1021/ja056518q
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