Ab Initio Calculations for Kinetic Modeling of Halocarbons

Abstract

The thermochemistry and reaction kinetics of halogenated hydrocarbons have been investigated by ab initio methods in order to improve our understanding of their flame chemistry and likely roles in flame suppression. Bond additivity corrections at the G2, G2(MP2), CBS-4 and CBS-Q levels of theory were developed for fluorinated and chlorinated C 1 and C 2 species, including saturated and unsaturated compounds. The resulting enthalpies of formation are in excellent agreement with experimental values. Transition states for the reactions of H atoms with hydrofluoromethanes were characterized at up to the G2 level of theory, and application of transition state theory yielded rate constants in good accord with experimental results. A similar analysis for H and OH reactions with CH 3 I also agrees with the known thermochemistry and kinetics. These investigations provide insight into the major product channels and the temperature dependence of the rate constants. The implications for flame suppression by haloalkanes are discussed.During the past two decades, it has become apparent that the release of volatile chlorofluorocarbons (CFCs) and halon fire suppressants (e.g. CF 3 Br, CF 2 ClBr, CF 2 Br-CF 2 Br) is a major cause of depletion of the stratosphere's ozone layer (7-5). Hence, there have been numerous restrictions placed upon the industrial use of CFCsand halons, which has led to concerted efforts to find new, "ozone-friendly" replacements (4). Potential replacement agents include hydrofluorocarbons and iodocarbons, which are degraded in the troposphere and, hence, pose no significant risk to the ozone layer. They also possess low global warming potentials.The effectiveness of a proposed flame suppressant can be reliably predicted via kinetic modeling provided accurate thermochemical and kinetic data are available for the hundreds of reactions associated with the suppressant and its interactions with

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