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 ab straction reactions from CF3CH2F by chlorine or fluorine atoms in gas phase are reported as a function of temperature using the discharge fl ow/mass spectrometric technique (DF/MS). The reactions were investigat ed under pseudo-first-order conditions with Cl or F atoms in large exc ess with respect to the haloethane. The temperature dependence of the rate coefficients is expressed in the Arrhenius form. For the reaction Cl + CF3CH2F, the Arrhenius expression over the temperature range 298 -423 K was found to be (3.2 +/- 0.3) x 10(-12) exp [-(2300 +/- 70)/T] and for the reaction F + CF3CH2F over the range 296-381 K, (6.5(-1.6)( +2.1)) x 10(-11) exp[-(1100 +/- 100)/T]. The units of the rate constan ts are cm(3) molecule(-1) s(-1), and the quoted uncertainties are with in the 95% confidence range. Ab initio molecular orbital calculations for the H-abstraction from CF3CH2F by F and Cl were performed. The cha racteristics (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 bee n 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.