Reaction pathways and excited states in H2 O2 +OH→H O2 + H2 O: A new ab initio investigation

The mechanism of the hydrogen abstraction reaction H2 O2 +OH→H O2 + H2 O in gas phase was revisited using density functional theory and other highly correlated wave function theories. We located two pathways for the reaction, both going through the same intermediate complex OH- H2 O2, but via two distinct transition state structures that differ by the orientation of the hydroxyl hydrogen relative to the incipient hydroperoxy hydrogen. The first two excited states were calculated for selected points on the pathways. An avoided crossing between the two excited states was found on the product side of the barrier to H transfer on the ground state surface, near the transition states. We report on the calculation of the rate of the reaction in the gas phase for temperatures in the range of 250-500 K. The findings suggest that the strong temperature dependence of the rate at high temperatures is due to reaction on the low-lying excited state surface over a barrier that is much larger than on the ground state surface.