STRUCTURAL DETERMINATION OF THE LOW TEMPERATURE PHASES OF GLOBULAR ORGANIC MOLECULES BY POWDER X-RAY DIFFRACTION A.J.MORA^1,2 and A.N.FITCH³ Department of Chemistry, Keele University, Keele, Staffordshire, ST5 2BG, UK ²Departamento de Química, Facultad de Ciencias, Universidad de los Andes, Mérida, 5101, Venezuela ³European Synchrotron Radiation Facility, BP220, 38043 Grenoble Cedex, France.

Globular molecules are often orientationally disordered at room temperature and exist in the so-called "plastic state". It is only at low temperatures (or high pressures) that they order. These ordered phases highlight the subtle interplay between steric and van-der-Waals interactions, neither of which are strong because of the globular nature of the molecules. Although carefully studied by calorimetric and spectroscopic techniques, the arrangement of the molecules at low temperature is often unknown, since crystals tend to fragment during the phase transitions. High resolution powder diffraction provides an alternative to the more traditional single-crystal approach. In particular, we have used this technique to study the ordered crystal structures of bicyclic organic molecules using the synchrotron source at Daresbury Laboratory, UK. The table shows the cell parameters, space group and transition temperature to ordered phases for some of the molecules we have investigated. We present in more detail the structural solution and Rietveld refinements of 3-azabicyclo[3.2.2.]nonane, bicyclo[3.3.1.]nonan-9-one and the racemic solid solution of R- and S-camphor.