Rotational tunneling of NH4 and NH3D and dipolar interaction in (NH3.6D0.4)2PtCl6

At 10% deuteration ammonium hexachloroplatinate still exhibits the cubic phase Fm3m of the protonated material at all temperatures. Rotational tunneling of the ammonium isomers NH4+ and NH3D+ was measured by neutron spectroscopy. At temperatures T⩾18 K the energies and intensities of the observed tunneling transitions agree with a statistical occurrence of ammonium isomers and a cubic environment. Below T=18 K there is no change of the NH4 spectrum while the NH3D transition adopts an unresolved broad intensity distribution before it transforms into a new structured low temperature spectrum. The intermediate spectrum is interpreted as fingerprint of a potential distribution of statistically distributed disordered NH3D ions interacting by long-range dipolar coupling. The low-temperature spectrum finds an explanation as frozen minimum energy configurations of coupled NH3D dipoles. A dipole moment of NH3D of 0.05 D can be derived from the difference of tunnel splittings.