Abstract :
Is the hindered rotation of NH3D+ in ammonium solids one or three dimensional? Results of a recent inelastic neutron scattering study of NH3D+ in (NH4)2PdCl6 indicate that NH3D+ is a one-dimensional rotor in this solid. The tunneling frequencies of NH3D+ in a tetrahedral field have been computed using only the first term (J=3) in the rotational potential function. The results indicate that the motion of NH3D+ is three dimensional. The tunneling frequencies of NH3D+ in the cubic (NH4)2PdCl6 and in the low temperature monoclinic phase of (ND4)2PdCl6 have been computed and compared to the tunneling frequencies that had been observed in [(NH4)2x(ND4)2(1−x)]PdCl6 (x=0, 0.1, 0.3, and 0.5). The results indicate that the motion of NH3D+ in cubic ammonium solids is three dimensional, but in the monoclinic phase the motion is one dimensional. The tunneling frequency of NH3D+ in NH4ClO4 has also been computed. The computed results indicate that one-dimensional motion can always be expected when NH3D+ is in a crystalline field of C1 symmetry. Rules are presented for which site symmetries will yield one-dimensional rotation for NH3D+.
