THEORETICAL PREDICTION OF THE STRUCTURE AND INFRARED-SPECTRUM OF THE MOLECULE ION COMPLEXES NH3-H-, NH3-D-, AND ND3-H-

The potential energy and dipole moment functions of the molecule-ion c omplex NH3-H-, and its isotopomers, NH3-D- and ND3-H-, have been calcu lated by the CEPA-2 method. From these functions we have computed the vibration-rotation-inversion states for J=0 and J=1, and the rovibrati onal transition probabilities between them. The complexes are found to have a nearly rigid structure in the ground state, with the H- or D- ion localized near a hydrogen (or deuterium) atom of the ammonia, and a small probability of tunneling between the three equivalent equilibr ium positions. For the vibrationally excited states, however, the prob ability of this threefold tunneling increases considerably. The umbrel la inversion of the ammonia is nearly quenched by the presence of the ion. The character of the rovibrational excitations is determined, and is found to be affected by the isotope substitutions. In order to inv estigate whether it is possible to evaluate the rovibrational transiti on probabilities for other total J transitions from the present calcul ations, an approximate expression for the vibrational transition proba bilities has been derived. The formula appears to be valid for the ort ho species, for the para species it is found to be rather crude. (C) 1 995 American Institute of Physics.