Physical Chemistry Chemical Physics vol:7 issue:6 pages:1187-1193
The rate constants for the combination reactions CF3 + CF3 and CF3+ F at 290 K and helium pressures of approximate to1-6 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 k(1)(infinity) = (1.8 +/- 0.6) x 10(-12) cm(3) 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 k(2)(0)(He) = (1.47 +/- 0.24) x 10(-28) cm(6) s(-1), differing substantially from the only available previous determination; a variational transition state theoretical treatment is shown to support our data.