Study discovers formation of Criegee intermediates from photochemical oxidation of alkenes

Organic peroxy radicals (RO₂) and Criegee intermediates (CI, carbonyl oxides) are key reactive species in atmospheric chemistry and play crucial roles in the formation of secondary organic aerosol (SOA).

Recently, a research group from the Institute of Earth Environment of the Chinese Academy of Sciences (IEECAS) investigated the formation mechanisms of Criegee intermediates from the OH-initiated oxidation of ethylene (C₂H₄), propylene (C₃H₆) and 2-methylpropene (2-CH₃-C₃H₅) in the presence of O₂ by using quantum chemical and kinetic modeling methods. The paper is published in the journal Atmospheric Environment.

They found that the dominant reaction pathway of HO-RO₂· + ·OH was the barrierless formation of ROOOH on the singlet photoelectron spectroscopy (PES), with its decomposition into RO and HO₂ radicals being the lowest-energy pathway. The stability of ROOOH increased with increasing methyl substituents. The trioxide intermediate did not form on the triplet PES. H-abstraction from the –CH_x group, which formed carbonyl oxides, was favorable for HO-RO₂ radicals.

For the reactions HO-RO₂· + HO-RO·, the dominant pathway was the barrierless formation of ROOOR on the singlet PES, with dissociation back to the separate reactants being the lowest-energy pathway. The number and position of methyl substituents had a minor impact on the stability of ROOOR.

The barrier and endothermicity for the formation of carbonyl oxides from the self-reaction of HO-RO₂ radicals decreased with increasing methyl substituents. The structures of HO-RO₂ radicals strongly influenced the barrier of carbonyl oxides’ formation.

This study enhances the understanding of the traditional pathways involved in the transformation mechanisms of peroxy radicals and Criegee intermediates. It has implications for improving the accuracy of numerical models and assessing the impact of anthropogenic emissions on SOA formation.