The tunneling frequencies of NH4+ and NH3D+ in (NH4)2PdCl6

Abstract Is the hindered rotation of NH 3 D + in ammonium solids one or three dimensional? Results of a recent inelastic neutron scattering study of NH 3 D + in (NH 4 ) 2 PdCl 6 indicate that NH 3 D + is a one-dimensional rotor in this solid. The tunneling frequencies of NH 3 D + 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 NH 3 D + is three dimensional. The tunneling frequencies of NH 3 D + in the cubic (NH 4 ) 2 PdCl 6 and in the low temperature monoclinic phase of (ND 4 ) 2 PdCl 6 have been computed and compared to the tunneling frequencies that had been observed in [(NH 4 ) 2 x (ND 4 ) 2(1− x ) ]PdCl 6 ( x =0, 0.1, 0.3, and 0.5). The results indicate that the motion of NH 3 D + in cubic ammonium solids is three dimensional, but in the monoclinic phase the motion is one dimensional. The tunneling frequency of NH 3 D + in NH 4 ClO 4 has also been computed. The computed results indicate that one-dimensional motion can always be expected when NH 3 D + is in a crystalline field of C 1 symmetry. Rules are presented for which site symmetries will yield one-dimensional rotation for NH 3 D + .