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, 21, 20296

The reaction, BrO + HO2 → HOBr + O2, is exothermic and can produce O2 in both its ground state (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.