Unexpected Reactivity of Amidogen Radical in the Gas Phase Degradation of Nitric Acid

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.

Metal-free catalysis for the reaction of nitrogen dioxide dimer with phenol: An unexpected favorable source of nitrate and aerosol precursors in vehicle exhaust

Atmospheric reaction mechanism and dynamics of phenol with nitrogen dioxide dimer were explored by the density functional theory and high-level quantum chemistry combined with statistical kinetic calculations within 220-800 K. The nitric acid and phenyl nitrite, the typical aerosol precursors, are the preponderant products because of the low formation free energy barrier (∼8.7 kcal/mol) and fast rate constants (∼10^-15 cm³ molecule^-1 s^-1 at 298 K). Phenyl nitrate is the minor product and it would be also formed from the transformation of phenyl nitrite in NO₂-rich environment. More importantly, kinetic effects and catalytic mechanism of a series of metal-free catalysts (H₂O, NH₃, CH₃NH₂, CH₃NHCH₃, HCOOH, and CH₃COOH) on the title reaction were investigated at the same level. The results indicate that CH₃NH₂ and CH₃NHCH₃ can not only catalyze the title reaction by lowering the free energy barrier (about 1.4-6.5 kcal/mol) but also facilitate the production of organic ammonium nitrate via acting as a donor-acceptor of hydrogen. Conversely, the other species are non-catalytic upon the title reaction. The stabilization energies and donor-acceptor interactions in alkali-catalyzed product complexes were explored, which can provide new insights to the properties of aerosol precursors. Moreover, the lifetime of phenol determined by nitrogen dioxide dimer in the presence of dimethylamine may compete with that of determined by OH radicals, indicating that nitrogen dioxide dimer is responsible for the elimination of phenol in the polluted atmosphere. This work could help us thoroughly understand the removal of nitrogen oxides and phenol as well as new aerosol precursor aggregation in vehicle exhaust.

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.

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.

New mechanistic pathways for the formation of organosulfates catalyzed by ammonia and carbinolamine formation catalyzed by sulfuric acid in the atmosphere

Sulfuric acid exerts a remarkable catalytic role in the H₂SO₄ + HCHO + NH₃ reaction that leads to the formation of carbinolamine.

Atmospheric Spectroscopy and Photochemistry at Environmental Water Interfaces

The air-water interface is ubiquitous in nature, as manifested in the form of the surfaces of oceans, lakes, and atmospheric aerosols. The aerosol interface, in particular, can play a crucial role in atmospheric chemistry. The adsorption of atmospheric species onto and into aerosols modifies their concentrations and chemistries. Moreover, the aerosol phase allows otherwise unlikely solution-phase chemistry to occur in the atmosphere. The effect of the air-water interface on these processes is not entirely known. This review summarizes recent theoretical investigations of the interactions of atmosphere species with the air-water interface, including reactant adsorption, photochemistry, and the spectroscopy of reactants at the water surface, with an emphasis on understanding differences between interfacial chemistries and the chemistries in both bulk solution 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 atmosphere.

Atmospheric reactions of glyoxal with NO2 and NH2 radicals: Hydrogen abstraction mechanism and natural bond orbital analysis

Glyoxal can be important in atmospheric chemistry in terms of its ability to convert to secondary organic aerosols. In this study, the glyoxal-breaking reaction by two atmospheric active radicals, NO2 and NH2, has been investigated at the B3LYP and M06-2X levels in connection with 6-311++G(d,p) basis set. The formation of the most stable adducts from glyoxal with NO2/NH2 radical requires two hydrogen atom transfers. The accuracy of the predicted mechanisms in describing the hydrogen transfers was confirmed by atoms-in-molecules calculations and natural bond orbital analysis. The calculated results predict that hydrogen transfer process in both reactions at the M06-2X level is favourable from the kinetic and thermodynamic points of view. In the natural bond orbital analysis, the stabilization energy, E(2), delocalization corrections, at the B3LYP level is much higher than the same results at the M06-2X level (nearly twice). The activation thermodynamic parameters show that the first steps of the two reactions have lower barrier energy than the second steps. The Gibbs free energy values estimate that adducts of both the reactions at the mentioned method are spontaneous. The whole reaction of glyoxal + NH2 is more favourable than the whole reaction of glyoxal + NO2. The rate constants were calculated for the mentioned pathways using transition state theory for bimolecular steps and the fitted equations are reported.

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

The excited triplet electronic state of SO₂ (a³B₁) reacts with water through a proton coupled electron transfer (pcet) mechanism rather than via a conventional hydrogen atom transfer (hat) process.

Catalytic effect of (H2O) n (n = 1-3) on the HO2 + NH2 → NH3 + 3O2 reaction under tropospheric conditions

The effects of (H2O) n (n = 1-3) clusters on the HO2 + NH2 → NH3 + 3O2 reaction have been investigated by employing high-level quantum chemical calculations with M06-2X and CCSD(T) theoretical methods, and canonical variational transition (CVT) state theory with small curvature tunneling (SCT) correction. The calculated results show that two kinds of reaction, HO2⋯(H2O) n (n = 1-3) + NH2 and H2N⋯(H2O) n (n = 1-3) + HO2, are involved in the (H2O) n (n = 1-3) catalyzed HO2 + NH2 → NH3 + 3O2 reaction. Due to the fact that HO2⋯(H2O) n (n = 1-3) complexes have much larger stabilization energies and much higher concentrations than the corresponding complexes of H2N⋯(H2O) n (n = 1-3), the atmospheric relevance of the former reaction is more obvious with its effective rate constant of about 1-11 orders of magnitude faster than the corresponding latter reaction at 298 K. Meanwhile, due to the effective rate constant of the H2O⋯HO2 + NH2 reaction being respectively larger by 5-6 and 6-7 orders of magnitude than the corresponding reactions of HO2⋯(H2O)2 + NH2 and HO2⋯(H2O)3 + NH2, the catalytic effect of (H2O) n (n = 1-3) is mainly taken from the contribution of the water monomer. In addition, the enhancement factor of the water monomer is 10.06-13.30% within the temperature range of 275-320 K, which shows that at whole calculated temperatures, a positive water effect is obvious under atmospheric conditions.

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.

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

The amidogen radical abstracts the hydrogen from nitric acid through a proton coupled electron transfer mechanism rather than by an hydrogen atom transfer process.