Unimolecular Kinetics of Stabilized CH 3 CHOO Criegee Intermediates: syn -CH 3 CHOO Decomposition and anti -CH 3 CHOO Isomerization.

The kinetics of the unimolecular decomposition of the stabilized Criegee intermediate syn -CH 3 CHOO has been investigated at temperatures between 297 and 331 K and pressures between 12 and 300 Torr using laser flash photolysis of CH 3 CHI 2 /O 2 /N 2 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 -CH 3 CHOO 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 -CH 3 CHOO are provided for use in atmospheric models, and atmospheric implications are discussed.