Rate Coefficients for the Gas Phase Reactions of CF3CH2F (HFC-134a) with Chlorine and Fluorine Atoms: Experimental and ab Initio Theoretical Studies

The first direct measurements of rate coefficients for the hydrogen abstraction reactions from CF 3 CH 2 F by chlorine or fluorine atoms in gas phase are reported as a function of temperature using the discharge flow/mass spectrometric technique (DF/MS). The reactions were investigated under pseudo-first-order conditions with Cl or F atoms in large excess with respect to the haloethane. The temperature dependence of the rate coefficients is expressed in the Arrhenius form. For the reaction Cl + CF 3 CH 2 F, the Arrhenius expression over the temperature range 298−423 K was found to be (3.2 ± 0.3) × 10 -12 exp [−(2300 ± 70)/ T ] and for the reaction F + CF 3 CH 2 F over the range 296−381 K, (6. ) × 10 -11 exp[−(1100 ± 100)/ T ]. The units of the rate constants are cm 3 molecule -1 s -1, and the quoted uncertainties are within the 95% confidence range. Ab initio molecular orbital calculations for the H-abstraction from CF 3 CH 2 F by F and Cl were performed. The characteristics (energies, structural parameters, and frequencies) of the reactants, products, and transition states were determined at the MP2/6-31G(d,p) level of theory. Single-point energy calculations have been performed at MP4/6-31G(d,p), MP2/6-311G(d,p), and MP2/6-311G(2d,2p) levels. ISO-M energy corrections have been applied leading to reaction enthalpies and activation energies in a good accord with experimental findings. Calculated rate constants at room temperature including one-dimensional tunneling are in agreement with experiment.