The recombination of CH(2)Cl and CH(2)F radicals generates vibrationally excited CH(2)ClCH(2)Cl, CH(2)FCH(2)F, and CH(2)ClCH(2)F molecules with about 90 kcal mol(-1) of energy in a room temperature bath gas. New experimental data for CH(2)ClCH(2)F have been obtained that are combined with previously published studies for C(2)H(4)Cl(2) and C(2)H(4)F(2) to define reliable rate constants of 3.0 x 10(8) (C(2)H(4)F(2)), 2.4 x 10(8) (C(2)H(4)Cl(2)), and 1.9 x 10(8) (CH(2)ClCH(2)F) s(-1) for HCl and HF elimination. The product branching ratio for CH(2)ClCH(2)F is approximately 1. These experimental rate constants are compared to calculated statistical rate constants (RRKM) to assign threshold energies for HF and HCl elimination. The calculated rate constants are based on transition-state models obtained from calculations of electronic structures; the energy levels of the asymmetric, hindered, internal rotation were directly included in the state counting to obtain a more realistic measure for the density of internal states for the molecules. The assigned threshold energies for C(2)H(4)F(2) and C(2)H(4)Cl(2) are both 63 +/- 2 kcal mol(-1). The threshold energies for CH(2)ClCH(2)F are 65 +/- 2 (HCl) and 63 +/- 2 (HF) kcal mol(-1). These threshold energies are 5-7 kcal mol(-1) higher than the corresponding values for C(2)H(5)Cl or C(2)H(5)F, and beta-substitution of F or Cl atoms raises threshold energies for HF or HCl elimination reactions. The treatment presented here for obtaining the densities of states and the entropy of activation from models with asymmetric internal rotations with high barriers can be used to judge the validity of using a symmetric internal-rotor approximation for other cases. Finally, threshold energies for the 1,2-fluorochloroethanes are compared to those of the 1,1-fluorochloroethanes to illustrate substituent effects on the relative energies of the isomeric transition states.

The recombination of CF(2)Cl with CH(2)Cl and CFCl(2) with CH(2)F were employed to generate CF(2)ClCH(2)Cl* and CFCl(2)CH(2)F* molecules with 381 and 368 kJ mol(-1), respectively, of vibrational energy in a room-temperature bath gas. The unimolecular reactions of these molecules, which include HCl elimination, HF elimination, and isomerisation by interchange of chlorine and fluorine atoms, were characterized. The three rate constants for CFCl(2)CH(2)F were 2.9×10(7), 0.87×10(7) and 0.04×10(7) s(-1) for HCl elimination, isomerisation and HF elimination, respectively. The isomerisation reaction must be included to have a complete characterization of the unimolecular kinetics of CFCl(2)CH(2)F. The rate constants for HCl elimination and HF elimination from CF(2)ClCH(2)Cl were 14×10(7) and 0.37×10(7) s(-1), respectively. Isomerisation that has a rate constant less than 0.08×10(7) s(-1) is not important. These experimental rate constants were matched to calculated statistical rate constants to assign threshold energies, which are 264, 268, and 297 kJ mol(-1), respectively, for isomerisation, HCl elimination, and HF elimination for CFCl(2)CH(2)F and 314, 251, and 289 kJ mol(-1) in the same order for CF(2)ClCH(2)Cl. Density functional theory was used to evaluate the models that were needed for the statistical rate constants; the computational method was B3PW91/6-31G(d',p'). Threshold energies for the unimolecular reactions of CF(2)ClCH(2)Cl and CFCl(2)CH(2)F are compared to those for CF(2)ClCH(3) and CFCl(2)CH(3) to illustrate the elevation of threshold energies by F- or Cl-atom substitution at the beta carbon atom (identified by C(H)). The DFT calculations systematically underestimate the threshold energy for HCl elimination.