ROTATIONAL ENERGY TRANSFERS IN ION-MOLECULE SCATTERING .2. ANISOTROPIC POTENTIALS FOR CH4-H+ AND NH3-H+

Calculations have been carried out for the two title molecules, interacting with simple protons, in order to construct symmetry-adapted, scattering-oriented forms of the lowest electronic potential energy surfaces (PES) associated with the direct collisional processes where charge-exchange channels can be disregarded. The computational level of sophistication involved the (6-311G**) basis set quality for the GTOs employed and yielded HF-SCF total wavefunctions in which the target molecule was treated as a rigid rotor. A large number of orientations and of relative distances of the proton from the centre of mass of the molecule was employed in order to obtain the various coefficients of the corresponding, symmetry-adapted, multipolar expansions. The long-range region of interaction was also included by using the corresponding analytic forms of the coefficients with the lowest inverse power dependence of the relative distance and by matching them via smooth interpolation to the inner region given by the SCF results. The general shapes of the resulting hypersurfaces and their bearing on dynamically-induced energy transfer mechanisms are analyzed and discussed. Marked differences are found between the relative anisotropies of the two PES produced by the present calculations.