Watching a Hydroperoxyalkyl Radical (•QOOH) Dissociate

A prototypical hydroperoxyalkyl radical (•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 •QOOH unimolecular dissociation rates over a wide range of energies were found to be in excellent accord with those predicted theoretically utilizing state-of-the-art electronic structure characterizations of the transition state barrier region. Unimolecular decay was enhanced by significant heavy-atom tunneling involving O-O elongation and C-C-O angle contraction along the reaction pathway. Master equation modeling was used to obtain fully a priori prediction of the pressure-dependent thermal unimolecular dissociation rates for the •QOOH intermediate, again increased by heavy-atom tunneling, which are required for global models of atmospheric and combustion chemistry. Master equation rate constants were determined using the following files, which are a collation of all of the requisite, theoretical electronic structure and energy transfer data obtained for this research into files for the master equation kinetics code, MESS.