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Thermal Decomposition of CF3 and the Reaction of CF2 + OH → CF2O + H

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journal contribution
posted on 10.01.2008 by N. K. Srinivasan, M.-C. Su, J. V. Michael, A. W. Jasper, S. J. Klippenstein, L. B. Harding
The reflected shock tube technique with multipass absorption spectrometric detection (at a total path length of ∼1.75 m) of OH-radicals at 308 nm has been used to study the dissociation of CF3-radicals [CF3 + Kr → CF2 + F + Kr (a)] between 1803 and 2204 K at three pressures between ∼230 and 680 Torr. The OH-radical concentration buildup resulted from the fast reaction F + H2O → OH + HF (b). Hence, OH is a marker for F-atoms. To extract rate constants for reaction (a), the [OH] profiles were modeled with a chemical mechanism. The initial rise in [OH] was mostly sensitive to reactions (a) and (b), but the long time values were additionally affected by CF2 + OH → CF2O + H (c). Over the experimental temperature range, rate constants for (a) and (c) were determined from the mechanistic fits to be kCF3+Kr = 4.61 × 10-9 exp(−30020 K/T) and kCF2+OH = (1.6 ± 0.6) × 10-10, both in units of cm3 molecule-1 s-1. Reaction (a), its reverse recombination reaction reaction (−a), and reaction (c) are also studied theoretically. Reactions (c) and (−a) are studied with direct CASPT2 variable reaction coordinate transition state theory. A master equation analysis for reaction (a) incorporating the ab initio determined reactive flux for reaction (−a) suggests that this reaction is close to but not quite in the low-pressure limit for the pressures studied experimentally. In contrast, reaction (c) is predicted to be in the high-pressure limit due to the high exothermicity of the products. A comparison with past and present experimental results demonstrates good agreement between the theoretical predictions and the present data for both (a) and (c).