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, 23, DOI: 10.1039/D0CP00222D

Abstract

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.