Catalytic effect of a single water molecule on the atmospheric reaction of HO2 + OH: fact or fiction? A mechanistic and kinetic study

Abstract

The catalytic effect of a single water molecule was investigated by studying the reaction of HO₂ with OH at the CCSD(T)/aug-cc-pVTZ//CCSD/6-311G(d,p) level of theory. Three hydrogen abstraction channels were found for the reaction of HO₂ with OH in the absence of a water molecule. The reaction channel for ³O₂ formation was dominant. The computed rate constant for the reaction was 2.2 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ at 298 K and was in good agreement with previously reported values. In the presence of a water molecule, the two reactions of the complex HO⋯H₂O with HO₂ and the complex HO₂⋯H₂O with OH were investigated. The calculated results show that the reaction mechanisms become much more complex, but the dominant product does not change. Although a water molecule plays a role in the positive catalytic effect of decreasing the energy of the transition state barrier within the temperature range of 216.7–298.2 K, the effective rate constants of HO₂⋯H₂O + OH and HO₂ + HO⋯H₂O reactions are respectively slower by 5–9 and 2–3 orders of magnitude than that of the unassisted reaction in the absence of a water molecule. Thus, we can predict that the title reaction’s rate enhancement by a single water molecule does not occur under tropospheric conditions.