The Unimolecular Reactions of CF<sub>3</sub>CHF<sub>2</sub> 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
2017-09-19T00:00:00Z (GMT) by
The recombination of CF<sub>3</sub> and CHF<sub>2</sub> radicals in a room-temperature bath gas was used to prepare vibrationally excited CF<sub>3</sub>CHF<sub>2</sub>* molecules with 101 kcal mol<sup>–1</sup> 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<sub>3</sub>, CF<sub>3</sub>(F)C: and C<sub>2</sub>F<sub>4</sub> product concentrations by gas chromatography using a mass spectrometer as the detector. The singlet CF<sub>3</sub>(F)C: concentration was measured by trapping the carbene with <i>trans</i>-2-butene. The experimental rate constants are 3.6 × 10<sup>4</sup>, 4.7 × 10<sup>4</sup>, and 1.1 × 10<sup>4</sup> s<sup>–1</sup> 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 (<i>E</i><sub>0</sub>) of 88 ± 2, 88 ± 2, and 87 ± 2 kcal mol<sup>–1</sup> 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<sub>3</sub>(F)C: and HF in the exit channel with a dissociation energy of ∼5 kcal mol<sup>–1</sup>. Hydrogen-bonded complexes between HF and the H atom migration transition state of CH<sub>3</sub>(F)C: and the F atom migration transition state of CF<sub>3</sub>(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.
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