Role of (H2O)n (n = 2–3) Clusters on the HO2 + O3 Reaction: A Theoretical Study

The influence of a single water molecule on the reaction of BrO + HO2

The influence of a single water molecule on the BrO + HO2 hydrogen extraction reaction has been explored by taking advantage of CCSD(T)/aug-cc-pVTZ//B3LYP/6-311 +  + G(d,p) method. The reaction in the absence of water have two distinct kinds of H-extraction channels to generate HOBr + O2 (1Δg) and HBr + O3, and the channel of generation of HOBr + O2 (1Δg) dominated the BrO + HO2 reaction. The rate coefficient of the most feasible channel for the BrO + HO2 reaction in the absence of water is estimated to be 1.44 × 10-11 cm3 molecule-1 s-1 at 298.15 K, which is consistent with the experiment. The introduction of water made the reaction more complex, but the products are unchanged. Four distinct channels, beginning with HO2…H2O with BrO, H2O…HO2 with BrO, BrO…H2O with HO2, H2O…BrO with HO2 are researched. The most feasible channels, stemming from H2O…HO2 with BrO, and BrO…H2O with HO2, are much slower than the reaction of BrO + HO2 without water, respectively. Thus, the existence of water molecule takes a negative catalytic role for BrO + HO2 reaction.

Effect of (H2O)n (n = 1 and 2) on HOCl + Cl reaction

In the present work, we investigate the effect of water molecules (H₂O and (H₂O)₂) on HOCl + Cl˙ → ClO˙ + HCl (R1), and HOCl + Cl˙ → OH˙ + Cl₂ (R2) reactions using quantum chemical and kinetics calculations. The present investigation suggests that a water molecule decreases the energy barrier of both reactions significantly, compared to uncatalyzed reaction. However, the effective rate constants for the water catalyzed path for both channels (R1 and R2) were found to be lower than the bimolecular rate constant of the uncatalyzed path. Further, it was found that the R2 reaction will dominate over the R1 reaction, with or without catalyst. Interestingly, the uncatalyzed title reaction was found to be two times faster than the HOCl + OH˙ reaction, but in the presence of water, HOCl + OH˙ becomes the dominant reaction compared to the HOCl + Cl˙ reaction in the atmosphere. In addition, the concentration of bimolecular complexes formed in the presence of a catalyst are found to be higher than the precursor molecule of the uncatalyzed reaction, which suggests that in the presence of catalyst, the HOCl + Cl˙ reaction would favor the catalyzed path rather than the uncatalyzed path.

Computational Study on the Reaction of β-Hydroxyethylperoxy Radical with HO2 and Effects of Water Vapor

The reaction of β-hydroxyethylperoxy radical (β-HEP) and HO₂ with and without water was studied using quantum chemistry and kinetic calculations. The main products are HOCH₂CH₂OOH and ³O₂ for the reaction with and without water, while all other reaction channels can be neglected. The rate coefficients of the reaction follow negative temperature dependence. The pseudo-second-order rate coefficients are 2-4 orders of magnitude smaller for the reaction with saturated water vapor, indicating the negligible contribution of water in this reaction. This is probably also true for other peroxy radicals (except for HO₂), indicating that a large part of previous results on the water enhancement of reaction rate coefficients might have overestimated the influence of water.

Formation of Formamide from HCN + H2O: A Computational Study on the Roles of a Second H2O as a Catalyst, as a Spectator, and as a Reactant

Formamide (NH₂CHO), being the smallest and fundamental building block of life (with a peptide linkage), has recently been able to attract much interests, in the field of astrochemistry, astrophysics, and astrobiology. In this work, using quantum mechanical computations, reactions between HCN and H₂O, leading to the formation of formamide, have been analyzed. For the first time, an alternative and competing reaction channel, which proceeds via a geminal diol intermediate, for the formation of formamide, has been proposed. In this alternative channel, an extra water molecule (second H₂O) was found to be acting as a reactant, in the second step of the reaction path. Effects of second H₂O molecule in the reaction paths, providing catalytic assistance to the reaction or behaving like a spectator (concept is introduced for the first time for this reaction), have also been analyzed. Usefulness of spectator behavior is highlighted for the reactions happening on the rigid water-ice surfaces, where the water-ice may not be getting involved for any catalytic assistance. In light of catalytic assistances provided by the second H₂O, prominent effects in reducing the barrier heights drastically (even for the second step of the reaction, the barrier height was found to be below the reactants), through a hydrogen relay transport mechanism, were observed. In addition to the mechanism studies, interstellar feasibilities of all the reaction channels and their significances are discussed in detail.

Atmospheric chemistry of the self-reaction of HO2 radicals: stepwise mechanism versus one-step process in the presence of (H2O)n (n = 1-3) clusters

The effects of water on radical-radical reactions are of great importance for the elucidation of the atmospheric oxidation process of free radicals. In the present work, the HO2 + HO2 reactions with (H2O)n (n = 1-3) have been investigated using quantum chemical methods and canonical variational transition state theory with small curvature tunneling. We have explored both one-step and stepwise mechanisms, in particular the stepwise mechanism initiated by ring enlargement. The calculated results have revealed that the stepwise mechanism is the dominant one in the HO2 + HO2 reaction that is catalyzed by one water molecule. This is because its pseudo-first-order rate constant (kRWM1') is 3 orders of magnitude larger than that of the corresponding one-step mechanism. Additionally, the value of kRWM1' at 298 K has been found to be 4.3 times larger than that of the rate constant of the HO2 + HO2 reaction (kR1) without catalysts, which is in good agreement with the experimental findings. The calculated results also showed that the stepwise mechanism is still dominant in the (H2O)2 catalyzed reaction due to its higher pseudo-first-order rate constant, which is 3 orders of magnitude larger than that of the corresponding one-step mechanism. On the other hand, the one-step process is much faster than the stepwise mechanism by a factor of 105-106 in the (H2O)3 catalyzed reaction. However, the pseudo-first-order rate constants for the (H2O)2 and (H2O)3-catalyzed reactions are lower than that of the H2O-catalyzed reaction by 3-4 orders of magnitude, which indicates that the water monomer is the most efficient one among all the catalysts of (H2O)n (n = 1-3). The present results have provided a definitive example that water and water clusters have important influences on atmospheric reactions.

Catalytic effect of (H2O)n (n = 1–3) clusters on the HO2 + SO2 → HOSO + 3O2 reaction under tropospheric conditions

The HO₂ + SO₂ → HOSO + ³O₂ reaction without and with (H₂O)n (n = 1–3) have been investigated using CCSD(T)/CBS//M06-2X/aug-cc-pVTZ methods, and canonical variational transition state theory with small curvature tunneling.

The Role of (H₂O)1-2 in the CH₂O + ClO Gas-Phase Reaction

Mechanism and kinetic studies have been carried out to investigate whether one and two water molecules could play a possible catalytic role on the CH₂O + ClO reaction. Density functional theory combined with the coupled cluster theory were employed to explore the potential energy surface and the thermodynamics of this radical-molecule reaction. The reaction proceeded through four different paths without water and eleven paths with water, producing H + HCO(O)Cl, Cl + HC(O)OH, HCOO + HCl, and HCO + HOCl. Results indicate that the formation of HCO + HOCl is predominant both in the water-free and water-involved cases. In the absence of water, all the reaction paths proceed through the formation of a transition state, while for some reactions in the presence of water, the products were directly formed via barrierless hydrogen transfer. The rate constant for the formation of HCO + HOCl without water is 2.6 × 10-16 cm³ molecule-1 s-1 at 298.15 K. This rate constant is decreased by 9-12 orders of magnitude in the presence of water. The current calculations hence demonstrate that the CH₂O + ClO reaction is impeded by water.

Assessing Intermolecular Interactions in Guest-Free Clathrate Hydrate Systems

Recently, empty hydrate structures sI, sII, sH, and others have been proposed as low-density ice structures by both experimental observations and computer simulations. Some of them have been synthesized in the laboratory, which motivates further investigations on the stability of such guest-free clathrate structures. Using semiempirical and ab initio-based water models, as well as dispersion-corrected density functional theory approaches, we predict their stability, including cooperative many-body effects, in comparison with reference data from converged wave function-based DF-MP2 electronic structure calculations. We show that large basis sets and counterpoise corrections are required to improve convergence in the interaction/binding energies for such systems. Therefore, extrapolation schemes based on triple/quadruple and quadruple/quintuple ζ quality basis sets are used to reach high accuracy. Eleven different water structures corresponding to dodecahedron, edge sharing, face sharing, and fused cubes, as a part of the WATER27 database, as well as cavities from the sI, sII, and sH clathrate hydrates formed by 20, 24, 28, and 36 water molecules, are employed, and new benchmark energies are reported. Using these benchmark sets of interaction energies, we assess the performance of both analytical models and direct DFT calculations for such clathrate-like systems. In particular, seven popular water models (TIP4P/ice, TIP4P/2005, q-TIP4P/F, TTM2-F, TTM3-F, TTM4-F, and MB-pol) available in the literature, and nine density functional approximations (3 meta-GGAs, 3 hybrids, and 3 range separated functionals) are used to investigate their accuracy. By including dispersion corrections, our results show that errors in the interaction energies are reduced for most of the DFT functionals. Despite the difficulties faced by current water models and DFT functionals to accurately describe the interactions in such water systems, we found some general trends that could serve to extend their applicability to larger systems.

Role of the (H2O)n (n = 1–3) cluster in the HO2 + HO → 3O2 + H2O reaction: mechanistic and kinetic studies

Upon incorporation of the catalyst (H₂O)_n (n = 1–3) into the reaction HO₂ + HO → H₂O + ³O₂, the catalytic effects of water, water dimer, and water trimer mainly arise from the contribution of a single molecule of water vapor.

Mechanistic investigation of the atmospheric reaction of NH2 with NO2 and study of the catalytic effects of water molecule on kinetic path

The gas-phase reaction mechanism of NO2 with NH2 on the singlet potential energy surface is investigated at the B3LYP, MP2, CCSD(T) and G3B3 levels of calculation in connection with the 6-311[Formula: see text]G(3df, 3pd) basis sets. Seven different kinds of products have been obtained. The results reveal that P7 [N2[Formula: see text]2O] is the main product in thermodynamic viewpoint with the Gibbs free energy of [Formula: see text][Formula: see text]kcal/mol. The product P2 due to less number of transition states and low level barrier height of transition states is introduced as the kinetic product. The calculation of rate constants in different temperatures confirms our predication for P7 adduct in comparison with other products. In the first step, the reactions without catalytic effects are considered. All of these reactions begin with pre-reactive collision complex CR [NO2–NH2]. This complex has been produced from barrier-less process via collision of reactants. From variation of bond lengths and angles or dissociation of this complex, other species are produced. In the second step, the catalytic effect of water and water dimer molecules on the reaction rate of kinetic path, P2, have been attended. Calculation of the rate constant was done in the temperature range from 500[Formula: see text]K to 3000[Formula: see text]K. Our results showed that the rate constant of path P2 increases with temperature and the number of water molecules. So, we can conclude that the number of water molecules corresponding to relative humidity of atmosphere has catalytic effect on the mentioned reaction rate.

The catalytic effect of water, water dimers and water trimers on H2S +3O2formation by the HO2+ HS reaction under tropospheric conditions

Catalyst X (X = H₂O, (H₂O)₂and (H₂O)₃) is incorporated into the channel of H₂S +³O₂formation and the catalytic effect of water, water dimers and water trimers is mainly taken from the contribution of a single water vapor molecule.