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

Abstract

The recombination of CH2Cl and CH2F radicals generates vibrationally excited CH2ClCH2Cl, CH2FCH2F, and CH2ClCH2F molecules with about 90 kcal mol−1 of energy in a room temperature bath gas. New experimental data for CH2ClCH2F have been obtained that are combined with previously published studies for C2H4Cl2 and C2H4F2 to define reliable rate constants of 3.0 × 108 (C2H4F2), 2.4 × 108 (C2H4Cl2), and 1.9 × 108 (CH2ClCH2F) s−1 for HCl and HF elimination. The product branching ratio for CH2ClCH2F 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 C2H4F2 and C2H4Cl2 are both 63 ± 2 kcal mol−1. The threshold energies for CH2ClCH2F 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 C2H5Cl or C2H5F, 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