KINETICS OF THE REACTIONS OF F- WITH CF3BR AND CF3I AS A FUNCTION OF TEMPERATURE, KINETIC-ENERGY, INTERNAL TEMPERATURE, AND PRESSURE

The reactions of F- with CF3Br and CF3I have been studied at several t emperatures as functions of pressure and ion-neutral average center-of -mass kinetic energy ((KE(cm))). Rate constants and product branching fractions were measured by using a variable temperature-selected ion f low drift tube apparatus. Both title reactions involve two product pat hways: displacement and association, the latter being the major channe l under most experimental conditions. The rate constants for displacem ent are governed by total energy, regardless of the relative contribut ions by translational and internal energy, indicating that the displac ement reactions behave statistically. In contrast, the rate constants for association depend strongly on the internal energy of CF(3)X (rota tions and vibrations) compared with the relatively weak effect of tran slational collision energy. For CF3Br the rate constants for the assoc iation channel increase with increasing pressure while those for displ acement are independent of pressure, indicating that the additional re activity leading to more association at higher pressure appears not to compete with the displacement pathway. This lack of competition may b e the result of two different reaction intermediates. The more efficie nt CF3I reaction is at or near the high-pressure limit for association , showing no pressure dependence in both the association and displacem ent channels. Therefore, there appears to be no competition between th ese channels for CF3I as well, but the data are less definitive in thi s case.