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.

Catalytic effect of (H2O) n (n = 1-3) on the HO2 + NH2 → NH3 + 3O2 reaction under tropospheric conditions.

The effects of (H2O) n (n = 1-3) clusters on the HO2 + NH2 → NH3 + 3O2 reaction have been investigated by employing high-level quantum chemical calculations with M06-2X and CCSD(T) theoretical methods, and canonical variational transition (CVT) state theory with small curvature tunneling (SCT) correction. The calculated results show that two kinds of reaction, HO2⋯(H2O) n (n = 1-3) + NH2 and H2N⋯(H2O) n (n = 1-3) + HO2, are involved in the (H2O) n (n = 1-3) catalyzed HO2 + NH2 → NH3 + 3O2 reaction. Due to the fact that HO2⋯(H2O) n (n = 1-3) complexes have much larger stabilization energies and much higher concentrations than the corresponding complexes of H2N⋯(H2O) n (n = 1-3), the atmospheric relevance of the former reaction is more obvious with its effective rate constant of about 1-11 orders of magnitude faster than the corresponding latter reaction at 298 K. Meanwhile, due to the effective rate constant of the H2O⋯HO2 + NH2 reaction being respectively larger by 5-6 and 6-7 orders of magnitude than the corresponding reactions of HO2⋯(H2O)2 + NH2 and HO2⋯(H2O)3 + NH2, the catalytic effect of (H2O) n (n = 1-3) is mainly taken from the contribution of the water monomer. In addition, the enhancement factor of the water monomer is 10.06-13.30% within the temperature range of 275-320 K, which shows that at whole calculated temperatures, a positive water effect is obvious under atmospheric conditions.