Bimolecular reaction of molecular oxygen with overtone excited HOOH: Implications for recycling HO2 in the atmosphere

Abstract

We report ab initio calculations of important regions of the potential energy surface for the reaction: O₂(³Σ⁻_g) + HOOH → 2 HO₂. The reaction is endothermic by 37.5 ± 3 kcal mol⁻¹, as calculated at the QCISD(T)/6-311++G(3df,3pd)//QCISD/6-311G(2df,2p) level of theory. The predicted mechanism is via a simple hydrogen abstraction, with a barrier of 35.3 kcal mol⁻¹, followed by formation of a six-membered complex, which breaks up into two HO₂ radicals. If an OH stretching type local vibrational mode of the hydrogen peroxide molecule is excited to the overtone level v_OH = 4, the reaction becomes energetically feasible. This overtone-driven reaction will occur in the atmosphere; we estimate the amount of “extra” HO_x production by this mechanism to be about 20–30% of the production by H₂O₂ ultraviolet absorption when the sun is low in the sky, and about 1% when the sun is overhead.