Unimolecular Reactions of Chemically Activated CF₂BrCF₂CH₃and CF₂BrCF₂CD₃:  Evidence for 1,2-FBr Interchange

Vibrationally excited CF2BrCF2CH3 and CF2BrCF2CD3 molecules were prepared with 96 kcal mol(-1) energy at room temperature by the recombination of CF2BrCF2 and CH3 (CD3) radicals. The observed unimolecular reactions are 1,2-BrF interchange to give CF3CFBrCH3 (CD3) molecules and 2,3-FH (FD) elimination; the rate constants are 2.2 x 10(5) (1.5 x 10(5)) s(-1) and 2.0 x 10(5) (0.75 x 10(5)) s(-1), respectively. The CF3CFBrCH3 (CD3) molecules rapidly, relative to the reverse reaction, eliminate HBr or DBr to give the observed product CF3CF=CH2 (CD2). Density functional theory at the B3PW91/6-311+G(2d,p) level was used to obtain vibrational frequencies and moments of inertia of the molecule and transition states for subsequent calculations of statistical rate constants for CF2BrCF2CH3 and CF2BrCF2CD3. Matching experimental and calculated rate constants gave threshold energies of 62 and 66 kcal mol(-1) for 1,2-BrF interchange and 2,3-FH elimination, respectively. The BrF interchange reaction is compared to CIF interchange from CF2ClCF2CH3 and CF2-ClCHFCH3.