Clustering effects on the reactivity of alkoxy radicals: Rate coefficients of ³(RO...OR) complexes accounting for multiple conformers †

Abstract

Atmospheric peroxy (RO₂·) and alkoxy (RO·) radical species are crucial intermediates in the formation of secondary organic aerosol (SOA). Recent computational work has demonstrated that recombination reactions of peroxy radicals (RO₂· + RO₂·) proceed through triplet complexes consisting of two alkoxy radicals (³(RO...OR)). To understand how peroxy recombination reactions branch into different product channels, it is thus necessary to thoroughly investigate the reactions of these triplet alkoxy complexes. Although the reactions of free alkoxy radicals have been extensively studied, the reactivity of triplet alkoxy complexes remains relatively less explored. In this study, we have therefore developed a systematic conformer sampling workflow for ³(RO...OR), and applied it to four typical alkoxy systems (AceO, β-ISOPO, PhCH₂O, and PhC(O)O). Rate coefficients (k) of key reactions have been calculated using multi-conformer and lowest-conformer transition state theory (MC-TST and LC-TST), allowing the quantitative evaluation of conformer effects. Our results demonstrate that the presence of the other RO· in the complex has a noticeable effect on k values. For the β-ISOPO system in particular, the predicted β-scission k values in the ³(RO...OR) complex are over 100 times higher than for the free radical.