ALMOST FREE ROTATION OF NH3 MOLECULES IN CRYSTALS - OBSERVATION FROM A MAXIMUM-ENTROPY RECONSTRUCTION OF THE PROTON DENSITY IN [CO(NH3)(6)](PF6)(2)

Accurate density data yield the primary information required for a the rmodynamic model of molecular disorder. A detailed analysis of the str ucture from neutron Bragg data shows that the protons of the ammonia g roups perform nearly unhindered rotations at room temperature as well as at 30 K. The nuclear density distribution is derived from a combina tion of conventional crystallographic split-atom density interpolation and maximum-entropy density reconstruction. The crystallographic dens ity interpolation allows unique phases to be determined, i.e. phases t hat are independent of details of the model, for the measured Fourier components. MaxEnt reconstruction eliminates series-termination effect s and provides for higher quality in the spatial resolution compared w ith standard Fourier maps. The proton density obtained in [CO(NH3)(6)] (PF6)(2) at both temperatures is concentrated on a plane perpendicular to the fourfold crystal axis and is found to be nearly circular, with a weak tetragonal contribution superimposed. This is a strong indicat ion of nearly free uniaxial rotation in this compound. The proton dens ity is analysed in terms of an anharmonic orientational potential, whi ch couples rotational and translational motion. The nearly unhindered rotation in this compound is a consequence of the quasi-eightfold symm etry built up by the next-neighbour F atoms surrounding the NH3 groups in their plane of rotation.