Can a single water molecule really affect the HO2 + NO2 hydrogen abstraction reaction under tropospheric conditions?

The effect of a single water molecule on the HO2 + NO2 hydrogen abstraction reaction has been investigated by employing B3LYP and CCSD(T) theoretical approaches with the aug-cc-pVTZ basis set. The reaction without water has three types of reaction channels on both singlet and triplet potential energy surfaces, depending on how the HO2 radical approaches NO2. These correspond to the formation of trans-HONO + O2, cis-HONO + O2 and HNO2 + O2. Our calculated results show that triplet reaction channels are favorable and their total rate constant, at 298 K, is 2.01 × 10(-15) cm(3) molecule(-1) s(-1), which is in good agreement with experimental values. A single water molecule affects each one of these triplet reaction channels in the three different reactions of H2O···HO2 + NO2, HO2···H2O + NO2 and NO2···H2O + HO2, depending on the way the water interacts. Interestingly, the water molecule in these reactions not only acts as a catalyst giving the same products as the naked reaction, but also as a reactant giving the product of HONO2 + H2O2. The total rate constant of the H2O···HO2 + NO2 reaction is estimated to be slower than the naked reaction by 6 orders of magnitude at 298 K. However, the total rate constants of the HO2···H2O + NO2 and NO2···H2O + HO2 reactions are faster than the naked reaction by 4 and 3 orders of magnitude at 298 K, respectively. Their total effective rate constant is predicted to be 1.2 times that of the corresponding total rate constant without water at 298 K, which is in agreement with the prediction reported by Li et al. (science, 2014, 344, 292-296).