posted on 2021-11-29, 13:03authored byMeenu Upadhyay, Markus Meuwly
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.