Molecular insights into the formation of Criegee intermediates from β-hydroxyperoxy radicals

Organic peroxy radicals (RO₂) and Criegee intermediates (CI, carbonyl oxides) are key reactive species in atmospheric chemistry, and can proceed various reactions influencing the recycling of radicals and the formation of aerosol particles. Carbonyl oxides have recently detected in the OH-initiated autoxidation reactions of unsaturated hydrocarbons. However, their formation mechanisms remain elusive. Herein, the β-hydroxyperoxy radicals (HO-RO₂) are selected as the model compounds to study the mechanism of their transformation to carbonyl oxides. Potential formation pathways of carbonyl oxides, including unimolecular decomposition, bimolecular reactions with OH radicals, HO-RO and HO-RO₂ radicals, are studied by using quantum chemical methods. The results show that the unimolecular decomposition of HO-RO₂ radicals undergoes through the direct cleavage of C-C bond to produce carbonyl oxides, but they are strongly endothermic. For the reactions of HO-RO₂ with OH radicals, the preferable pathway is the barrierless formation of ROOOH on the single PES, and their stability increases with increasing the number of methyl substituents. On the triplet PES, the formation of carbonyl oxides from H-abstraction reaction in the –CH_x group is favorable for the unsubstituted and a methyl substituted HO-RO₂ radicals. For the reactions of HO-RO₂ with HO-RO radicals, the dominant pathway is the barrierless formation of ROOOR on the singlet PES, with dissociation back to the separate reactants being the lowest-energy pathway. The formation of carbonyl oxides is preferable on the triplet PES, and the methyl substitution is beneficial for decreasing the reaction barriers. The barrier for the formation of carbonyl oxides from the self-reaction of HO-RO₂ radicals significantly decrease with increasing the number of methyl substituents. The self-reaction of dimethyl substituted HO-RO₂ radicals forming (CH₃)₂COO is able to compete effectively with the bimolecular reactions with HO₂ radicals. These findings enhance our understanding of the formation of carbonyl oxides from the photochemical oxidation of alkenes.