Role of the (H 2 O) n (n = 1-3) cluster in the HO 2 + HO → 3 O 2 + H 2 O reaction: mechanistic and kinetic studies

To study the catalytic effects of (H O) (n = 1-3), the mechanisms of the reaction HO + HO → O + H O without and with (H O) (n = 1-3) have been investigated theoretically at the CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ level of theory, coupled with rate constant calculations using the conventional transition state theory. Our results show that upon incorporation of (H O) (n = 1-3) into the channel of H O + O formation, two different reactions, i.e. HO + HO (H O) (n = 1-3) and HO + HO(H O) (n = 1-3), have been observed, and these two reactions are competitive with each other. The catalytic effects of (H O) (n = 1-3) mainly arise from the contribution of a single water vapor molecule; this is because the effective rate constants with water are respectively larger by 2-3 and 3-4 orders of magnitude than those of the reactions with (H O) and (H O) . Furthermore, the catalytic effects of the water monomer mainly arise from the H OHO + HO reaction, and the enhancement factor of this reaction is obvious within the temperature range of 240.0-425.0 K, with the branching ratio (k'(RW)/k ) of 17.27-80.77%. Overall, the present results provide a new example of how water and water clusters catalyze gas phase reactions under atmospheric conditions.