Quantum chemical and RRKM/master equation studies of cyclopropene ozonolysis

The cycloalkene–ozone reaction impacts atmospheric chemistry as a non-photochemical source of hydroxyl radical (OH) and as a source of secondary organic aerosol. Unlike in the ozonolysis of acyclic alkenes, the Criegee Intermediates (CI’s) formed in cycloalkene ozonolysis retain all of the energy released in the reaction, enabling these CI’s to access reaction pathways with high activation barriers. We used CBS-QB3, MCG3, RRKM/master equation, and transition state theory (TST) calculations to perform a detailed analysis of cyclopropene ozonolysis, treating all possible conformers of all intermediates and transition structures (TS’s). The total TST rate constant for O3 cycloaddition to cyclopropene is 2.3 × 10−14 cm3 molecule−1 s−1, with ∼90% of the reaction proceeding through the endo TS. We predict that ∼35% of chemically activated syn CI’s will cyclize to dioxiranes, even though the barriers against dioxirane formation are 5–12 kcal mol−1 higher than the barriers against the 1,4-hydrogen shift that leads to vinyl hydroperoxides and OH. This helps reduce the predicted OH yield for cyclopropene ozonolysis to 44%. We also predict ∼20% of either the endo primary ozonide (PO) or its syn CI derivatives will isomerize to the exo PO or anti CI’s. Further work is required to predict the extent to which these reactivity patterns apply to the ozonolysis of larger cycloalkenes.