Theoretical study of the formation of acetone in the OH-initiated atmospheric oxidation of alpha-pinene

A mechanism is proposed for the formation of acetone in the OH-initiated at mospheric oxidation of alpha -pinene. In a first step, addition of the OH r adical onto the alpha -pinene double bond forms a chemically activated tert iary radical P1OH(dagger). This activated radical can then for a certain fr action break its four-membered ring, leading to a 6-hydroxymenthen-8-yl rad ical, which is subsequently converted to a 6-hydroxymenthen-8-oxy radical b y reaction with O-2 and NO, and elimination of an NO2 molecule. Finally, th e 6-hydroxymenthen-8-oxy radical forms acetone by beta C-C bond rupture. Fo r each of these steps, competing reactions are considered, as well as the s ite and stereospecificity of the reaction itself. To quantify the acetone y ield, quantum chemical calculations were combined with RRKM-Master Equation analyses for most of the reactions; other branching ratios were estimated from available literature data. The total yield of acetone was obtained by propagating the relevant product fractions of each step in the mechanism. W e find an acetone yield of 8.5%, in good agreement with available experimen tal data. The uncertainty interval is estimated at 4-16%. It should be emph asized that only the nascent, chemically activated P1OH(dagger); radicals c ontribute to the crucial ring-breaking isomerization step.