Methyl vinyl carbonyl oxide is an important intermediate in the reaction of isoprene and ozone and may be responsible for most of the (*)OH formed in isoprene ozonolysis. We use CBS-QB3 calculations and RRKM/master equation simulations to characterize all the pathways leading to the formation of this species, all the interconversions among its four possible conformers, and all of its irreversible isomerizations. Our calculations, like previous studies, predict (*)OH yields consistent with experiment if thermalized syn-methyl carbonyl oxides form (*)OH quantitatively. Natural bond order analysis reveals that the vinyl group weakens the C=O bond of the carbonyl oxide, making rotation about this bond accessible to this chemically activated intermediate. The vinyl group also allows one conformer of the carbonyl oxide to undergo electrocyclization to form a dioxole, a species not previously considered in the literature. Dioxole formation, which has a CBS-QB3 reaction barrier of 13.9 kcal/mol, is predicted to be favored over vinyl hydroperoxide formation, dioxirane formation, and collisional stabilization. Our calculations also predict that two dioxole derivatives, 1,2-epoxy-3-butanone and 3-oxobutanal, should be major products of isoprene ozonolysis.

The yields of OH radicals from the ozonolysis of ethene and trans-2-butene under dry and humid conditions (0% and 65% relative humidity, respectively) were measured by the small-ratio relative rate technique. The OH yields from both alkenes are found to be independent of the humidity within the error limits of the experiments. This suggests that the OH yield from the reaction of Criegee intermediates with water is less than 30%. These findings are supported by Master equation calculations that are consistent with the major product from the reaction of Criegee intermediates with water being the hydroxyalkyl hydroperoxide (RCH-(OH)OOH), with a smaller contribution from OH. Implications of these results for both the fundamental reaction mechanism of ozone-alkene reactions and the fate of Criegee intermediates in the troposphere are discussed.

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