Unimolecular decomposition rates of a methyl-substituted Criegee intermediate syn-CH₃CHOO

Criegee intermediates play important roles in atmospheric chemistry. Methyl Criegee intermediate, CH₃CHOO, has two conformers, syn- and anti-conformers. Syn-CH₃CHOO would undergo fast unimolecular decomposition to form OH radical via 1,4 H-atom transfer. In this work, unimolecular decomposition of syn-CH₃CHOO was probed in real time with UV absorption spectroscopy at 278-318 K and 100-700 torr. We used water vapor as the scavenger of anti-CH₃CHOO to distinguish the absorption signals of the two conformers. After removing the contributions from reactions with radical byproducts, reaction with water vapor and wall loss, we obtained the unimolecular reaction rate coefficient of syn-CH₃CHOO (at 300 torr), which increases from (67 ± 15) s^-1 at 278 K, (146 ± 31) s^-1 at 298 K, to (288 ± 81) s^-1 at 318 K with an Arrhenius activation energy of ca. 6.4 kcal mol^-1 and a weak pressure dependence for 100-700 torr. Compared to previous studies, this work provides temperature dependent unimolecular rates of syn-CH₃CHOO at higher pressures, which are more relevant to atmospheric conditions.