Effects of a Single Water Molecule on the OH + H₂O₂ Reaction

The effect of a single water molecule on the reaction between H₂O₂ and HO has been investigated by employing MP2 and CCSD­(T) theoretical approaches in connection with the aug-cc-PVDZ, aug-cc-PVTZ, and aug-cc-PVQZ basis sets and extrapolation to an ∞ basis set. The reaction without water has two elementary reaction paths that differ from each other in the orientation of the hydrogen atom of the hydroxyl radical moiety. Our computed rate constant, at 298 K, is 1.56 × 10^–12 cm³ molecule^–1 s^–1, in excellent agreement with the suggested value by the NASA/JPL evaluation. The influence of water vapor has been investigated by considering either that H₂O₂ first forms a complex with water that reacts with hydroxyl radical or that H₂O₂ reacts with a previously formed H₂O·OH complex. With the addition of water, the reaction mechanism becomes much more complex, yielding four different reaction paths. Two pathways do not undergo the oxidation reaction but an exchange reaction where there is an interchange between H₂O₂·H₂O and H₂O·OH complexes. The other two pathways oxidize H₂O₂, with a computed total rate constant of 4.09 × 10^–12 cm³ molecule^–1 s^–1 at 298 K, 2.6 times the value of the rate constant of the unassisted reaction. However, the true effect of water vapor requires taking into account the concentration of the prereactive bimolecular complex, namely, H₂O₂·H₂O. With this consideration, water can actually slow down the oxidation of H₂O₂ by OH between 1840 and 20.5 times in the 240–425 K temperature range. This is an example that demonstrates how water could be a catalyst in an atmospheric reaction in the laboratory but is slow under atmospheric conditions.