Atmospheric Chemistry of the Self-Reaction of HO2 Radical: Stepwise Mechanism Versus One-Step Process in the Presence of (H2O)n (n = 1-3) Clusters
The effects of water on the radical-radical reactions are of great importance of the elucidation of 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 the ring enlargement. The calculated results have revealed that the stepwise mechanism is the dominant one in HO2 + HO2 reaction which is catalyzed by one water molecule. This is because its pseudo-first-order rate constant (k'RWM1) is larger by 3 orders of magnitude than that of the corresponding one-step mechanism. Additionally, the value of k'RWM1 at 298 K has been found to be larger by 4.3 times than that of the rate constant of HO2 + HO2 reaction (kR1) without catalysts, which is in good agreement with the experimental findings. The calculated results also showed that the stepwise mechanism is still dominant in (H2O)2 catalyzed reaction due to its higher pseudo-first-order rate constant, which is larger by 3 orders of magnitude than that of the corresponding one-step mechanism. On the other hand, one-step process is much faster than that of 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 slower than that of 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.