Analysis of the kinetics of the thermal and chemically activated elimination of HF from 1,1,1-trifluoroethane: the C?C bond dissociation energy and the heat of formation of 1,1,1-trifluoroethane

The thermal, unimolecular elimination of HF from CH₃CF₃ was studied by three different groups over the temperature range 1000° to 1800°K. While the reported kinetic parameters varied greatly, it is shown here that these data may be satisfactorily correlated in terms of a four-center transition state. This correlation results in ΔE = 69.2 kcal/mol, and log (k/s^?1) = 14.6 – 72.6/θ. These results may then be combined with the kinetics of the chemically activated elimination of HF from CH₃CF₃ formed by the recombination of methyl and trifluoromethyl radicals. The data from three different laboratories are shown to be in excellent agreement. These data, combined with extant thermal data, yield as a best value DH(CH₃? CF₃) = 99.6 ± 1.1 kcal/mol. This gives the unexpectedly high value of DH₂₉₈°(CH₃? CF₃) = 101.2 ± 1.1 kcal/mol. It is suggested that dipoledipole interactions, primarily in CH₃CF₃, account for this surprisingly strong C? C bond dissociation energy. These results also yield δH(CH₃CF₃; g, 298) = ?178.6 ± 1.5 kcal/mol.