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

We report ab initio calculations of important regions of the potential energy surface for the reaction: O-2((3)Sigma(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 HOx 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.