Role of the (H2O)n (n = 1-3) cluster in the HO2 + HO →3O2 + H2O reaction: mechanistic and kinetic studies.

To study the catalytic effects of (H2O)n (n = 1-3), the mechanisms of the reaction HO2 + HO →3O2 + H2O without and with (H2O)n (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 (H2O)n (n = 1-3) into the channel of H2O + 3O2 formation, two different reactions, i.e. HO + HO2(H2O)n (n = 1-3) and HO2 + HO(H2O)n (n = 1-3), have been observed, and these two reactions are competitive with each other. The catalytic effects of (H2O)n (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 (H2O)2 and (H2O)3. Furthermore, the catalytic effects of the water monomer mainly arise from the H2OHO2 + 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)/ktot) 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.