The Unimolecular Reactions of CFCHF Studied by Chemical Activation: Assignment of Rate Constants and Threshold Energies to the 1,2-H Atom Transfer, 1,1-HF and 1,2-HF Elimination Reactions, and the Dependence of Threshold Energies on the Number of F-Atom Substituents in the Fluoroethane Molecules.

The recombination of CF and CHF radicals in a room-temperature bath gas was used to prepare vibrationally excited CFCHF* molecules with 101 kcal mol of vibrational energy. The subsequent 1,2-H atom transfer and 1,1-HF and 1,2-HF elimination reactions were observed as a function of bath gas pressure by following the CHF, CF(F)C: and CF product concentrations by gas chromatography using a mass spectrometer as the detector. The singlet CF(F)C: concentration was measured by trapping the carbene with trans-2-butene. The experimental rate constants are 3.6 × 10, 4.7 × 10, and 1.1 × 10 s for the 1,2-H atom transfer and 1,1-HF and 1,2-HF elimination reactions, respectively. These experimental rate constants were matched to statistical RRKM calculated rate constants to assign threshold energies (E) of 88 ± 2, 88 ± 2, and 87 ± 2 kcal mol to the three reactions. Pentafluoroethane is the only fluoroethane that has a competitive H atom transfer decomposition reaction, and it is the only example with 1,1-HF elimination being more important than 1,2-HF elimination. The trend of increasing threshold energies for both 1,1-HF and 1,2-HF processes with the number of F atoms in the fluoroethane molecule is summarized and investigated with electronic-structure calculations. Examination of the intrinsic reaction coordinate associated with the 1,1-HF elimination reaction found an adduct between CF(F)C: and HF in the exit channel with a dissociation energy of ∼5 kcal mol. Hydrogen-bonded complexes between HF and the H atom migration transition state of CH(F)C: and the F atom migration transition state of CF(F)C: also were found by the calculations. The role that these carbene-HF complexes could play in 1,1-HF elimination reactions is discussed.