Isomerisation of CF2ClCH2Cl and CFCl2CH2F by interchange of Cl and F atoms with analysis of the unimolecular reactions of both molecules.

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.