STRUCTURE OF C-60 - PARTIAL ORIENTATIONAL ORDER IN THE ROOM-TEMPERATURE MODIFICATION OF C-60

Using published synchrotron X-ray data, the room-temperature scattering density distribution of pure C60 has been parametrized in terms of a combination of eight oriented symmetry-related images of the molecule, and of a freely spinning molecule. Corresponding populations are 61 and 39%. The oriented part of the model is obtained, in good approximation, by imposing m3mBAR symmetry on the energetically more favourable major orientation in the low-temperature structure of C60. The model was refined using angle restraints to impose the icosahedral molecular symmetry and displacement-factor restraints to restrict thermal movements to rigid-body translations and librations. Translational displacement factors are in the range 0.0 1 7-0.023 Al. The orientational probability density distribution obtained from the model shows maxima for C60 orientations possessing 3m crystallographic site symmetry. It also relatively large for the C60 orientations with cubic site symmetry m3BAR. The smallest energy barrier for reorientation between different 3mBAR orientations via an ml orientation appears to be less than 2kJmol-1. On average, 75% of the intermolecular contacts of the oriented molecules are longer than those observed in the low-temperature structure, the other 25% are less favourable. The second orientation of C60 found in the low-temperature structure could not be identified at room temperature.