Catalytic effect of (H(2)O) (n) (n = 1-3) on the HO(2) + NH(2) → NH(3) + (3)O(2) reaction under tropospheric conditions.

The effects of (H(2)O) (n) (n = 1-3) clusters on the HO(2) + NH(2) → NH(3) + (3)O(2) 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, HO(2)⋯(H(2)O) (n) (n = 1-3) + NH(2) and H(2)N⋯(H(2)O) (n) (n = 1-3) + HO(2), are involved in the (H(2)O) (n) (n = 1-3) catalyzed HO(2) + NH(2) → NH(3) + (3)O(2) reaction. Due to the fact that HO(2)⋯(H(2)O) (n) (n = 1-3) complexes have much larger stabilization energies and much higher concentrations than the corresponding complexes of H(2)N⋯(H(2)O) (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 H(2)O⋯HO(2) + NH(2) reaction being respectively larger by 5-6 and 6-7 orders of magnitude than the corresponding reactions of HO(2)⋯(H(2)O)(2) + NH(2) and HO(2)⋯(H(2)O)(3) + NH(2), the catalytic effect of (H(2)O) (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.