Rotation/precession of NH3 groups in Hofmann clathrates

Vibrational spectra, crystallography and rotational spectra of the Hofmann clathrate Ni(NH3)(2)Ni(CN)(4) . 2C(6)D(6) are all consistent with the displacement of the N atom during the free rotation of the NH3 groups. Molecular dynamics calculations using a slightly modified universal force field reproduce the measured inelastic neutron scattering spectrum and crystal structure. An examination of the H and N atom trajectories from a simulation show that the NH3 rotation is best described as a rigid rotor in which the H3 triangle turns around the Ni-N axis whilst the Ni-N axis precesses round the crystallographic z-direction.