Dissociation, relaxation, and incubation in the pyrolysis of 1,1,1-trifluoroethane and ethane

We report a shock-tube, laser-schlieren investigation of the molecular dissociation of the title reaction trifluoroethane, CF 3CH3 → CH2CF2 + HF, over very high temperatures, 1600--2400 K, and a wide range of sub-atmospheric pressures, 15--550 Torr. The density gradients are well fit by a simple two-reaction mechanism and accurate dissociation rates obtained.; The results are compared with a kinfinity calculated from a G3B3 TS for this molecular elimination, which is a superb fit to the available lower-T data and a reliable extrapolation of kinfinity to high temperatures. The derived rate constants show a very deep falloff from this extrapolation but surprisingly little variation with pressure. This peculiarity is so severe that RRKM calculations dramatically fail to account for the behavior. The dissociation seems to be a clear example of an intrinsic non-RRKM process (non-statistical dissociation). This conclusion is strongly supported by the observation of double vibrational relaxation at both dissociating and non-dissociating temperatures, an unambiguous demonstration of slow IVR.; Using a simple model with division into two groups of states, the deep falloff is found to be consistent with a rate-controlling slow IVR, not with low collision efficiency. The model suggests an IVR rate of ∼10 8 s-1 for dissociation energies.; The above mentioned non-RRKM dissociation in CF3CH3 led us to study the thermal dissociation of ethane, which has long been known to exhibit double relaxation at room-T, in two complementary sets of shock-tube experiments that together cover 1400--2200 K and a pressure range of 70 to 5700 torr. These experiments used both laser-schlieren and W absorption of CH3 as diagnostics.; The fact that ethane exhibits a double vibrational relaxation has been confirmed for high temperatures, demonstrating slow IVR in this molecule, and both relaxation times have been determined for 1000--1500 K. Some estimates of dissociation incubation times were possible over about 1900--2100K. An RRKM model calculation using the earlier Klippenstein-Harding theoretical model provides a remarkably accurate description of the complete range of data with <DeltaE>down = 120(T/300)0.9 cm -1. Over 1400--2200 K the kinfinity from this model is 8.03 x 1028 T-3.52 exp (-95346 cal/mol/RT) s-1. It is suggested that the success of this model shows that there is no non-RRKM reaction in this molecule.