Unraveling the Electronically Excited State Dynamics of Hydroperoxyalkyl (•QOOH) Radicals via Quantum Chemical and Surface Hopping Studies

Organic hydroperoxy (•QOOH) radicals are key reactive intermediates generated during the oxidation of volatile organic compounds in both combustion systems and the Earth’s atmosphere. Their atmospheric and combustion lifetimes are governed by competition between unimolecular decomposition and bimolecular reactions, with the relative importance of these pathways strongly dependent on molecular structure and temperature. Here, we combine non-adiabatic molecular dynamics simulations with high-level quantum chemical calculations to investigate the excited-state photochemistry of the prototypical •QOOH species, the hydroperoxy-tert-butyl radical, following excitation to electronically excited states accessible in the near-ultraviolet. Our results reveal that photodissociation is the dominant fate of electronically excited •QOOH radicals, primarily forming •OH + epoxide and HO2• + 2-methylpropene (2MP) products. Intramolecular hydrogen-atom transfer leading to organoperoxy radicals is predicted to be a minor photochemical pathway. These findings suggest that solar photolysis may represent a previously overlooked daytime sink for relatively long-lived •QOOH intermediates in the troposphere, with potential implications for atmospheric oxidation chemistry and secondary organic aerosol formation.