Triplet State Radical Chemistry: Significance of the Reaction of 3SO2 with HCOOH and HNO3.

The triplet excited states of sulfur dioxide can be accessed in the UV region and have a lifetime large enough that they can react with atmospheric trace gases. In this work, we report high level ab initio calculations for the reaction of the a3B1 and b3A2 excited states of SO2 with weak and strong acidic species such as HCOOH and HNO3, aimed to extend the chemistry reported in previous studies with nonacidic H atoms (water and alkanes). The reactions investigated in this work are very versatile and follow different kinds of mechanisms, namely, proton-coupled electron transfer (pcet) and conventional hydrogen atom transfer (hat) mechanisms. The study provides new insights into a general and very important class of excited-state-promoted reactions, opening up interesting chemical perspectives for technological applications of photoinduced H-transfer reactions. It also reveals that atmospheric triplet chemistry is more significant than previously thought.

Controlling the Diradical Character of Thiele Like Compounds.

Organic diradicals play an important role in many fields of chemistry, biochemistry, and materials science. In this work, by means of high-level theoretical calculations, we have investigated the effect of representative chemical substituents in p-quinodimethane (pQDM) and Thiele's hydrocarbons with respect to the singlet-triplet energy gap, a feature characterizing their diradical character. We show how the nature of the substituents has a very important effect in controlling the singlet-triplet energy gap so that several compounds show diradical features in their ground electronic state. Importantly, steric effects appear to play the most determinant role for pQDM analogues, with minor effects of the substituents in the central ring. For Thiele like compounds, we found that electron-withdrawing groups in the central ring favor the quinoidal form with a low or almost null diradical character, whereas electron-donating group substituents favor the aromatic-diradical form if the electron donation does not exceed 6-π electrons. In this case, if there is an excess of electron donation, the diradical character is reduced. The electronic spectrum of these compounds is also calculated, and we predict that the most intense bands occur in the visible region, although in some cases characteristic electronic transition in the near-IR region may appear.

Fast Sulfate Formation Initiated by the Spin-Forbidden Excitation of SO2 at the Air-Water Interface.

The multiphase oxidation of SO2 to sulfate in aerosol particles is a key process in atmospheric chemistry. However, there is a large gap between the observed and simulated sulfate concentrations during severe haze events. To fill in the gaps in understanding SO2 oxidation chemistry, a combination of experiments and theoretical calculations provided evidence for the direct, spin-forbidden excitation of SO2 to its triplet states using UVA photons at an air-water interface, followed by reactions with water and O2 that facilitate the rapid formation of sulfate. The estimated reaction energy for the whole process, 3SO2 + H2O + 1/2O2 → HSO4- + H+ (298 K, 1 M), was ΔGr = -107.8 kcal·mol-1. Moreover, calculations revealed that this was a multistep reaction involving submerged, small energy barriers (∼10 kcal·mol-1). These results indicate that photochemical oxidation of SO2 at the air-water interface with solar actinic light may be an important unaccounted source of sulfate aerosols under polluted haze conditions.

Photosensitization mechanisms at the air-water interface of aqueous aerosols.

Photosensitization reactions are believed to provide a key contribution to the overall oxidation chemistry of the Earth's atmosphere. Generally, these processes take place on the surface of aqueous aerosols, where organic surfactants accumulate and react, either directly or indirectly, with the activated photosensitizer. However, the mechanisms involved in these important interfacial phenomena are still poorly known. This work sheds light on the reaction mechanisms of the photosensitizer imidazole-2-carboxaldehyde through ab initio (QM/MM) molecular dynamics simulations and high-level ab initio calculations. The nature of the lowest excited states of the system (singlets and triplets) is described in detail for the first time in the gas phase, in bulk water, and at the air-water interface, and possible intersystem crossing mechanisms leading to the reactive triplet state are analyzed. Moreover, the reactive triplet state is shown to be unstable at the air-water surface in a pure water aerosol. The combination of this finding with the results obtained for simple surfactant-photosensitizer models, together with experimental data from the literature, suggests that photosensitization reactions assisted by imidazole-2-carboxaldehyde at the surface of aqueous droplets can only occur in the presence of surfactant species, such as fatty acids, that stabilize the photoactivated triplet at the interface. These findings should help the interpretation of field measurements and the design of new laboratory experiments to better understand atmospheric photosensitization processes.

Tight electrostatic regulation of the OH production rate from the photolysis of hydrogen peroxide adsorbed on surfaces.

Recently, experimental and theoretical works have reported evidence indicating that photochemical processes may significantly be accelerated at heterogeneous interfaces, although a complete understanding of the phenomenon is still lacking. We have carried out a theoretical study of interface and surface effects on the photochemistry of hydrogen peroxide (H2O2) using high-level ab initio methods and a variety of models. Hydrogen peroxide is an important oxidant that decomposes in the presence of light, forming two OH radicals. This elementary photochemical process has broad interest and is used in many practical applications. Our calculations show that it can drastically be affected by heterogeneous interfaces. Thus, compared to gas phase, the photochemistry of H2O2 appears to be slowed on the surface of apolar or low-polar surfaces and, in contrast, hugely accelerated on ionic surfaces or the surface of aqueous electrolytes. We give particular attention to the case of the neat air-water interface. The calculated photolysis rate is similar to the gas phase, which stems from the compensation of two opposite effects, the blue shift of the n→σ* absorption band and the increase of the absorption intensity. Nevertheless, due to the high affinity of H2O2 for the air-water interface, the predicted OH production rate is up to five to six orders of magnitude larger. Overall, our results show that the photochemistry of H2O2 in heterogeneous environments is greatly modulated by the nature of the surface, and this finding opens interesting new perspectives for technological and biomedical applications, and possibly in various atmospheres.

Reactivity of Undissociated Molecular Nitric Acid at the Air-Water Interface.

Recent experiments and theoretical calculations have shown that HNO3 may exist in molecular form in aqueous environments, where in principle one would expect this strong acid to be completely dissociated. Much effort has been devoted to understanding this fact, which has huge environmental relevance since nitric acid is a component of acid rain and also contributes to renoxification processes in the atmosphere. Although the importance of heterogeneous processes such as oxidation and photolysis have been evidenced by experiments, most theoretical studies on hydrated molecular HNO3 have focused on the acid dissociation mechanism. In the present work, we carry out calculations at various levels of theory to obtain insight into the properties of molecular nitric acid at the surface of liquid water (the air-water interface). Through multi-nanosecond combined quantum-classical molecular dynamics simulations, we analyze the interface affinity of nitric acid and provide an order of magnitude for its lifetime with regard to acid dissociation, which is close to the value deduced using thermodynamic data in the literature (∼0.3 ns). Moreover, we study the electronic absorption spectrum and calculate the rate constant for the photolytic process HNO3 + hν → NO2 + OH, leading to 2 × 10-6 s-1, about twice the value in the gas phase. Finally, we describe the reaction HNO3 + OH → NO3 + H2O using a cluster model containing 21 water molecules with the help of high-level ab initio calculations. A large number of reaction paths are explored, and our study leads to the conclusion that the most favorable mechanism involves the formation of a pre-reactive complex (HNO3)(OH) from which product are obtained through a coupled proton-electron transfer mechanism that has a free-energy barrier of 6.65 kcal·mol-1. Kinetic calculations predict a rate constant increase by ∼4 orders of magnitude relative to the gas phase, and we conclude that at the air-water interface, a lower limit for the rate constant is k = 1.2 × 10-9 cm3·molecule-1·s-1. The atmospheric significance of all these results is discussed.

The Aqueous Surface as an Efficient Transient Stop for the Reactivity of Gaseous NO2 in Liquid Water.

The heterogeneous reaction of NO2 with water on diverse surfaces is broadly considered as a possible source of atmospheric HONO in dark conditions, but the associated mechanisms are not fully understood. We report data from first-principles simulations showing that the lifetime of the putative reactive NO2 dimer on the surface of pure water droplets is too small to host the whole process. One infers from our results that the hydrolysis of NO2 in clouds must be catalyzed by organic or inorganic species adsorbed on the droplets.

Photoinduced Oxidation Reactions at the Air-Water Interface.

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.

Two-step reaction mechanism reveals new antioxidant capability of cysteine disulfides against hydroxyl radical attack.

Cysteine disulfides, which constitute an important component in biological redox buffer systems, are highly reactive toward the hydroxyl radical (•OH). The mechanistic details of this reaction, however, remain unclear, largely due to the difficulty in characterizing unstable reaction products. Herein, we have developed a combined approach involving mass spectrometry (MS) and theoretical calculations to investigate reactions of •OH with cysteine disulfides (Cys-S-S-R) in the gas phase. Four types of first-generation products were identified: protonated ions of the cysteine thiyl radical (+Cys-S•), cysteine (+Cys-SH), cysteine sulfinyl radical (+Cys-SO•), and cysteine sulfenic acid (+Cys-SOH). The relative reaction rates and product branching ratios responded sensitively to the electronic property of the R group, providing key evidence to deriving a two-step reaction mechanism. The first step involved •OH conducting a back-side attack on one of the sulfur atoms, forming sulfenic acid (-SOH) and thiyl radical (-S•) product pairs. A subsequent H transfer step within the product complex was favored for protonated systems, generating sulfinyl radical (-SO•) and thiol (-SH) products. Because sulfenic acid is a potent scavenger of peroxyl radicals, our results implied that cysteine disulfide can form two lines of defense against reactive oxygen species, one using the cysteine disulfide itself and the other using the sulfenic acid product of the conversion of cysteine disulfide. This aspect suggested that, in a nonpolar environment, cysteine disulfides might play a more active role in the antioxidant network than previously appreciated.

Molecular reactions at aqueous interfaces.

This Review aims to critically analyse the emerging field of chemical reactivity at aqueous interfaces. The subject has evolved rapidly since the discovery of the so-called 'on-water catalysis', alluding to the dramatic acceleration of reactions at the surface of water or at its interface with hydrophobic media. We review critical experimental studies in the fields of atmospheric and synthetic organic chemistry, as well as related research exploring the origins of life, to showcase the importance of this phenomenon. The physico-chemical aspects of these processes, such as the structure, dynamics and thermodynamics of adsorption and solvation processes at aqueous interfaces, are also discussed. We also present the basic theories intended to explain interface catalysis, followed by the results of advanced ab initio molecular-dynamics simulations. Although some topics addressed here have already been the focus of previous reviews, we aim at highlighting their interconnection across diverse disciplines, providing a common perspective that would help us to identify the most fundamental issues still incompletely understood in this fast-moving field.

A New Mechanism of Acid Rain Generation from HOSO at the Air-Water Interface.

The photochemistry 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 dissociated at the air-water interface. Accordingly, this radical might play an important role in acid rain formation. Potential implications are discussed.

Photochemistry of HOSO radical in the gas phase.

The photochemistry of HOSO in the near- and deep-UV spectral range has been studied in the gas phase using the multireference configuration interaction MRCI+Q/aug-cc-pV(T+d)Z level of theory. HOSO is found to be a nonplanar radical in its ground electronic state with a torsion angle calculated to be 49.7°. The lowest three doublet electronic states are characterized by a large transition dipole moment and are implicated in the photodissociation of HOSO in the gas phase to generate SO and OH as products. Sulfur dioxide and hydrogen products may also result after UV absorption to reach the first excited state, and this channel competes with the production of OH and SO.

Formation of a stable biradical triplet state cation versus a closed shell singlet state cation by oxidation of adducts of 3,6-dimethoxycarbazole and polychlorotriphenylmethyl radicals.

We report an experimental and theoretical study of two stable radical adducts of the triphenylmethyl series, 1 and 2, whose composition and molecular structure are distinguished by the content and position of chlorine atoms in phenyls. The electrochemical study through cyclic voltammetry of these open layer species shows the existence of two reversible processes, related to reduction and oxidation, to stable charged species. The chemical oxidation of both radical adducts gives rise to stable cations, whose fundamental state has a biradical triplet electronic structure or a closed shell singlet character, depending on the electronic conjugation between the donor and acceptor electron moieties. The presence of chlorines adjacent to the nitrogen in 1 breaks the conjugation between both halves, facilitating the formation of a triplet electronic state of the cation, while the absence of chlorines in these positions in 2 facilitates partial conjugation and stabilizes the closed shell singlet electronic state of the cation.

Theoretical Investigation of the Photoexcited NO2 +H2 O reaction at the Air-Water Interface and Its Atmospheric Implications.

The atmospheric role of photochemical processes involving NO2 beyond its dissociation limit (398 nm) is controversial. Recent experiments have confirmed that excited NO2 * beyond 420 nm reacts with water according to NO2 * +H2 O→HONO+OH. However, the estimated kinetic constant for this process in the gas phase is quite small (k≈10-15 -3.4×10-14  cm3 molecule-1 s-1 ) suggesting minor atmospheric implications of the formed radicals. In this work, ab initio molecular dynamics simulations of NO2 adsorbed at the air-water interface reveal that the OH production rate increases by about 2 orders of magnitude with respect to gas phase, attaining ozone reference values for NO2 concentrations corresponding to slightly polluted rural areas. This finding substantiates the argument that chemistry on clouds can be an additional source of OH radicals in the troposphere and suggests directions for future laboratory experimental studies.

Triplet state promoted reaction of SO2 with H2O by competition between proton coupled electron transfer (pcet) and hydrogen atom transfer (hat) processes.

The SO2 + H2O reaction is proposed to be the starting process for the oxidation of sulfur dioxide to sulfate in liquid 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 calculations 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 compounds with water, where Y is a chalcogen atom and X is either an atom or a functional group, which opens up interesting chemical perspectives in technological applications of photoinduced H-transfer.

Tropospheric oxidation of methyl hydrotrioxide (CH3OOOH) by hydroxyl radical.

We have employed high level theoretical methods to investigate the oxidation of methyl hydrotrioxide by hydroxyl radical, which is of interest in atmospheric chemistry research. The reaction can proceed by abstraction of either the terminal hydrogen atom of OOH group producing CH3O, O2 and H2O, or one hydrogen atom of the CH3 group forming H2CO, HO2 and H2O. The rate constants for both reactions at 298 K are computed to be 4.7 × 10-11 and 2.1 × 10-12 cm3 molecule-1 s-1, respectively, that is, the abstraction of terminal hydrogen atom of the OOH group is about 22 times faster than that of a hydrogen atom of the CH3 group. The rate constant for the overall CH3OOOH + OH reaction is computed to be 4.9 × 10-11 cm3 molecule-1 s-1. Our calculations predict branching ratios between 99.0 and 93.9% for the formation of methoxy radical plus molecular oxygen and water, and between 1.0 and 6.1% for the formation of formaldehyde plus hydroperoxyl radical and water, in the 225-325 K temperature range. The lifetime of CH3OOOH in the troposphere is predicted to range from of 1.8 hours at 225 K, up to 3.9 hours at 275 K and decreasing to 0.2 hours at 310 K. CH3OOO and CH2OOOH radicals have been also investigated.

Photochemistry of SO2 at the Air-Water Interface: A Source of OH and HOSO Radicals.

The photochemistry of sulfur dioxide in the near UV-vis energy range has been studied in aqueous environments. The combination of previously reported experimental measurements with accurate quantum chemical calculations achieved in this work reveals that the process represents an important source of OH radicals in the troposphere. It implicates the reaction of the lowest triplet excited state of SO2 with a water molecule. When the process occurs in the gas-phase, photochemical OH production is only significant under high humidity conditions and high SO2 concentrations as those measured in polluted urban areas. However, the OH production rate increases by several orders of magnitude when the process takes place at the surface of water droplets. The present study indicates therefore that the atmospheric importance of sulfur dioxide goes beyond its well-known role as acid rain and aerosol formation precursor.

Reactivity of hydropersulfides toward the hydroxyl radical unraveled: disulfide bond cleavage, hydrogen atom transfer, and proton-coupled electron transfer.

Hydropersulfides (RSSH) are highly reactive as nucleophiles and hydrogen atom transfer reagents. These chemical 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 chemical nature of RSSH. We have performed a high-level theoretical 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 constant: 4.55 × 1012 M-1 s-1). Our results highlight the complexity in RSSH reaction chemistry, the extent of which is closely modulated by the inductive effect of the substituents in the case of the oxidation by hydroxyl radicals.

The Atmospheric Oxidation of HONO by OH, Cl, and ClO Radicals.

The atmospheric oxidation of nitrous acid by hydroxyl radical, chlorine atom, and chlorine monoxide radical was investigated with high-level theoretical methods. Nitrous acid has two conformers (cis and trans), and we found a reaction path for the oxidation of each of these conformers with the radicals considered. In all cases, the oxidation of the cis conformer is much more favorable than the oxidation of the trans conformer. Interestingly all transition states in these oxidation processes follow a proton-coupled electron-transfer mechanism. Our computed rate constant at 298 K for the reaction of cis-HONO + ·OH is 4.83 × 10-12 cm3 molecule-1 s-1, in excellent agreement with their experimental values (4.85 × 10-12 and 6.48 × 10-12 cm3 molecule-1 s-1). For the trans-HONO + ·OH reaction our calculated rate constant at 298 K is 9.05 × 10-18 cm3 molecule-1 s-1, and we computed an effective rate constant for the oxidation of the whole nitrous acid by hydroxyl radical of 1.81 × 10-12 cm3 molecule-1 s-1. For the oxidation of nitrous acid by chlorine atom we predict greater rate constants (7.38 × 10-11, 3.33 × 10-15, and 2.76 × 10-11 cm3 molecule-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 oxidation of nitrous acid, especially in marine boundary areas, and to the formation of tropospheric ozone. For the oxidation of nitrous acid by chlorine monoxide we predict rate constants roughly 6 orders of magnitude smaller than the oxidation by chlorine atom, and therefore we consider that this process should play a minor role in the troposphere.

Relevance of the DFT method to study expanded porphyrins with different topologies.

Meso-aryl expanded porphyrins present a structural versatility that allows them to achieve different topologies with distinct aromaticities. Several studies appeared in the literature studying these topological switches from an experimental and theoretical point of view. Most of these publications include density functional theory calculations, being the B3LYP the most used methodology. In this work, we show that the selection of the functional has a critical role on the geometric, energetic, and magnetic results of these expanded porphyrins, and that the use of an inadequate methodology can even generate spurious stationary points on the potential energy surface. To illustrate these aspects, in this article we have studied different molecular distortions of two expanded porphyrins, [32]-heptaphyrin and [26]-hexaphyrin using 11 DFT functionals and performing single point energy calculations at the local pair natural orbital coupled cluster DLPNO-CCSD(T) method, which have been carried out for benchmarking purposes. For some selected functionals, the dispersion effects have also been evaluated using the D3-Grimme's dispersion correction with Becke-Johnson damping. Our results let us to conclude that the CAM-B3LYP, M05-2X, and M06-2X functionals are the methodologies that provide a more consistent description of these topological switches, while other methods, such as B3LYP, BPE, and BP86, show a biased description. © 2017 Wiley Periodicals, Inc.