A Kinetic Study on the Reaction of CHF₃ with H at High Temperatures

Abstract

The reaction of CHF3 (HFC-23) with H atoms has been investigated by using a shock tube−atomic resonance absorption spectroscopy technique over the temperature range 1100−1350 K and the total concentration range 5.5 × 1018−8.5 × 1018 molecules cm-3. Ethyl iodide was used as a precursor of hydrogen atoms. The rate coefficient for the reaction CHF3 + H → CF3 + H2 (1a) was determined from the decay profiles of H-atom concentration to be k1a = 10-9.80±0.10 exp[−(64.6 ± 2.3) kJ mol-1/RT] cm3 molecule-1 s-1 (error limits at the 1 standard deviation level), which is 50−60% smaller than the value recommended by the NIST group. The rate coefficient was also calculated with the transition-state theory (TST). Structural parameters and vibrational frequencies of the reactants and transition state required for the TST calculation were obtained from an ab initio molecular orbital (MO) calculation. The energy barrier, E0⧧, which is the most sensitive parameter in the calculation, was adjusted until the TST rate coefficient most closely matched the observed one. This fitting yielded E0⧧ = 59.0 kJ mol-1, in excellent accord with the barrier of 62.0 kJ mol-1 calculated with the ab initio MO method at the G2(MP2) level.