THE NATURE OF MONOMER INVERSION IN THE AMMONIA DIMER

A model is presented for calculating the splittings due to umbrella in version of the monomers in (NH3)(2). Input to the model are the six-di mensional dimer bound state wave functions for rigid monomers, calcula ted previously [E.H.T. Olthof, A. van der Avoird, and P.E.S Wormer, J. Chem. Phys. 101, 8430 (1994)]. This model is based on first-order (qu asi) degenerate perturbation theory and adaptation of the wave functio ns to the-group chain G(36)subset of G(72)subset of G(144). The umbrel la inversion splittings depend sensitively on the intermolecular poten tial from which the bound state wave functions are obtained. A complet e interpretation of the observed splining pattern [J.G. Loeser, C.A. S chmuttenmaer, R.C. Cohen, M.J. Elrod, D.W. Steyert, R.J. Saykally, R.E . Bumgarner, and G.A. Blake, J. Chem. Phys. 97, 4727 (1992)] and quant itative agreement with the measured splittings, which range over three orders of magnitude, are obtained from the potential that reproduces the far-infrared spectrum of (NH3)(2) and the dipole moment and nuclea r quadrupole splittings of (NH3)(2) and (ND3)(2). The umbrella inversi on splittings of (ND3)(2) are predicted.