Is the NH3–NH3 riddle solved?

Abstract

Vibration–rotation–tunnelling (VRT) splittings have been computed for the dimer (NH₃)₂ by the use of four different model potentials, which have different barriers to internal rotations and to the interchange of the donor and the acceptor in the hydrogen bond. The six-dimensional nuclear motion problem is solved variationally for J= 0 and J= 1 in a symmetry adapted basis consisting of analytic radial functions and 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 potential that has an equilibrium hydrogenbonded structure close to linear, but a low interchange barrier (24 cm^–1). Although even the mixed ortho–para states have large amplitude motions with this potential, our calculations on (ND₃)₂ still explain the near absence of shifts in the nuclear quadrupole splittings and the observed change in the dipole moment upon isotope substitution.