The kinetics of the CF3 + CF3 and CF3 + F combination reactions at 290 K and at He-pressures of ≈1–6 Torr

Abstract

The rate constants for the combination reactions CF₃ + CF₃ and CF₃ + F at 290 K and helium pressures of ≈1–6 Torr have been determined, using clean chemical sources of CF₃, by means of discharge flow-molecular beam sampling-threshold ionisation mass spectrometry (DF/MB-TIMS). For the mutual reaction of CF₃, no pressure dependence could be observed over the 1–6 Torr pressure range, indicating that the obtained rate constant of k₁^∞ = (1.8 ± 0.6) × 10⁻¹² cm³ s⁻¹ is the high pressure limit. This result, which agrees with the lowest values in literature but is ca. five times smaller than the most recent data, is fully in line with the known trend in the mutual reaction rate constant for the series CH₃; CH₂F; and CHF₂. The reaction of CF₃ with F was found to exhibit a clear pressure dependence in the 0.5 to 6 Torr range. Using a Troe fall-off formalism, the low-pressure limit rate constant was determined as k₂⁰(He) = (1.47 ± 0.24) × 10⁻²⁸ cm⁶ s⁻¹, differing substantially from the only available previous determination; a variational transition state theoretical treatment is shown to support our data.