Direct Kinetic Measurements of a Cyclic Criegee Intermediate; Unimolecular Decomposition of c-(CH₂)₅COO

We report the first direct kinetic measurements of a cyclic stabilized Criegee Intermediate. We have measured the unimolecular reaction rate coefficient of cyclohexanone oxide (c-(CH2)5COO) in the temperature 213–296 K and pressure 7–50 Torr ranges using absorption spectrometry. The c-(CH2)5COO was produced by the photolysis of c-(CH2)5CIBr at 213 nm in the presence of O2. We compare the measured fast c-(CH2)5COO unimolecular rate coefficient, 1998 ± 147 s–1 at 296 K, with the literature calculations for the structurally similar E-nopinone oxide formed in β-pinene ozonolysis. The kuni(c-(CH2)5COO)/kuni(E-nopinone oxide) ratio calculated using transition-state theory and density functional theory agrees well with this comparison. We have also measured the bimolecular rate coefficient of the reaction between c-(CH2)5COO and trifluoroacetic acid at 253 K and 10 Torr and obtained the value (8.7 ± 1.0) × 10–10 cm3 molecule–1 s–1. This very large value agrees with previous kinetic measurements for reactions between stabilized Criegee intermediates and halogenated organic acids.