Low Temperature Structure and Supramolecular Interactions of the C60-Cubane Cocrystal

The rotor-stator phase of the fullerene-cubane cocrystal, C-60 center dot C8H8, has an orientational ordering phase transition at around 140 K. On the basis of a simulated annealing analysis of the powder X-ray diffraction data, we determined the previously unknown crystal structure of the orthorhombic oriented phase: the space group is Pnma, the positions and the orientations of the constituent molecules significantly changed relative to the rock salt structure of the ambient temperature phase. The experimental structure is confirmed by cohesive energy calculations based on a pair potential method. Related structures, allowed by symmetry considerations, can be excluded by both the experiments and the calculations. The stability of the oriented phase is predominantly controlled by the nearest neighbor fullerene-fullerene and fullerene-cubane interactions.