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

The effects of (H₂O) (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 = 1-3) + NH₂ and H₂N⋯(H₂O) (n = 1-3) + HO₂, are involved in the (H₂O) (n = 1-3) catalyzed HO₂ + NH₂ → NH₃ + ³O₂ reaction. Due to the fact that HO₂⋯(H₂O) (n = 1-3) complexes have much larger stabilization energies and much higher concentrations than the corresponding complexes of H₂N⋯(H₂O) (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 = 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.