Thermal and Vibrationally Activated Decomposition of the syn-CH<sub>3</sub>CHOO Criegee Intermediate
Meenu Upadhyay, Markus Meuwly
Abstract
The full reaction pathway between the syn-CH3CHOO Criegee Intermediate via vinyl hydroxyperoxide (VHP) to CH2COH+OH is followed for vibrationally excited and thermally prepared reactants. Reactivity along the entire pathway was characterized from an aggregate of more than 10 μs of reactive MD simulations using energy functions with accuracies at the Møller–Plesset second order level of theory. Reaction times for OH elimination are on the nanosecond time scale, and the energy dependence of the rates is consistent with experiments in the jet. The actual rates depend on the O–O dissociation energy (DeOO = 31.5 kcal/mol at the MP2/aug-cc-pVTZ level or DeOO = 23.5 kcal/mol closer to earlier CASPT2 calculations). Also, the initial preparation of the reactants influences both the VHP formation/OH elimination rates and the OH yields. For most conditions with initial vibrational excitation 80% or more of syn-CH3CHOO progress to OH elimination on the 5 ns time scale. However, for internally cold conformational ensembles generated at low temperature (50 K) only 20% to 30% of VHP eliminate OH on the 5 ns time scale which increases to 55% to 67% depending on excitation energy from simulations on the 15 ns time scale. For thermal preparation of syn-CH3CHOO, which is relevant in the atmosphere, 35% of the trajectories lead to OH-elimination within 1 ns. This compares with experimentally reported yields of 24% to 64% in a collisional environment. The estimated thermal rate at 300 K is 103 s–1, extrapolated from results at 500 to 900 K, is consistent with the experimentally measured rate of 182 ± 66 s–1.