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

Organic peroxy radicals (RO 2 ) 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-RO 2 ) 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-RO 2 radicals, are studied by using quantum chemical methods. The results show that the unimolecular decomposition of HO-RO 2 radicals undergoes through the direct cleavage of C-C bond to produce carbonyl oxides, but they are strongly endothermic. For the reactions of HO-RO 2 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-RO 2 radicals. For the reactions of HO-RO 2 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-RO 2 radicals significantly decrease with increasing the number of methyl substituents. The self-reaction of dimethyl substituted HO-RO 2 radicals forming (CH 3 ) 2 COO is able to compete effectively with the bimolecular reactions with HO 2 radicals. These findings enhance our understanding of the formation of carbonyl oxides from the photochemical oxidation of alkenes . • The barrierless formation of ROOOH is preferable on the single PES. • The stability of ROOOH increases with increasing the number of methyl substituents. • The formation of carbonyl oxides is preferable on the triplet PES. • The barriers of the self-reactions of HO-RO 2 are sensitive to the number of methyl.