Unimolecular HO2 Loss from Peroxy Radicals Formed in Autoxidation Is Unlikely under Atmospheric Conditions

A concerted HO2 loss reaction from a peroxy radical (RO2), formed from the addition of O2 to an alkyl radical, has been proposed as a mechanism to form closed-shell products in the atmospheric oxidation of organic molecules. We investigate this reaction computationally with four progressively oxidized radicals. Potential energy surfaces of the O2 addition and HO2 loss reactions were calculated at ROHF-RCCSD­(T)-F12a/VDZ-F12//ωB97xD/aug-cc-pVTZ level of theory and the master equation solver for multienergy well reactions (MESMER) was used to calculate Bartis-Widom phenomenological rate coefficients. The rate coefficients were also compared with the unimolecular rate coefficients of the HO2 loss reaction calculated with transition state theory at atmospheric temperature and pressure. On the basis of our calculations, the unimolecular concerted HO2 loss is unlikely to be a major pathway in the formation of highly oxidized closed-shell molecules in the atmosphere.