Watching a hydroperoxyalkyl radical (•QOOH) dissociate

• Published in: Science (American Association for the Advancement of Science) 2021-08, Vol.373 (6555)
• Main Authors: Hansen, Anne S., Bhagde, Trisha, Moore, Kevin B., Moberg, Daniel R., Jasper, Ahren W., Georgievskii, Yuri, Vansco, Michael F., Klippenstein, Stephen J., Lester, Marsha I.
• Format: Article
• Language: English
• Subjects: INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
• ISSN: 0036-8075; 1095-9203

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 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 •QOOH intermediate—again increased by heavy-atom tunneling—which are required for global models of atmospheric and combustion chemistry.