Theoretical Study of the Formation of Acetone in the OH-Initiated Atmospheric Oxidation of α-Pinene

A mechanism is proposed for the formation of acetone in the OH-initiated atmospheric oxidation of α-pinene.
In a first step, addition of the OH radical onto the α-pinene double bond forms a chemically activated tertiary
radical P1OH†. This activated radical can then for a certain fraction break its four-membered ring, leading to
a 6-hydroxymenthen-8-yl radical, which is subsequently converted to a 6-hydroxymenthen-8-oxy radical by
reaction with O2 and NO, and elimination of an NO2 molecule. Finally, the 6-hydroxymenthen-8-oxy radical
forms acetone by β CC bond rupture. For each of these steps, competing reactions are considered, as well
as the site and stereospecificity of the reaction itself. To quantify the acetone yield, 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. We find an acetone yield of 8.5%, in good
agreement with available experimental data. The uncertainty interval is estimated at 4−16%. It should be
emphasized that only the nascent, chemically activated P1OH† radicals contribute to the crucial ring-breaking
isomerization step.