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)(5)COO) in the temperature 213-296 K and pressure 7-50 Torr ranges using absorption spectrometry. The c-(CH2)(5)COO was produced by the photolysis of c-(CH2)(5)CIBr at 213 nm in the presence of O-2. We compare the measured fast c-(CH2)(5)COO unimolecular rate coefficient, 1998 +/- 147 s(-1) at 296 K, with the literature calculations for the structurally similar E-nopinone oxide formed in beta-pinene ozonolysis. The k(uni)(c-(CH2)(5)COO)/k(uni)(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)(5)COO and trifluoroacetic acid at 253 K and 10 Torr and obtained the value (8.7 +/- 1.0) x 10(-10) cm(3) molecule(-1) s(-1). This very large value agrees with previous kinetic measurements for reactions between stabilized Criegee intermediates and halogenated organic acids