Unimolecular rate constants for HX or DX elimination (X = F, Cl) from chemically activated CF3CH2CH2Cl, C2H5CH2Cl, and C2D5CH2Cl:: Threshold energies for HF and HCl elimination

Vibrationally activated CF3CH2CH2Cl Molecules were prepared with 94 kcal mol(-1) of vibrational energy by the combination of CF3CH2 and CH2Cl radicals and with 101 kcal mol(-1) of energy by the combination of CF3 and CH2CH2Cl radicals at room temperature. The unimolecular rate constants for elimination of HCl from CF3CH2CH2Cl were 1.2 x 10(7) and 0.24 x 10(7) s(-1) with 101 and 94 kcal mol(-1), respectively. The product branching ratio, k(HCl)/k(HF), was 80 +/- 25. Activated CH3CH2CH2Cl and CD3CD2CH2Cl molecules with 90 kcal mol(-1) of energy were prepared by recombination Of C2H5 (or C2D5) radicals with CH2Cl radicals. The unimolecular rate constant for HCl elimination was 8.7 x 10(7) s(-1), and the kinetic isotope effect was 4.0. Unified transition-state models obtained from density-functional theory calculations, with treatment of torsions as hindered internal rotors for the molecules and the transition states, were employed in the calculation of the RRKM rate constants for CF3CH2CH2Cl and CH3CH2CH2Cl. Fitting the calculated rate constants from RRKM theory to the experimental values provided threshold energies, E-0, of 58 and 71 kcal mol(-1) for the elimination of HCl or HF, respectively, from CF3CH2CH2Cl and 54 kcal mol(-1) for HCl elimination from CH3CH2CH2Cl. Using the hindered-rotor model, threshold energies for HF elimination also were reassigned from previously published chemical activation data for CF3CH2CH3, CF3CH2CF3, CH3CH2CH2F, CH3CHFCH3, and CH3CF2CH3. In an appendix, the method used to assign threshold energies was tested and verified using the combined thermal and chemical activation data for C2H5Cl, C2H5F, and CH3CF3.