Effect of a single water molecule on the HO2 + ClO reaction

Abstract

The catalytic effect of a single water molecule on the HO₂ + ClO reaction has been investigated at the CCSD(T)/aug-cc-pVTZ//B3LYP-D3/aug-cc-pVDZ level of theory. Four H-abstraction paths and two kinds of products, among which the paths for HOCl + O₂ formation are dominant, have been found for the HO₂ + ClO reaction without water. The rate constant of the most favorable path for the reaction without water is computed to be 4.53 × 10⁻¹² cm³ molecule⁻¹ s⁻¹ at room temperature, in good agreement with the experiment. In the presence of a water molecule, although the reaction becomes more complex, the dominant products do not change. Four main channels, starting from HO₂⋯H₂O + ClO, H₂O⋯HO₂ + ClO, ClO⋯H₂O + HO₂ and H₂O⋯ClO + HO₂, are investigated. The most favorable paths, reactions between H₂O⋯HO₂ and ClO, and between ClO⋯H₂O and HO₂, are 7–10 and 6–9 orders of magnitude slower than the reaction in the absence of water, respectively. It is concluded that the presence of a single water molecule does not play an important role in enhancing the HO₂ + ClO reaction under tropospheric conditions.