Computational exploration of the six-dimensional vibration–rotation–tunneling dynamics of (NH3)2

Abstract

In order to address the well-known problem that the nearly cyclic structure of (NH3)2 deduced from microwave spectra differs greatly from the hydrogen-bonded equilibrium structure obtained from ab initio calculations, we have calculated the vibration–rotation–tunneling (VRT) states of this complex, and explicitly studied the effects of vibrational averaging. The potential used is a spherical expansion of a site–site potential which was extracted from ab initio data. The six-dimensional VRT wave functions for all the lowest states with J=0 and J=1 were expanded in products of radial (van der Waals stretch) functions and free-rotor states for the internal and overall rotations, which were first adapted to the complete nuclear permutation inversion group G36. Although the (expanded) potential is too approximate to expect quantitative agreement with the observed microwave and far-infrared spectra, we do find several interesting features: The 14N quadrupole splittings and the dipole moment of the complex, which are indicative of the orientational distributions of the NH3 monomers, are substantially affected by vibrational averaging. The interchange tunneling of the two monomers is not quenched. In the ortho–ortho and para–para states, of A and E symmetry, this tunneling manifests itself in a very different manner than in the ortho–para states of G symmetry. In contrast with the interpretation of Nelson et al. [J. Chem. Phys. 87, 6364 (1987)], we believe that the Gα and Gβ states observed by these authors correspond to a single VRT state which is split by (hindered) NH3 monomer inversion.