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.