Kinetics of the Reactions of F- with CF3Br and CF3I as a Function of Temperature, Kinetic Energy, Internal Temperature, and Pressure

Abstract

Abstract : The reactions of F- with CF3Br and CF3I have been studied at several temperatures as functions of pressure and ion-neutral average center-of-mass kinetic energy ((KE sub cm)). Rate constants and product branching fractions were measured by using a variable temperature-selected ion flow drift tube apparatus. Both title reactions involve two product pathways: displacement and association, the latter being the major channel under most experimental conditions. The rate constants for displacement are governed by total energy, regardless of the relative contributions by translational and internal energy, indicating that the displacement reactions behave statistically. In contrast, the rate constants for association depend strongly on the internal energy of CF3X (rotations and vibrations) compared with the relatively weak effect of translational collision energy. For CF3Br the rate constants for the association channel increase with increasing pressure while those for displacement are independent of pressure, indicating that the additional reactivity leading to more association at higher pressure appears not to compete with the displacement pathway. This lack of competition may be the result of two different reaction intermediates. The more efficient CF3I reaction is at or near the high-pressure limit for association, showing no pressure dependence in both the association and displacement channels. Therefore, there appears to be no competition between these channels for CF3I as well, but the data are less definitive in this case.