Computational Investigation of the Formation of Peroxide (ROOR) Accretion Products in the OH- and NO3-Initiated Oxidation of alpha-Pinene

The formation of accretion products ("dimers") from recombination reactions of peroxyl radicals (RO2) is a key 5 3 step in the gas-phase generation of low-volatility vapors, leading to atmospheric aerosol particles. We have recently demonstrated that S 1 this recombination reaction very likely proceeds via an intermediate complex of two alkoxy radicals (RO center dot center dot center dot OR') and that the accretion product pathway involves an intersystem crossing (ISC) of this complex from the triplet to the singlet surface. However, ISC rates have hitherto not been computed for large and chemically complex RO center dot center dot center dot OR' systems actually relevant to atmospheric aerosol formation. Here, we carry out systematic conformational sampling and ISC rate calculations on (3)(RO center dot center dot center dot OR') clusters formed in the recombination reactions of different diastereomers of the first-generation peroxyl radicals originating in both OH- and NO3 -initiated reactions of alpha-pinene, a key biogenic hydrocarbon for atmospheric aerosol formation. While we find large differences between the ISC rates of different diastereomer pairs, all systems have ISC rates of at least 10(6) s(-1), and many have rates exceeding 10(10) s(-1). Especially the latter value demonstrates that accretion product formation via the suggested pathway is a competitive process also for alpha-pinene-derived RO2 and likely explains the experimentally observed gas-phase formation of C-20 compounds in alpha-pinene oxidation.