Unimolecular HCl and HF Elimination Reactions of 1,2-Dichloroethane, 1,2-Difluoroethane,and 1,2-Chlorofluoroethane: Assignment of Threshold Energies

摘要

The recombination of CH_2Cl and CH_2F radicals generates vibrationally excited CH_2ClCH_2Cl, CH_2FCH_2F, and CH_2ClCH_2F molecules with about 90 kcal mol~(-1) of energy in a room temperature bath gas. New experimental data for CH_2ClCH_2F have been obtained that are combined with previously published studies for C_2H_4Cl_2 and C_2H_4F_2 to define reliable rate constants of 3.0 × 10~8 (C_2H_4F_2), 2.4 × 10~8 (C_2H_4Cl_2), and 1.9 × 10~8 (CH_2ClCH_2F) s~(-1) for HCl and HF elimination. The product branching ratio for CH_2ClCH_2F 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 transitionstate 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_2H_4F_2 and C_2H_4Cl_2 are both 63 ± 2 kcal mol~(-1). The threshold energies for CH_2ClCH_2F 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_2H_5Cl or C_2H_5F, and β-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.