DISORDERED STRUCTURE OF THE CUBIC PHASE OF QUINUCLIDINE AT 295-K

Single-crystal X-ray diffraction data were collected and interpreted for the plastic phase of quinuclidine at 295 K. The unit-cell is face-centered cubic with a equals 8. 913 A, space group Fm3m. Assuming a rigid molecular skeleton, several types of orientational disorder were investigated: isotropic reorientations of the molecules about their center of gravity and hindered reorientations between equally weighted orientations. For the second model, there is a fair agreement between observed and calculated structure amplitudes (residual R prime equals 4. 5%); the packing has been found by a Monte Carlo method coupled with a rigid-group least-squares refinement. The threefold molecular and crystal axis do not coincide and there are 24 possible orientations for each molecule.