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

Peroxyl radicals (RO [Formula: see text]) 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 [Formula: see text] sink reactions. We assume that the photolysis channel is always RO [Formula: see text] + hν => 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 [Formula: see text] , C(O)HCH(2)O [Formula: see text] , CH(3)CH(2)O [Formula: see text] , HC(O)O [Formula: see text] and CH(3)C(O)O [Formula: see text]. 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 [Formula: see text]. 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 [Formula: see text] to formation of [Formula: see text] H and CH(2)O or HO [Formula: see text] 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.