Crystal structure and bonding of ordered C60

TL;DR: In this paper, the packing configuration of C60 molecules is revealed in an anticlockwise manner around the [111] direction by ∼98° from the ideal Fm3¯ configuration, which is due to an optimized ordering scheme in which electron-rich short inter-pentagon bonds face the electron-poor pentagon centres of adjacent C60 units.

Abstract: STRUCTURAL studies1–.3 of crystalline C60 (ref. 4) have indicated that at room temperature the C60 molecules are orientationally disordered and the crystal structure may be regarded as a face-centred cubic configuration of C60 spheres. Below 249 K, however, the molecules become orientationally ordered3 and a simple cubic lattice results, corresponding to a symmetry change from Fm3¯ to Pa3¯. Here we present the results of a neutron powder diffraction study of the low-temperature ordered structure, which reveals the packing configuration of the C60 molecules. The C60 units are rotated in an anticlockwise manner around the [111] direction by ∼98° from the ideal Fm3¯ configuration. This apparently arbitrary rotation in fact results from an optimized ordering scheme in which electron-rich short (1.391-A) inter-pentagon bonds face the electron-poor pentagon centres of adjacent C60 units. The high symmetry of the C60 molecule allows these interactions to be optimized identically for all twelve nearest neighbours, a possibility that is by no means intuitively obvious. The bonds common to a given pentagon are somewhat longer (1.455 A). The high degree of bonding optimization and the absence of bonding frustration accounts for the high ordering temperature of 249 K (ref. 5).