Scattering of NH3 by ortho- and para-H2: Expansion of the potential and collisional propensity rules

We present the general expansion of the interaction potential between a rigid, but invertible symmetric top and a linear molecule. The development is derived from the average over electronic coordinates of the fundamental electrostatic interaction. Our expansion for the electrostatic potential takes into account explicitly the symmetry under inversion of the position of all the nuclei, which was not included in earlier expansions. A numerical fit of calculated ab initio points for the NH3--H2 system suggests the presence of a similar inversion symmetry for the total interaction potential. We then use our expansion to derive, in the Born and energy sudden limits, propensity rules for collisions of NH3 with ortho- and para-hydrogen. Although at a formal level these propensity rules are equally valid for both hydrogen rotomers, close coupling calculations show that they are considerably weakened for o-H2. This is a manifestation of the additional anisotropy in the interaction between NH3 and a non-spherical collision partner. Notwithstanding, evidence of these propensity rules is clear, in contrast to earlier calculations based on a preliminary version of the NH3--H2 interaction potential used here