Characterization of the 1,1-HF Elimination Reaction from the Competition between the 1,1-HF and 1,2-DF Unimolecular Elimination Reactions of CD₃CD₂CHF₂

The recombination of CHF₂ and C₂D₅ radicals was used to produce CD₃CD₂CHF₂* molecules with 96 kcal mol^–1 of vibrational energy in a room temperature bath gas. The formation of CD₃CDCHF and CD₃CDCDF was used to identify the 1,2-DF and 1,1-HF unimolecular elimination channels; CD₃CDCDF is formed by isomerization of the singlet-state CD₃CD₂CF carbene. The total unimolecular rate constant is 1.6 × 10⁶ s^–1, and the branching ratio for 1,1-HF elimination is 0.25. Threshold energies of 64 ± 2 and 73 ± 2 kcal mol^–1 were assigned to the 1,2-DF and 1,1-HF reaction channels. The E and Z isomers of 1-fluoropropene were observed for each reaction; approximately 30% of the CD₃CDCDF molecules derived from 1,1-HF elimination retained enough energy to undergo cis–trans isomerization. Electronic structure calculations with density-functional theory were used to characterize the transition-state structures and the H atom migration barrier for CD₃CD₂CF. 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 C₂H₅CHF₂. The results from an earlier study of CD₃CHF₂ and CH₃CHF₂ are also reinterpreted to assign a threshold energy of 74 kcal mol^–1 for the 1,1-HF elimination reaction. Because CHF₂CHF₂* is generated in the photolysis system, the 1,1-and 1,2-HF-elimination reactions of CHF₂CHF₂* are discussed. The 1,1-HF channel was identified by trapping the CF₂HCF carbene with cis-butene-2.