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

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.

Treatment of Olefin plant spent caustic by combination of Fenton-like and foam fractionation methods in a bench scale

Spent Merox caustic (SMC) is a hazardous waste that is produced during the Merox desulfurization process in the petroleum refinery industry and should be treated before discharging to environment. In the present study, treatment of SMC was investigated by three methods including Fenton-like process, foam fractionation, and a combination of both processes. Immobilized TiO₂/Fe⁰ on modified silica nanoparticles was used as a heterogeneous Fenton-like catalyst. The chemical and physical characteristics of the catalyst were determined using Fourier-transform infrared spectroscopy, X-ray diffraction, diffuse reflectance spectroscopy, thermogravimetric analysis, differential scanning calorimetry, and transmission electron microscopy techniques. The treatment performance of the combined method was measured as a cost-effective method with chemical oxygen demand (COD) removal percentage. The effect of parameters including pH, gas flow rate, surfactant type and concentration of hydrogen peroxide, catalyst, and chelate were investigated. It is found that the prepared heterogeneous catalyst has high activity for the treatment of SMC. In addition, the results showed that the combined method achieved 97.6 ± 0.5% COD removal, while the measured values for Fenton or foam fractionation methods alone did not exceed 85.5 ± 1% and 47.2 ± 0.4%, respectively. The advantage of combination process over foam fractionation was the use of an advanced oxidation process in the separating column to eliminate or reduce the secondary phase contamination load. Besides, the role of the column in the effective contact of contaminants with the rising bubbles improved the degradation performance of the proposed process and reduced the consumption of hydrogen peroxide by 46% compared to the Fenton-like method.

Atmospheric reaction of hydrazine plus hydroxyl radical

Understanding the mechanism of hydrazine oxidation reaction by OH radical along with the rate constants of all possible pathways leads to explain the fate of hydrazine in the atmosphere. In this article, the comprehensive mechanisms and kinetics of the hydrazine plus hydroxyl radical reaction have been investigated theoretically at different temperatures and pressures. To achieve the main goals, a series of high levels of quantum chemical calculations have been widely implemented in reliable channels of the H-abstraction, S_N2, and addition/elimination reactions. The energy profile of all pathways accompanied by the molecular properties of the involved stationary points has been characterized at the MP2, M06-2X, and CCSD(T)/CBS levels. To estimate accurate barrier energies of the H-abstraction channels, large numbers of the CCSD (T) calculations in conjunction with various augmented basis sets have been implemented. The direct dynamic calculations have been carried out using the validated M06-2X/maug-cc-pVTZ level, and also by the CCSD(T) (energies) + MP2 (partition functions) level. The pressure-dependent rate constants of the barrierless pathways have been investigated by the strong collision approach. Therefore, the main behaviors of the N₂H₄ + OH reaction have been explored according to the influences of temperature and pressure on the computed rate coefficients within the well-behaved theoretical frameworks of the TST, VTST, and RRKM theories. It has been found that the H-abstraction mechanism (to form N₂H₃) is dominant relative to the S_N2 reaction and OH-addition to the N center of N₂H₄ moiety (to form H₂NOH + NH₂). The computed high pressure limit rate constant of the main reaction pathway, k(298.15) = 7.31 × 10^–11 cm³ molecule⁻¹ s⁻¹, has an excellent agreement with the experimental value (k (298.15) = (6.50 ± 1.3) × 10^–11 cm³ molecule⁻¹ s⁻¹) recommended by Vaghjiani. Also, the atmospheric lifetime of hydrazine degradation by OH radicals has been demonstrated to be 32.80 to 1161.11 h at the altitudes of 0–50 km. Finally, the disagreement in the calculated rate constants between the previous theoretical study and experimental results has been rectified.

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.

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.

Theoretical Insights into the Electron Capture Behavior of H₂SO₄···N₂O Complex: A DFT and Molecular Dynamics Study

Both sulfuric acid (H₂SO₄) and nitrous oxide (N₂O) play a central role in the atmospheric chemistry in regulating the global environment and climate changes. In this study, the interaction behavior between H₂SO₄ and N₂O before and after electron capture has been explored using the density functional theory (DFT) method as well as molecular dynamics simulation. The intermolecular interactions have been characterized by atoms in molecules (AIM), natural bond orbital (NBO), and reduced density gradient (RDG) analyses, respectively. It was found that H₂SO₄ and N₂O can form two transient molecular complexes via intermolecular H-bonds within a certain timescale. However, two molecular complexes can be transformed into OH radical, N₂, and HSO₄⁻ species upon electron capture, providing an alternative formation source of OH radical in the atmosphere. Expectedly, the present findings not only can provide new insights into the transformation behavior of H₂SO₄ and N₂O, but also can enable us to better understand the potential role of the free electron in driving the proceeding of the relevant reactions in the atmosphere.

Investigation of nucleation kinetics in H2SO4 vapor through modeling of gas phase kinetics coupled with particle dynamics

We have developed a new model utilizing our existing kinetic gas phase models to simulate experimental particle size distributions emerging in dry supersaturated H₂SO₄ vapor homogeneously produced by rapid oxidation of SO₂ through stabilized Criegee-Intermediates from 2-butene ozonolysis. We use a sectional method for simulating the particle dynamics. The particle treatment in the model is based on first principles and takes into account the transition from the kinetic to the diffusion-limited regime. It captures the temporal evolution of size distributions at the end of the ozonolysis experiment well, noting a slight underrepresentation of coagulation effects for larger particle sizes. The model correctly predicts the shape and the modes of the experimentally observed particle size distributions. The predicted modes show an extremely high sensitivity to the H₂SO₄ evaporation rates of the initially formed H₂SO₄ clusters (dimer to pentamer), which were arbitrarily restricted to decrease exponentially with increasing cluster size. In future, the analysis presented in this work can be extended to allow a direct validation of quantum chemically predicted stabilities of small H₂SO₄ clusters, which are believed to initiate a significant fraction of atmospheric new particle formation events. We discuss the prospects and possible limitations of the here presented approach.

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.

Mechanistic and kinetic study on the catalytic hydrolysis of COS in small clusters of sulfuric acid

The catalytic hydrolysis of carbonyl sulfide (COS) and the effect of small clusters of H₂O and H₂SO₄ have been studied by theoretical calculations. The addition of H₂SO₄ could increase the enthalpy change (ΔH<0) and decrease relative energy of products (relative energy<0), resulting in hydrolysis reaction changed from an endothermic reaction to an exothermic reaction. Further, H₂SO₄ decreases the energy barrier by 5.25 kcal/mol, and it enhances the catalytic hydrolysis through the hydrogen transfer effect. The (COS + H₂SO₄-H₂O) reaction has the lowest energy barrier of 29.97 kcal/mol. Although an excess addition of H₂O and H₂SO₄ increases the energy barrier, decreases the catalytic hydrolysis, which is consistent with experimental observations. The order of the energy barriers for the three reactions from low to high are as follows: COS + H₂SO₄-H₂O < COS + H₂O + H₂SO₄-H₂O < COS + H₂O+(H₂SO₄)₂. Kinetic simulations show that the addition of H₂SO₄ can increase the reaction rate constants. Consequently, adding an appropriate amount of sulfuric acid promotes the catalytic hydrolysis of COS both kinetically and thermodynamically.

Exploring the role of a single water molecule in the tropospheric reaction of glycolaldehyde with an OH radical: a mechanistic and kinetics study

This work reveals that though a single-water molecule decelerates the atmospheric reaction between the glycolaldehyde and OH radical, however, it facilitates the cis–​trans interconversion along the hydrogen-abstraction pathways.

Exploring water catalysis in the reaction of thioformic acid with hydroxyl radical: a global reaction route mapping perspective

Hydrogen abstraction pathways, in the gas-phase reaction of tautomers of thioformic acid (TFA), TFA(thiol), and TFA(thione), with hydroxyl radical in the presence and absence of single water molecule acting as a catalyst, is investigated with high-level quantum mechanical calculations at CCSD(T)/6-311++G(2d,2p)//MP2/6-311++G(2d,2p), CCSD(T)/6-311++G(d,p)//DFT/BHandHLYP/6-311++G(d,p), and DFT/B3LYP/6-311++G(2df,2p) levels of the theory. A systematic and automated search of the potential energy surface (PES) for the reaction pathways is performed using the global reaction route mapping (GRRM) method that employs an uphill walking technique to search prereaction complexes and transition states. The computations reveal significant lowering of the PES and substantial reduction in the activation energy for the hydrogen abstraction pathway in the presence of water, thereby proving water as an efficient catalyst in the reaction of both the TFA tautomers with OH radical. The hydrogen-bonding interactions are observed to be responsible for the large catalytic effect of water. Notably, in the case of TFA(thiol), formyl hydrogen abstraction is observed to be kinetically more favorable, while acidic hydrogen abstraction is observed to be thermodynamically more feasible. Interestingly, in the case of TFA(thione), reaction pathways involving only formyl hydrogen abstraction were observed to be feasible. The water-catalyzed hydrogen abstraction reaction of TFA with hydroxyl radical, investigated in this work, can provide significant insights into the corresponding reaction in the biological systems.

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.

The gas-phase reaction of methane sulfonic acid with the hydroxyl radical without and with water vapor

The gas phase reaction between methane sulfonic acid (CH3SO3H; MSA) and the hydroxyl radical (HO), without and with a water molecule, 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 constant for the oxidation process is 8.3 × 10(-15) cm(3) s(-1) molecule(-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 molecule 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 calculations show that 99.5% of the reaction involves this interchange of the water molecule and, consequently, water vapor does not play any role in the oxidation reaction of methane sulfonic acid by the hydroxyl radical.

New Insight into the Gas-Phase Bimolecular Self-Reaction of the HOO Radical

The singlet and triplet potential energy surfaces (PESs) for the gas-phase bimolecular self-reaction of HOO*, a key reaction in atmospheric environments, have been investigated by means of quantum-mechanical electronic structure methods (CASSCF and CASPT2). All the reaction pathways on both PESs consist of a first step involving the barrierless formation of a prereactive doubly hydrogen-bonded complex, which is a diradical species lying about 8 kcal/mol below the energy of the reactants at 0 K. The lowest energy reaction pathway on both PESs is the degenerate double hydrogen exchange between the HOO* moieties of the prereactive complex via a double proton transfer mechanism involving an energy barrier of only 1.1 kcal/mol for the singlet and 3.3 kcal/mol for the triplet at 0 K. The single H-atom transfer between the two HOO* moieties of the prereactive complex (yielding HOOH + O2) through a pathway keeping a planar arrangement of the six atoms involves a conical intersection between either two singlet or two triplet states of A' and A" symmetries. Thus, the lowest energy reaction pathway occurs via a nonplanar cisoid transition structure with an energy barrier of 5.8 kcal/mol for the triplet and 17.5 kcal/mol for the singlet at 0 K. The simple addition between the terminal oxygen atoms of the two HOO* moieties of the prereactive complex, leading to the straight chain H2O4 intermediate on the singlet PES, involves an energy barrier of 7.3 kcal/mol at 0 K. Because the decomposition of such an intermediate into HOOH + O2 entails an energy barrier of 45.2 kcal/mol at 0 K, it is concluded that the single H-atom transfer on the triplet PES is the dominant pathway leading to HOOH + O2. Finally, the strong negative temperature dependence of the rate constant observed for this reaction is attributed to the reversible formation of the prereactive complex in the entrance channel rather than to a short-lived tetraoxide intermediate.