STRUCTURE, INTERNAL MOBILITY, AND SPECTRUM OF THE AMMONIA DIMER - CALCULATION OF THE VIBRATION-ROTATION-TUNNELING STATES

We have obtained a potential for (NH3)(2) by calculating the six-dimen sional vibration-rotation-tunneling (VRT) states from a model potentia l with some variable parameters, and adjusting some calculated transit ion frequencies to the observed far-infrared spectrum. The equilibrium geometry is strongly bent away from a linear hydrogen bonded structur e. Equivalent minima with the proton donor and acceptor interchanged a re separated by a barrier of only 7 cm(-1). The barriers to rotation o f the monomers about their C-3 axes are much higher. The VRT levels fr om this potential agree to about 0.25 cm(-1) with all far-infrared fre quencies of (NH3)(2) observed for K=0, \K\=1, and \K\=2 and for all th e symmetry species: A(i)=ortho-ortho, E(i)=para-para, and G=ortho-para . Moreover, the dipole moments and the nuclear quadrupole splittings a gree well with the values that are observed for the G states. The pote ntial has been explicitly transformed to the center-of-mass coordinate s of (ND3)(2) and used to study the effects of the deuteration on the VRT states. The observed decrease of the dipole moment and the (small) changes in the nuclear quadrupole splittings are well reproduced. It follows from our calculations that the ammonia dimer is highly nonrigi d and that vibrational averaging effects are essential. Seemingly cont radictory effects of this averaging on its properties are the conseque nce of the different hindered rotor behavior of ortho and para monomer s.