Large Pressure Effects Caused by Internal Rotation in the <i>s-cis-syn</i>-Acrolein Stabilized Criegee Intermediate at Tropospheric Temperature and Pressure

Criegee intermediates are important atmospheric oxidants, and quantitative kinetics for stabilized Criegee intermediates are key parameters for atmospheric modeling but are still limited. Here we report barriers and rate constants for unimolecular reactions of <i>s-cis-syn</i>-acrolein oxide (scsAO), in which the vinyl group makes it a prototype for Criegee intermediates produced in the ozonolysis of isoprene. We find that the MN15-L and M06-2X density functionals have CCSD(T)/CBS accuracy for the unimolecular cyclization and stereoisomerization of scsAO. We calculated high-pressure-limit rate constants by the dual-level strategy that combines (a) high-level wave function-based conventional transition-state theory (which includes coupled-cluster calculations with quasiperturbative inclusion of quadruple excitations because of the strongly multiconfigurational character of the electronic wave function) and (b) canonical variational transition-state theory with small-curvature tunneling based on a validated density functional. We calculated pressure-dependent rate constants both by system-specific quantum Rice-Ramsperger-Kassel theory and by solving the master equation. We report rate constants for unimolecular reactions of scsAO over the full range of atmospheric temperature and pressure. We found that the unimolecular reaction rates of this larger-than-previously studied Criegee intermediate depend significantly on pressure. Particularly, we found that falloff effects decrease the effective unimolecular cyclization rate constant of scsAO by about a factor of 3, but the unimolecular reaction is still the dominant atmospheric sink for scsAO at low altitudes. The large falloff caused by the inclusion of the stereoisomerization channel in the master equation calculations has broad implications for mechanistic analysis of reactions with competitive internal rotations that can produce stable rotamers.