DISORDER AND THE MOLECULAR PACKING OF C-60 BUCKMINSTERFULLERENE - A DIRECT ELECTRON-CRYSTALLOGRAPHIC ANALYSIS

The crystal structure of C60 buckminsterfullerene was determined at room temperature by a direct phasing analysis of single-crystal electron-diffraction intensity data. The initial electrostatic potential map is well fit by a regular icosahedron of C atoms but with an average rotational disorder corresponding to Fm3m symmetry. The static appearance of this directly determined map. however, does not refute the notion of uncorrelated molecular positions in the crystal lattice. indicated earlier by nuclear-magnetic-resonance spectroscopy and neutron scattering. Although the direct determination of crystallographic phases is correct. the occurrence of strong axial h00 reflections in the electron diffraction patterns appears to be a result of secondary scattering. Correction for this perturbation produces a good fit of the intensities to an isotropic spherical shell of C atoms. In fact, the static appearance of the initial potential map is artificial, owing to the use of only a limited set of phased structure factors in the Fourier transform carried out after the ab initio structure analysis.