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

Abstract

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.