Rate constant calculations for ion - symmetric top and ion - asymmetric top reactions.

Rate constant calculations for fast reactions between ions and various polyatomic molecules are presented. The method, which uses a rotationally adiabatic capture theory, has been applied previously to linear molecules and symmetric tops. It is extended here to the case where the interacting molecule is an asymmetric top. Results from calculations using an analytical potential (including charge-dipole + charge-induced dipole contributions) are compared with those obtained using an ab initio potential-energy surface for the H++ H2O and H++ NH3 reactions. Analytical formulae are derived for the rate constants in the limit of zero temperature by using perturbation theory. At high temperature the results are compared with those obtained using the infinite order sudden approximation (IOSA). Calculations for the ion-symmetric top and ion-asymmetric top interaction are also compared for the H+-+ H2O and H++ H2S reactions. The differences between the symmetric and asymmetric top rate constants calculated at low temperature are discussed and an explanation is proposed.