IS THE NH3-NH3 RIDDLE SOLVED

Vibration-rotation-tunnelling (VRT) splittings have been computed for the dimer (NH3)(2) by the use of four different model potentials, whic h have different barriers to internal rotations and to the interchange of the donor and the acceptor in the hydrogen bond. The six-dimension al nuclear motion problem is solved variationally for J = 0 and J = 1 in a symmetry adapted basis consisting of analytic radial functions an d rigid rotor functions depending on the five internal angles. Dipole moments, nuclear quadrupole splittings and the amount of quenching of the monomer umbrella inversions are also computed. Good agreement with the experimental data available for (NH3)(2) is obtained for a potent ial that has an equilibrium hydrogen-bonded structure close to linear, but a low interchange barrier (24 cm(-1)). Although even the mixed or tho-para states have large amplitude motions with this potential, our calculations on (ND3)(2) still explain the near absence of shifts in t he nuclear quadrupole splittings and the observed change in the dipole moment upon isotope substitution.