The catalytic effect of water, water dimers and water trimers on H2S + 3O2 formation by the HO2 + HS reaction under tropospheric conditions

Abstract

In this article, the reaction mechanisms of H₂S + ³O₂ formation by the HO₂ + HS reaction without and with catalyst X (X = H₂O, (H₂O)₂ and (H₂O)₃) have been investigated theoretically at the CCSD(T)/6-311++G(3df,2pd)//B3LYP/6-311+G(2df,2p) level of theory, coupled with rate constant calculations by using conventional transition state theory. Our results show that in the presence of catalyst X (X = H₂O, (H₂O)₂ and (H₂O)₃) into the channel of H₂S + ³O₂ formation, the reactions between the SH radical and HO₂⋯(H₂O)_n (n = 1–3) complexes are more favorable than the corresponding reactions of the HO₂ radical with HS⋯(H₂O)_n (n = 1–3) complexes due to the lower barrier of the former reactions and the higher concentrations of HO₂⋯(H₂O)_n (n = 1–3) complexes. Meanwhile, the catalytic effect of water, water dimers and water trimers is mainly taken from the contribution of a single water vapor molecule, since the total effective rate constant of HO₂⋯H₂O + HS and H₂O⋯HO₂ + HS reactions was, respectively, larger by 7–9 and 9–12 orders of magnitude than that of SH + HO₂⋯(H₂O)₂ and SH + HO₂⋯(H₂O)₃ reactions. Besides, the enhancement factor of water vapor is only 0.37% at 240 K, while at high temperatures, such as 425 K, the positive water vapor effect is enhanced up to 38.00%, indicating that at high temperatures the positive water effect is obvious under atmospheric conditions. Overall, these results show how water and water clusters catalyze the gas phase reactions under atmospheric conditions.