Cycloalkene ozonolysis: Collisionally mediated mechanistic branching

Master equation calculations on a computational potential energy surface reveal that collisional stabilization at atmospheric pressure becomes important in the gas-phase ozonolysis of endocyclic alkenes for a carbon number between 8 and 15. Because the reaction products from endocyclic ozonolysis are tethered, this system is ideal for consideration of collisional energy transfer, as chemical activation is confined to a single reaction product. Collisional stabilization of the Criegee intermediate precedes collisional stabilization of the primary ozonide by roughly an order of magnitude in pressure. The stabilization of the Criegee intermediate leads to a dramatic transformation in the dominant oxidation pathway from a radical-forming process at low carbon number to a secondary ozonide-forming process at high carbon number. Secondary ozonide formation is important even for syn-isomer Criegee intermediates, contrary to previous speculation. We use substituted cyclohexenes as analogues for atmospherically important mono- and sesquiterpenes, which are major precursors for secondary organic aerosol formation in the atmosphere. Combining these calculations with literature experimental data, we conclude that the transformation from chemically activated to collisionally stabilized behavior most probably occurs between the mono- and sesquiterpenes, thus causing dramatically different atmospheric behavior.