Characterization of the 1,1-HF Elimination Reaction from the Competition between the 1,1-HF and 1,2-DF Unimolecular Elimination Reactions of CD<sub>3</sub>CD<sub>2</sub>CHF<sub>2</sub>

The recombination of CHF<sub>2</sub> and C<sub>2</sub>D<sub>5</sub> radicals was used to produce CD<sub>3</sub>CD<sub>2</sub>CHF<sub>2</sub>* molecules with 96 kcal mol<sup>–1</sup> of vibrational energy in a room temperature bath gas. The formation of CD<sub>3</sub>CDCHF and CD<sub>3</sub>CDCDF was used to identify the 1,2-DF and 1,1-HF unimolecular elimination channels; CD<sub>3</sub>CDCDF is formed by isomerization of the singlet-state CD<sub>3</sub>CD<sub>2</sub>CF carbene. The total unimolecular rate constant is 1.6 × 10<sup>6</sup> s<sup>–1</sup>, and the branching ratio for 1,1-HF elimination is 0.25. Threshold energies of 64 ± 2 and 73 ± 2 kcal mol<sup>–1</sup> were assigned to the 1,2-DF and 1,1-HF reaction channels. The <i>E</i> and <i>Z</i> isomers of 1-fluoropropene were observed for each reaction; approximately 30% of the CD<sub>3</sub>CDCDF molecules derived from 1,1-HF elimination retained enough energy to undergo <i>cis–trans</i> isomerization. Electronic structure calculations with density-functional theory were used to characterize the transition-state structures and the H atom migration barrier for CD<sub>3</sub>CD<sub>2</sub>CF. Adjustment of the rate constants to account for kinetic-isotope effects suggest that the branching ratio would be 0.20 for 1,1-HF elimination from C<sub>2</sub>H<sub>5</sub>CHF<sub>2</sub>. The results from an earlier study of CD<sub>3</sub>CHF<sub>2</sub> and CH<sub>3</sub>CHF<sub>2</sub> are also reinterpreted to assign a threshold energy of 74 kcal mol<sup>–1</sup> for the 1,1-HF elimination reaction. Because CHF<sub>2</sub>CHF<sub>2</sub>* is generated in the photolysis system, the 1,1-and 1,2-HF-elimination reactions of CHF<sub>2</sub>CHF<sub>2</sub>* are discussed. The 1,1-HF channel was identified by trapping the CF<sub>2</sub>HCF carbene with <i>cis-</i>butene-2.