Kinetics of the BrO + HO2 reaction over the temperature range T = 246-314 K

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.

Reply to the 'comment on "impact of water on the BrO + HO2 gas-phase reaction: mechanism, kinetics and products"' by R. Chow, D. K. W. Mok, E. P. F. Lee and J. M. Dyke, Phys. Chem. Chem. Phys., 2021, , DOI

We reply to the comment on our recent paper entitled "Impact of water on the BrO + HO2 gas-phase reaction: mechanism, kinetics and products" by Chow et al. In their comment, the authors raised the differences between our results and their results in an earlier paper (R. Chow, D. K. W. Mok, E. P. F. Lee and J. M. Dyke, Phys. Chem. Chem. Phys., 2016, 18, 30554-30569), in terms of kinetics and potential energy surface, and they attributed these differences to the use of a small integration grid size and closed-shell wavefunctions for geometry optimizations in our study. Indeed, in our original manuscript, we did not ensure the proper use of UHF wavefunctions for singlet states, which led the singlet states to be treated with restricted M06-2X wavefunctions during optimizations. Furthermore, the default integration grid was used. New geometry optimizations have been performed where reactant complexes on the singlet surface were treated in their open-shell singlet states (ensured by using unrestricted-spin wave-functions) and using very tight convergence criteria, and new reaction rate constants have been calculated based on new energy barriers. No barrierless hydrogen abstraction reactions were observed as reported in our previous results and, consequently, the outer rate coefficient in the two-transition state approach (given by eqn (5) in Tsona et al., 2019) was determined by the collision theory. Overall rate constants exhibit a negative temperature dependency in the 200-400 K range. Despite the changes on the reaction energies and kinetics due to wrong UHF wavefunctions, our main conclusion that water has no net effect on the BrO + HO2 → BrOH + O2 reaction is still valid.

Comment on "Impact of water on the BrO + HO2 gas-phase reaction: mechanism, kinetics and products" by N. T. Tsona, S. Tang and L. Du, Phys. Chem. Chem. Phys., 2019, , 20296

The reaction, BrO + HO2 → HOBr + O2, is exothermic and can produce O2 in both its ground state (X[combining tilde]3∑g-) and its first excited state (ã1Δg). As a result, this reaction can proceed on both a singlet and a triplet potential energy surface. Recently, Tsona, Tang and Du published a paper entitled "Impact of water on the BrO + HO2 gas-phase reaction: mechanism, kinetics and products (Phys. Chem. Chem. Phys. 2019, 21, 20296-203072). The results of this work showed significant differences from those published earlier on this reaction by Chow et al. (Phys. Chem. Chem. Phys. 2016, 18, 30554-30569). Further calculations performed in this present work, combined with higher level calculations published by Chow et al. (Phys. Chem. Chem. Phys. 2016, 18, 30554-30569), demonstrate that the work of Tsona et al. is flawed because the integration grid size used in their lowest singlet and triplet calculations is too small, and a closed-shell wavefunction, rather than an open-shell wavefunction, has been used for the singlet surface. The major conclusion in the work of Tsona et al. that the lowest singlet and triplet channels are barrierless is shown to be incorrect. Also, the computed rate coefficients of Tsona et al. showed a positive temperature dependence, which is inconsistent with the experimentally observed negative temperature dependence, whereas the singlet rate coefficients computed by Chow et al. (Phys. Chem. Chem. Phys. 2016, 18, 30554-30569) showed a negative temperature dependence consistent with experiment.

Impact of water on the BrO + HO2 gas-phase reaction: mechanism, kinetics and products

The BrO + HO2 reaction, which participates in the cycle of ozone removal via BrOH formation, was explored both in the absence and in the presence of water using ab initio calculations. Two main sets of products, (i) HBr + O3 and (ii) BrOH + O2, are formed regardless of the presence of water, following a hydrogen abstraction mechanism. The HBr + O3 products are formed from the intermediate BrOOOH adduct, whereas BrOH + O2 are formed either from the intermediate OBrOOH adduct or via a barrierless hydrogen transfer from HO2 to BrO. Owing to the formation of molecular oxygen that can bear different spin configurations, the formation of BrOH + O2 products was examined both on the singlet and the triplet surfaces. Under relevant atmospheric temperatures and pressure, the formation of products (i) is energetically and kinetically less favorable than that of products (ii). The rate coefficient at 298 K for the HBr + O3 formation was determined to be 2.00 × 10-20 cm3 molecule-1 s-1, and found to decrease by 1-2 orders of magnitude when one or both reactants are clustered with water. For the formation of BrOH + O2, a rate coefficient of 2.21 × 10-11 cm3 molecule-1 s-1 is determined on both singlet and triplet surfaces in the absence of water. Though this rate coefficient slightly decreases for the hydrated reactions, the fractions of the reactants that are effectively complexed with water are not high enough to shift the overall BrOH + O2 formation rate. The current study further indicates that humidity plays a negligible role in ozone removal via the BrO + HO2 reaction.