Characterization of the 1,1-HCl Elimination Reaction of Vibrationally Excited CD₃CHFCl Molecules and Assignment of Threshold Energies for 1,1-HCl and 1,2-DCl plus 1,1-HF and 1,2-DF Elimination Reactions

Vibrationally excited CD₃CHFCl molecules with 96 kcal mol^–1 of energy were generated by the recombination of CD₃ and CHFCl radicals in a room-temperature bath gas. The four competing unimolecular decomposition reactions, namely, 1,1-HCl and 1,2-DCl elimination and 1,1-HF and 1,2-DF elimination, were observed, and the individual rate constants were measured. The product branching fractions are 0.60, 0.27, 0.09, and 0.04 for 1,2-DCl, 1,1-HCl, 1,2-DF, and 1,1-HF elimination, respectively. Electronic structure calculations were used to define models of the four transition states. The statistical rate constants calculated from these models were compared to the experimental rate constants. The assigned threshold energies with ±2 kcal mol^–1 uncertainty are 60, 72, 65, and 74 kcal mol^–1 for the 1,2-DCl, 1,1-HCl, 1,2-DF, and 1,1-HF reactions, respectively. The loose structure of the 1,1-HX transition states, which is exemplified by the order of magnitude larger pre-exponential factor relative to the 1,2-HX elimination reactions, compensates for the high threshold energy; thus, the 1,1-HX elimination reaction rates can compete with the 1,2-HX elimination reactions for high levels of vibrational excitation in CD₃CHFCl. The 1,1-HCl and 1,1-HF reactions are observed via the CD₂CDF and CD₂CDCl products formed from isomerization of the CD₃CF and CD₃CCl carbenes. These D-atom migration reactions are discussed, and the possibility of tunneling is evaluated. The transition states developed from the 1,1-HCl and 1,1-HF reactions of CD₃CHFCl are compared to models for the HCl and HF elimination reactions of CHF₂Cl, CHFCl₂, and CH₂FCl.