Theory of ion-molecule collisions: the effect of the anisotropy in the polarizability on the collision rate constant

A classical treatment is used to estimate the increase in ion-molecule collision rates attributable to the anisotropy of the molecular polarizability, for linear and symmetric top molecules. The treatment used is directly analogous to the average dipole orientation (ADO) theory and the average quadrupole orientation (AQO) theory. Rate constants calculated for a number of thermal energy proton transfer reactions from D+3 to neutral species at 300 K show that the anisotropy will have a relatively insignificant effect on these rates (0-2%). An expression is obtained for the maximum effect (in the limit of zero neutral rotational energy) of the anisotropy on reaction rates. It is shown that the anisotropy effect will usually not be important, except at very low temperatures, and will generally be smaller than the quadrupole effect. (Auth.)