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

Quantum chemical computations and transition state theory are employed to systematically research the influence of a single molecule water on the BrO + HONO reaction. Two distinct reactions, namely BrO + trans-HONO and BrO + cis-HONO are explored for the reaction in the absence of water, which is mainly decided by the configuration of HONO. With introduction a single water molecule to the reaction, the rate coefficient of the channel starting from BrO + cis-HONO and BrO + trans-HONO are 2.43 × 10^-19 and 5.22 × 10^-22 cm³ molecule^-1 s^-1, which is larger than the reaction in the absence of water. For further comprehend the impact of water on the BrO + HONO reaction, it is necessary to compute the effective rate coefficient by taking into account the concentration of water. The water-assisted effective rate coefficients for the BrO + HONO reaction are smaller than that the reaction in the absence of water. The reaction of BrO with cis-HONO is feasible both in absence and existence of water.

HO2 + NO2: Kinetics, Thermochemistry, and Evidence for a Bimolecular Product Channel

A master equation (ME) analysis of available experimental data has been carried out on the reaction HO₂ + NO₂ + M ⇋ HO₂NO₂ + M (1a)/(-1a). The analysis, based on the ME code MESMER, uses both the association and dissociation kinetic data from the literature, and provides improved thermochemistry on reaction 1a. Our preferred model assigns two low-frequency vibrations of HO₂NO₂ as hindered rotors and couples these to the external rotations. This model gives Δ_rH°₀(1a) = -93.9 ± 1.0 kJ mol^-1, which implies that Δ_fH°₀ HO₂NO₂ = -42.0 ± 1.0 kJ mol^-1 (uncertainties are 2σ). A significant contributor to the uncertainty derives from modeling the interaction between the internal and external rotors. Using this improved kinetics for reaction 1a/-1a, data at elevated temperatures, 353-423 K, which show no evidence of the expected equilibration, have been reanalyzed, indicating that an additional reaction is occurring that masks the equilibration. Based on a published ab initio study, this additional channel is assigned to the bimolecular reaction HO₂ + NO₂ → H-NO₂ + O₂ (1b); H-NO₂ is nitryl hydride and has not previously been directly observed in experiments. The output of the master equation analysis has been parametrized and Troe expressions are provided for an improved description of k_1a(p,T) and k_-1a(p,T).

DFT analysis on the removal of dimethylbenzoquinones in atmosphere and water environments: ·OH-initiated oxidation and captured by (TiO2)n clusters (n=1-6)

The elimination mechanisms and the dynamics of 2,5-dimethylbenzoquinone/2,6-dimethylbenzoquinone are performed by DFT under the presence of ·OH radical and TiO₂-clusters. The rate coefficients, calculated within the atmospheric and combustion temperature range of 200-2000 K, agree well with the experimental data. The subsequent reactions including the bond cleavage of quinone ring, O₂ addition or abstraction, the reactions of peroxy radical with NO yielding the precursor of organic aerosol are studied. Gaseous water molecule plays an important role in the transformation of alkoxy radical and exhibits a catalytic performance in the enol-ketone tautomerism. The lifetimes of 2,5-dimethylbenzoquinone/2,6-dimethylbenzoquinone are about 12.04-12.86 h at 298 K, which are in favor of the medium range transport of them in the atmosphere. Significantly, the water environment plays a negative role on the ·OH-degradation of dimethylbenzoquinone. Compared to the quinone ring, 2,5-dimethylbenzoquinone onto (TiO₂)_n clusters (n = 1-6) is easier to be absorbed by TiO₂-clusters through its oxygen site because of its strong chemisorption, which indicates that TiO₂-clusters are capable of trapping dimethylbenzoquinones effectively. The water environment could weaken the adsorption of 2,5-dimethylbenzoquinone onto (TiO₂)_n clusters (n = 1-6) by increasing the adsorption energy. This work reveals the removal of dimethylbenzoquinones and the formation of organic aerosol under polluted environments.

Effect of NH3 and HCOOH on the H2O2 + HO → HO2 + H2O reaction in the troposphere: competition between the one-step and stepwise mechanisms

The H₂O₂ + HO → HO₂ + H₂O reaction is an important reservoir for both radicals of HO and HO₂ catalyzing the destruction of O₃. Here, this reaction assisted by NH₃ and HCOOH catalysts was explored using the CCSD(T)-F12a/cc-pVDZ-F12//M06-2X/aug-cc-pVTZ method and canonical variational transition state theory with small curvature tunneling. Two possible sets of mechanisms, (i) one-step routes and (ii) stepwise processes, are possible. Our results show that in the presence of both NH₃ and HCOOH catalysts under relevant atmospheric temperature, mechanism (i) is favored both energetically and kinetically than the corresponding mechanism (ii). At 298 K, the relative rate for mechanism (i) in the presence of NH₃ (10, 2900 ppbv) and HCOOH (10 ppbv) is respectively 3–5 and 2–4 orders of magnitude lower than that of the water-catalyzed reaction. This is due to a comparatively lower concentration of NH₃ and HCOOH than H₂O which indicates the positive water effect under atmospheric conditions. Although NH₃ and HCOOH catalysts play a negligible role in the reservoir for both radicals of HO and HO₂ catalyzing the destruction of O₃, the current study provides a comprehensive example of how acidic and basic catalysts assisted the gas-phase reactions.

The H₂O₂ + HO → HO₂ + H₂O reaction is an important reservoir for both radicals of HO and HO₂ catalyzing the destruction of O₃.

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.

A single water molecule accelerating the atmospheric reaction of HONO with ClO

The role of a single water molecule on the atmospheric reaction of HONO + ClO is systematically investigated employing quantum chemical calculation combined with harmonic transition state theory. Two reaction pathways, cis-HONO + ClO and trans-HONO + ClO, are identified for the naked reaction, which depends on the configurations of HONO. When adding a single water molecule to this reaction, the rate constants of cis-HONO + ClO and trans-HONO + ClO pathways are 7.97 × 10^-21 and 2.29 × 10^-17 cm³ molecule^-1 s^-1, respectively, larger than the corresponding naked reaction. To further understand the role of water on the HONO + ClO reaction, it is necessary to calculate the effective rate constant by considering the concentration of water. It shows that the effective rate constants of water-assisted cis-HONO + ClO pathway are much smaller than those of the naked reaction, whereas the presence of water accelerates the trans-HONO + ClO at room temperature. This study demonstrates that water has a positive role in the pathway of trans-HONO + ClO by modifying the stabilities of reactant complexes and transition states through the hydrogen bond formation, which contributes to the sink of atmospheric HONO. In addition, the kinetic branching ratio indicates that the favorable reaction is the trans-HONO + ClO instead of the cis-HONO + ClO pathway, in contrast to the naked reaction. These results reveal the importance of water in the evaluation of the fate of active species in the atmosphere. Graphical Abstract.

Effects of water, ammonia and formic acid on HO2 + Cl reactions under atmospheric conditions: competition between a stepwise route and one elementary step

Quantum chemical calculations at M06-2X and CCSD(T) levels of theory have been performed to investigate the effects of H2O, NH3, and HCOOH on the HO2 + Cl → HCl + O2 reaction. The results show that catalyzed reactions with three catalysts could proceed through two different mechanisms, namely a stepwise route and one elementary step, where the former reaction is more favorable than the latter. Meanwhile, for the stepwise route, a single hydrogen atom transfer pathway in the presence of all catalysts has more advantages than the respective double hydrogen atom transfer pathway. Then, the relative impacts of catalysts under tropospheric conditions were investigated by considering the temperature dependence of the rate constants and the altitude dependence of catalyst concentrations. The calculated results show that at 0 km altitude, the HO2 + Cl → HCl + O2 reaction with catalysts, such as H2O, NH3, or HCOOH, cannot compete with the reaction without a catalyst, as the effective rate constant with a catalyst is smaller by 2-6 orders of magnitude than the naked reaction within the temperature range 280-320 K. The calculated results also show that at altitudes of 5, 10 and 15 km, the effective rate constant of the HCOOH-catalyzed reaction increases obviously with an increase in altitude. At 15 km altitude, its value is up to 9.63 × 10-11 cm3 per molecule per s, which is close to the corresponding value of the reaction without a catalyst, showing that the contribution of HCOOH to the HO2 + Cl → HCl + O2 reaction cannot be neglected at high altitudes. The new findings in this investigation are not only of great necessity and importance for elucidating the gas-phase reaction of HO2 with Cl in the presence of acidic, neutral and basic catalysts, but are also of great interest for understanding the importance of other types of hydrogen abstraction in the atmosphere.

Elucidating the mechanism and kinetics of the water-assisted reaction of nitrous acid with hydroxyl radical

The rate constant of the HONO + OH reaction is slightly increased by hydration.

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.

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.

Role of water on the H-abstraction from methanol by ClO

The influence of a single water molecule on the reaction mechanism and kinetics of hydrogen abstraction from methanol (CH₃OH) by the ClO radical has been investigated using ab initio calculations. The reaction proceeds through two channels: abstraction of the hydroxyl H-atom and methyl H-atom of CH₃OH by ClO, leading to the formation of CH₃O+HOCl (+H₂O) and CH₂OH+HOCl (+H₂O), respectively. In both cases, pre- and post-reactive complexes were located at the entrance and exit channel on the potential energy surfaces. Results indicate that the formation of CH₂OH+HOCl (+H₂O) is predominant over the formation of CH₃O+HOCl (+H₂O), with ambient rate constants of 3.07×10^-19 and 3.01×10^-23cm³/(molecule·sec), respectively, for the reaction without water. Over the temperature range 216.7-298.2K, the presence of water is seen to effectively lower the rate constants for the most favorable pathways by 4-6 orders of magnitude in both cases. It is therefore concluded that water plays an inhibitive role on the CH₃OH+ClO reaction under tropospheric conditions.

Effect of a single water molecule on the HO2 + ClO reaction

The catalytic effect of a single water molecule on the HO2 + ClO reaction has been investigated at the CCSD(T)/aug-cc-pVTZ//B3LYP-D3/aug-cc-pVDZ level of theory. Four H-abstraction paths and two kinds of products, among which the paths for HOCl + O2 formation are dominant, have been found for the HO2 + ClO reaction without water. The rate constant of the most favorable path for the reaction without water is computed to be 4.53 × 10-12 cm3 molecule-1 s-1 at room temperature, in good agreement with the experiment. In the presence of a water molecule, although the reaction becomes more complex, the dominant products do not change. Four main channels, starting from HO2H2O + ClO, H2OHO2 + ClO, ClOH2O + HO2 and H2OClO + HO2, are investigated. The most favorable paths, reactions between H2OHO2 and ClO, and between ClOH2O and HO2, are 7-10 and 6-9 orders of magnitude slower than the reaction in the absence of water, respectively. It is concluded that the presence of a single water molecule does not play an important role in enhancing the HO2 + ClO reaction under tropospheric conditions.

Catalysis and tunnelling in the unimolecular decay of Criegee intermediates

Semi-classical Transition State theory can be applied to catalysed atmospheric reactions, but reaction mode anharmonicity must be treated carefully.

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.

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.