| Abstract
| - 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 tertiaryradical P1OH†. This activated radical can then for a certain fraction break its four-membered ring, leading toa 6-hydroxymenthen-8-yl radical, which is subsequently converted to a 6-hydroxymenthen-8-oxy radical byreaction with O2 and NO, and elimination of an NO2 molecule. Finally, the 6-hydroxymenthen-8-oxy radicalforms acetone by β CC bond rupture. For each of these steps, competing reactions are considered, as wellas the site and stereospecificity of the reaction itself. To quantify the acetone yield, quantum chemicalcalculations were combined with RRKM-Master Equation analyses for most of the reactions; other branchingratios were estimated from available literature data. The total yield of acetone was obtained by propagatingthe relevant product fractions of each step in the mechanism. We find an acetone yield of 8.5%, in goodagreement with available experimental data. The uncertainty interval is estimated at 4−16%. It should beemphasized that only the nascent, chemically activated P1OH† radicals contribute to the crucial ring-breakingisomerization step.
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