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

Abstract

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 H₂O⋯HO₂ + 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_tot) 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.