The detailed mechanism of the OH-initiated atmospheric oxidation of α-pinene: a theoretical study

A detailed mechanism is developed for the OH-initiated atmospheric oxidation of α-pinene in the presence of NOx, based solely on quantitative structure–activity relationships and on theoretical quantum chemistry methods, combined with transition state theory calculations or RRKM master equation analyses. Thus, on objective theoretical grounds, the fate of some 40 organic (oxy) radical intermediates is predicted. Addition of OH onto the monoterpene double bond accounts for ∼90% of the reaction and leads to chemically activated P1OH† (∼44%) and P2OH† (∼44%) radicals. The subsequent chemistry of these radicals is described, and the quantitative importance of the pathways leading to first-generation products such as pinonaldehyde, acetone, formaldehyde, formic acid and nitrates is discussed. H-atom abstraction by OH from α-pinene is a minor route (∼12%) but contributes importantly to the overall yield of formaldehyde. Total product yields are obtained by propagating the product fractions of each step in the mechanism. Overall predicted product yields for the usual conditions of laboratory experiments are, on a molar basis: 35.7% pinonaldehyde, 18.8% formaldehyde, 19% organic nitrates, 17.9% acetone, 25.9% other (hydroxy)carbonyls, 17.2% C1 and C2 carboxylic acids and 30.7% CO2, in general agreement with measured yields. For the much lower NO levels of real atmospheric conditions, on the other hand, our theoretically predicted yields differ significantly: 59.5% pinonaldehyde, 12.6% formaldehyde, 13.1% organic nitrates, 11.9% acetone, 16.4% other (hydroxy)carbonyls, 8.7% CO2 and a negligible fraction of C1 and C2 carboxylic acids. Some specific effects relating to the hyperconjugation-stabilization of terpene-derived (bi)cyclic radicals are discussed.