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Proton density and orientational potential in nickelhexammine salts: a thermodynamic analysis of rotation-translation coupling

The scattering densities of the orientationally disordered protons and deuterons in Ni(NX3)6Y2 (X=H or D; Y=Br, I, NO3 or PF6) are obtained from neutron single-crystal diffraction data at room temperature. While we observe a nearly circular density distribution for Y=PF6, a nuclear density distribution with four maxima on a square shows up for Y=Br, I and NO3. This is a pronounced deviation from the circular distribution that is expected from uniaxial rotational diffusion or reorientational jump models for the dynamics of the orientational disordered ammonia groups. All observed density distributions are consistently explained as the consequence of rotation-translation coupling in an anharmonic crystal potential. The calculated potential parameters, i.e. anharmonicity and strength of the coupling, depend on the type of anion present in the crystal frame. The weak anisotropy in the Y=NO3 and PF6 compounds is the consequence of a nearly eightfold non-crystallographic symmetry of the atoms establishing the hindrance potential.