The Kinetics of the Reaction of H Atoms with C4F6

The rate constant k for the gas-phase reaction of atomic hydrogen with 1,1,2,3,4,4-hexafluoro-1,3-butadiene has been measured over the temperature range 290−1010 K by time-resolved atomic resonance fluorescence using H 2 O and NH 3 as precursors, at argon pressures of about 30−140 mbar. Over the range 290−620 K a positive activation energy was observed, k = (3.40 ± 0.34) × 10 -11 exp[(−16.7 ± 0.4) kJ mol -1 / RT ] cm 3 molecule -1 s -1, and k dropped rapidly between 620 and 700 K. Over the range 700−1010 K, k = (1.83 ± 0.53) × 10 -10 exp[(−34.1 ± 2.1) kJ mol -1 /RT] cm 3 molecule -1 s -1 where the statistical uncertainties are 1σ. Confidence limits for k are ± 16%. Several isomers of C 4 F 6 H were characterized by ab initio methods. Below about 600 K the dominant pathway is suggested to be addition of H atoms to form CF 2 H−CF•−CF CF 2 and/or CF 2 H−CF•−CF CF 2 . These adducts dissociate at elevated temperatures, and on the assumption that the major addition product at around 670 K is the more stable CF 2 H−CF·-CF CF 2, the C−H bond dissociation enthalpy is estimated via a third-law method to be about −111 kJ mol -1, with an estimated uncertainty of ± 7 kJ mol -1 .