A KINETIC-STUDY ON THE REACTION OF CHF3 WITH H AT HIGH-TEMPERATURES

The reaction of CHF3 (HFC-23) with H atoms has been investigated by us ing a shock tube-atomic resonance absorption spectroscopy technique ov er the temperature range 1100-1350 K and the total concentration range 5.5 x 10(18)-8.5 x 10(18) molecules cm(-3). Ethyl iodide was used as a precursor of hydrogen atoms. The rate coefficient for the reaction C HF3 + H --> CF3 + H-2 (1a) was determined from the decay profiles of H -atom concentration to be k(1a) = 10(-9.80+/-0.10) exp[-(64.6 +/- 2.3) kJ mol(-1)/RT] cm(3) molecule(-1) s(-1) (error limits at the 1 standa rd deviation level), which is 50-50% smaller than the value recommende d by the NIST group. The rate coefficient was also calculated with the transition-state theory(TST). Structural parameters and vibrational f requencies of the reactants and transition state required for the TST calculation were obtained from an ab initio molecular orbital (MO) cal culation. The energy barrier, E-0(double dagger), which is the most se nsitive parameter in the calculation, was adjusted until the TST rate coefficient most closely matched the observed one. This fitting yielde d E-0(double dagger) = 59.0 kJ mol(-1), in excellent accord with the b arrier of 62.0 kJ mol(-1) calculated with the ab initio MO method at t he G2(MP2) level.