Refinement strategies for fullerene structures: use of local, non-crystallographical point group symmetry

Abstract

The crystal structure of C70·Se8·CS2 was determined from single crystal X-ray diffraction data at 100 K. Due to some peculiarities of the space group Pca21 and the presence of local centres of symmetry, the crystal structure refinement turned out to be problematic. Instead of stabilizing the refinement procedure by means of a very high number of restraints about direct neighbour distances and -angles, and the shape of the probability density function of the carbon atoms of the fullerene molecules, the molecular structure of the C70 molecules was treated as a rigid body applying a local symmetry restraint in a local coordinate framework, and the TLS-formalism, using the JANA2000 program.The crystal structure may be derived from the NiAs structure type, with the fullerene molecules forming a highly distorted hexagonal closed packing. In order to enable lone-pair-p-interactions between the fullerene and the chalcogene molecules, the Se8-rings are shifted from the centres of the octahedral voids towards the triangular faces.