Is either direct photolysis or photocatalysed H-shift of peroxyl radicals a competitive pathway in the troposphere?

Abstract

Peroxyl radicals (RO O . ) are key intermediates in atmospheric chemistry, with relatively long lifetimes compared to most other radical species. In this study, we use multireference quantum chemical methods to investigate whether photolysis can compete with well-established RO O . sink reactions. We assume that the photolysis channel is always RO O . + h ν => RO + O( 3 P). Our results show that the maximal value of the cross-section for this channel is σ = 1.3 × 10 −18 cm 2 at 240 nm for five atmospherically representative peroxyl radicals: CH 3 O O . , C(O)HCH 2 O O . , CH 3 CH 2 O O . , HC(O)O O . and CH 3 C(O)O O . . These values agree with experiments to within a factor of 2. The rate constant of photolysis in the troposphere is around 10 −5 s −1 for all five RO O . . As the lifetime of peroxyl radicals in the troposphere is typically less than 100 s, photolysis is thus not a competitive process. Furthermore, we investigate whether or not electronic excitation to the first excited state (D 1 ) by infrared radiation can facilitate various H-shift reactions, leading, for example, in the case of CH 3 O O . to formation of O . H and CH 2 O or HO O . and CH 2 products. While the activation barriers for H-shifts in the D 1 state may be lower than in the ground state (D 0 ), we find that H-shifts are unlikely to be competitive with decay back to the D 0 state through internal conversion, as this has a rate of the order of 10 13 s −1 for all studied systems.