Unimolecular Kinetics of Stabilized CH₃CHOO Criegee Intermediates: syn-CH₃CHOO Decomposition and anti-CH₃CHOO...

The kinetics of the unimolecular decomposition of the stabilized Criegee intermediate syn-CH₃CHOO has been investigated at temperatures between 297 and 331 K and pressures between 12 and 300 Torr using laser flash photolysis of CH₃CHI₂/O₂/N₂ gas mixtures coupled with time-resolved broadband UV absorption spectroscopy. Fits to experimental results using the Master Equation Solver for Multi-Energy well Reactions (MESMER) indicate that the barrier height to decomposition is 67.2 ± 1.3 kJ mol^-1 and that there is a strong tunneling component to the decomposition reaction under atmospheric conditions. At 298 K and 760 Torr, MESMER simulations indicate a rate coefficient of 150_-81⁺¹⁷⁶ s^-1 when tunneling effects are included but only 5_-2⁺³ s^-1 when tunneling is not considered in the model. MESMER simulations were also performed for the unimolecular isomerization of the stabilized Criegee intermediate anti-CH₃CHOO to methyldioxirane, indicating a rate coefficient of 54_-21⁺³⁴ s^-1 at 298 K and 760 Torr, which is not impacted by tunneling effects. Expressions to describe the unimolecular kinetics of syn- and anti-CH₃CHOO are provided for use in atmospheric models, and atmospheric implications are discussed.