Atmospheric chemistry of the self-reaction of HO2 radicals: stepwise mechanism versus one-step process in the presence of (H2O)n (n = 1–3) clusters

Abstract

The effects of water on radical–radical reactions are of great importance for the elucidation of the atmospheric oxidation process of free radicals. In the present work, the HO₂ + HO₂ reactions with (H₂O)_n (n = 1–3) have been investigated using quantum chemical methods and canonical variational transition state theory with small curvature tunneling. We have explored both one-step and stepwise mechanisms, in particular the stepwise mechanism initiated by ring enlargement. The calculated results have revealed that the stepwise mechanism is the dominant one in the HO₂ + HO₂ reaction that is catalyzed by one water molecule. This is because its pseudo-first-order rate constant (k_RWM1′) is 3 orders of magnitude larger than that of the corresponding one-step mechanism. Additionally, the value of k_RWM1′ at 298 K has been found to be 4.3 times larger than that of the rate constant of the HO₂ + HO₂ reaction (k_R1) without catalysts, which is in good agreement with the experimental findings. The calculated results also showed that the stepwise mechanism is still dominant in the (H₂O)₂ catalyzed reaction due to its higher pseudo-first-order rate constant, which is 3 orders of magnitude larger than that of the corresponding one-step mechanism. On the other hand, the one-step process is much faster than the stepwise mechanism by a factor of 10⁵–10⁶ in the (H₂O)₃ catalyzed reaction. However, the pseudo-first-order rate constants for the (H₂O)₂ and (H₂O)₃-catalyzed reactions are lower than that of the H₂O-catalyzed reaction by 3–4 orders of magnitude, which indicates that the water monomer is the most efficient one among all the catalysts of (H₂O)_n (n = 1–3). The present results have provided a definitive example that water and water clusters have important influences on atmospheric reactions.