Nearly free quantum rotations of ammonia groups in Ni-Ni-Bz(d₆) Hofmann clathrate

In the inelastic neutron scattering spectra of Ni-Ni-Bz(d₆) obtained at 1.8 K the authors observed transitions differing only slightly, but systematically, from those between uniaxial free rotator levels of ammonia groups. They interpreted the deviations as the influence of a weak hindering potential. A twelve-fold cosine potential with a barrier height V₁₂ of 16+or-3 meV together with a rotational constant of 0.71+or-0.02 meV for NH₃ gave a satisfactory description of the experimental results. Spectra obtained at higher temperatures (up to 51 K) showed qualitatively the transition from quantum to classical behaviour.