On statistical and thermodynamic approaches to ion polar molecule collisions

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Abstract
Capture collision rate constants for ion polar molecule systems obtained from an average free energy function are shown to be equivalent to those obtained from a canonical activated complex theory formalism. Results provide a satisfactory least upper bound for observed reaction rate constants and compare well with Chesnavich, Su, and Bowers’ results obtained using microcanonical variational transition state theory. The average free energy approach is extended to the ion–quadrupole interaction, and capture collision rate constants are obtained providing a satisfactory least upper bound on experimental rate constants. A thermodynamic model for ion polar neutral collisions is developed and the difference between capture collision rate constants and momentum transfer collision rate constants rationalized in terms of a repulsive entropic force which prevents capture but produces isotropic scattering. The success of the results of Barker and Ridge in accounting for momentum transfer rates is thus explained and connected to the success of the average free energy result in accounting for reaction rate constants. It is argued that the agreement between theory and experiment indicates that rotational inelasticity plays a significant role in a gas phase transport property, i.e., the momentum transfer collision rate of ions in polar gases.