Theory of ion-polar molecule collisions. Comparison with experimental charge transfer reactions of rare gas ions to geometric isomers of difluorobenzene and dichloroethylene

Abstract

A classical theory for the ion-permanent dipole interaction is developed that takes into consideration the thermal rotational energy of the polar molecule. The theory is formulated in terms of an r-dependent average orientation angle θ (r) between the dipole and the line of centers of collision. The technique allows quantitative determination of the capture cross section as a function of ion-polar molecule relative velocity. In addition, capture limit rate constants are readily calculated both at thermal energies and as a function of relative energy. Charge transfer rate constants from various rare gas ions to difluorobenzene and dichloroethylene isomers have been measured at thermal energies using ion cyclotron resonance spectroscopy. Rate constants are considerably larger than predicted by the capture theories for both polar and nonpolar isomers indicating long range electron jump is a prevalent mechanism for charge transfer in these systems. The average-dipole-orientation theory developed in this paper adequately accounts for the effects of the permanent dipole moment on the rate constant.