Computational Studies of the Isomerization and Hydration Reactions of Acetaldehyde Oxide and Methyl Vinyl Carbonyl Oxide Keith T. Kuwata,* Matthew R. Hermes, Matthew J. Carlson, and Cheryl K. Zogg Department of Chemistry, Macalester College, Saint Paul, Minnesota 55105-1899 ReceiVed: June 10, 2010; ReVised Manuscript ReceiVed: July 26, 2010 Alkene ozonolysis is a major source of hydroxyl radical (•OH), the most important oxidant in the troposphere. Previous experimental and computational work suggests that for many alkenes the measured •OH yields should be attributed to the combined impact of both chemically activated and thermalized syn-alkyl Criegee intermediates (CIs), even though the thermalized CI should be susceptible to trapping by molecules such as water. We have used RRKM/master equation and variational transition state theory calculations to quantify the competition between unimolecular isomerization and bimolecular hydration reactions for the syn and anti acetaldehyde oxide formed in trans-2-butene ozonolysis and for the CIs formed in isoprene ozonolysis possessing syn-methyl groups. Statistical rate theory calculations were based on quantum chemical data provided by the B3LYP, QCISD, and multicoefficient G3 methods, and thermal rate constants were corrected for tunneling effects using the Eckart method. At tropospheric temperatures and pressures, all thermalized CIs with syn-methyl groups are predicted to undergo 1,4-hydrogen shifts from 2 to 8 orders of magnitude faster than they react with water monomer at its saturation number density. For thermalized anti acetaldehyde oxide, the rates of dioxirane formation and hydration should be comparable. I. Introduction The reaction of ozone with alkenes, known as ozonolysis, is a significant nonphotochemical source 1-6 of hydroxyl radical (•OH), the most important oxidant in the troposphere. 7,8 A comprehensive understanding of tropospheric chemistry thus depends on a complete and accurate characterization of the ozonolysis mechanism. Quantum chemical and statistical rate theory methods, if validated, can be powerful tools for con- structing such mechanisms. Scheme 1 presents the initial steps of ozonolysis for one of the alkenes considered in this paper, trans-2-butene. We chose to model trans-2-butene because of the availability of experimental data to validate our predictions. The large exothermicity (Δ r H° ≈ -55 kcal/mol) of primary ozonide formation leads to chemical activation of the resulting carbonyl oxide (or Criegee Intermediate, CI). For small alkenes such as trans-2-butene, a significant fraction of the nascent CI distribution possesses energy in excess of the barriers to unimolecular reaction, enabling the CI to isomerize promptly, that is, on the sub-microsecond time scale. 9,10 The particular unimolecular pathway taken by the CI is largely controlled by its conformation (Scheme 2). A chemically activated CI with an alkyl group syn to the peroxy bond preferentially isomerizes via a 1,4-hydrogen shift to a vinyl hydroperoxide. The hydroperoxide is likewise chemically activated and should therefore decompose promptly to form a vinoxy radical and •OH. 11-13 The syn-alkyl CI thus functions as an •OH precursor in the ozonolysis mechanism. In contrast, a chemically activated CI lacking a syn-alkyl group will typically isomerize to a dioxirane. 14-16 This dioxirane will preferentially isomerize to a carboxylic acid. 16 Although the acid is highly activated, it is likely 16-21 that this species does not undergo homolysis to form •OH. However, Kroll et al. 22 have reported experimental evidence that in the ozonolysis of some alkenes the acid derived from anti CI may release •OH. Nguyen et al. 23,24 have also reported computational evidence that dioxiranes may access a low-barrier intersystem crossing to a triplet bis(oxy) diradical that, in the case of the dioxirane derived from acetaldehyde oxide, would lead ultimately to CO 2 , •CH 3 , and •H. While consideration of this pathway for the CIs considered here is beyond the scope of this paper, it certainly merits future study. In the atmosphere, and in smog chamber experiments, the fraction of CI that does not isomerize promptly will be thermalized by collisions with background gas. The conforma- tional preferences shown in Scheme 2 apply even more rigidly to thermalized CI, as shown in reactions 1 and 2: Although the initial distribution of vinyl hydroperoxide 3 formed in the thermal reaction of 1 will not be as activated as that formed from chemically activated 1, all of 3 is nevertheless expected to decompose quantitatively to •OH and a vinoxy radical. 9 However, Johnson and Marston 25 have recently dis- cussed the possibility that some fraction of the vinyl hydroper- oxide formed in the ozonolysis of 2,3-dimethyl-2-butene may be collisionally stabilized under atmospheric conditions. We return to this possibility later. In contrast to chemically activated CI, the isomerization of thermalized CI takes place on the millisecond time scale or * To whom correspondence should be addressed. E-mail: kuwata@ macalester.edu. J. Phys. Chem. A 2010, 114, 9192–9204 9192 10.1021/jp105358v  2010 American Chemical Society Published on Web 08/11/2010