The reaction of CHF 3 (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 × 10 18 -8.5 × 10 18 molecules cm -3 . Ethyl iodide was used as a precursor of hydrogen atoms. The rate coefficient for the reaction CHF 3 + H f CF 3 + 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 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, E 0 q , 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 E 0 q) 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.

The reactions of CF2(X1A1) radicals with O(3P) and H atoms have been studied by using a shock tube/atomic resonance absorption spectroscopy technique over the temperature ranges 2000−2430 and 1450−1860 K and the total density range 6.1 × 1018 to 1.2 × 1019 molecules cm-3. Nitrous oxide and ethyl iodide were used as precursors of O(3P) and H atoms, respectively. Electronically ground state CF2(X1A1) radicals were produced through the thermal decomposition of chlorodifluoromethane. The rate coefficients for the reactions CF2(X1A1) + O(3P) and CF2(X1A1) + H were obtained from the decay profiles of O and H atom concentrations as k(CF2+O) = 10-10.39±0.07 and k(CF2+H) = 10-10.18±0.21 exp[−(19.0 ± 6.7) kJ mol-1/RT] cm3 molecule-1 s-1 (error limits at the two standard deviation level). Neither rate coefficient had any pressure dependence under the present experimental conditions. The G2-level ab initio molecular orbital calculation was also performed to examine the product channels for the CF2(X1A1) + O(3P) and CF2(X1A1) + H reactions. The theoretical calculation showed that the most energetically favorable pathways for CF2(X1A1) + O(3P) and CF2(X1A1) + H systems were the channels producing FCO + F and CF + HF, respectively. The G2 energy of the transition state for the channel CF2(X1A1) + O(3P) → FCO + F was 116 kJ mol-1 lower than that of the reactants CF2(X1A1) + O(3P), while the energy of the three-centered transition state for the channel CF2(X1A1) + H → CF + HF is 45 kJ mol-1 higher than that of the reactants CF2(X1A1) + H. These results could qualitatively explain the difference of the temperature dependence observed between k(CF2+O) and k(CF2+H).

The kinetics of the high-temperature reactions of CF 3 radicals with O( 3 P) and H atoms has been investigated experimentally and theoretically. The product channels of the CF 3 + O( 3 P) and CF 3 + H reactions were examined by calculating their branching fractions with the multichannel Rice-Ramsperger-Kassel-Marcus (RRKM) theory. Structural parameters, vibrational frequencies, and threshold energy required for the RRKM calculation were obtained from an ab initio MO calculation. The theoretical calculation showed that the productions of CF 2 O + F and CF 2 ( 1 A 1 ) + HF were the unique possible channels for the CF 3 + O( 3 P) and CF 3 + H reactions, respectively, and that the other channels such as deactivation were negligible for both the reactions. The rate coefficients for these reactions were experimentally determined by using a shock tubeatomic resonance absorption spectroscopy technique over the temperature ranges of 1900-2330 and 1150-1380 K and the total density ranges of 8.2 × 10 18 -1.2 × 10 19 and 6.1 × 10 18 -9.8 × 10 18 molecules‚cm -3 . Nitrous oxide and ethyl iodide were used as precursors of electronically ground-state oxygen and hydrogen atoms, respectively. Trifluoromethyl radicals were produced through the thermal dissociation of CF 3 I. The rate coefficients for the reactions CF 3 + O( 3 P) f CF 2 O + F (1b) and CF 3 + H f CF 2 ( 1 A 1 ) + HF (2c) were obtained from the decay profiles of O-and H-atom concentrations as k 1b ) (2.55(0.23) × 10 -11 and k 2c ) (8.86(0.32) × 10 -11 cm 3 molecule -1 s -1 (error limits at the one standard deviation level). Neither rate coefficient had any temperature or pressure dependence under the present experimental conditions; the values were in good agreement with some room-temperature data reported previously.