Watching A Hydroperoxyalkyl Radical (Center Dot Qooh) Dissociate

A prototypical hydroperoxyalkyl radical (center dot QOOH) intermediate, transiently formed in the oxidation of volatile organic compounds, was directly observed through its infrared fingerprint and energy-dependent unimolecular decay to hydroxyl radical and cyclic ether products. Direct time-domain measurements of center dot QOOH unimolecular dissociation rates over a wide range of energies were found to be in accord with those predicted theoretically using state- of-the-art electronic structure characterizations of the transition state barrier region. Unimolecular decay was enhanced by substantial heavy-atom tunneling involving O-O elongation and C-C-O angle contraction along the reaction pathway. Master equation modeling yielded a fully a priori prediction of the pressure-dependent thermal unimolecular dissociation rates for the center dot QOOH intermediate-again increased by heavy- atom tunneling-which are required for global models of atmospheric and combustion chemistry.