Orientational disorder as a function of temperature in the clathrate structure of hydroquinone and C60

In the compound [C6H6O2](3)C-60, hydroquinone (C6H6O2) forms a three-dimensional hydrogen-bonded network enclosing roughly spherical cages with point symmetry (3) over barm and a diameter of 13.2 Angstrom at 100 K. Although C-60 fits tightly into these cages, it shows threefold orientational disorder, the molecular site symmetry being 2/m. The disorder has been studied with single-crystal Mo K alpha X-ray data at four temperatures, 100, 200, 293 and 373 K. In the refinement, C-60 was restrained to the icosahedral molecular symmetry m(35) over bar and to rigid-body translational and librational displacements including third- and fourth-order cumulants to account for curvilinear atomic movements, R(|F|) = 3.2-4.7%. At 100 K, bond lengths in C-60 refine to the expected values 1.450 (1) and 1.388 (1) Angstrom. The ratio of these values increases with increasing temperature, but the radius of the molecule remains constant at 3.537 (2) Angstrom. The r.m.s. libration amplitudes of C-60 are relatively small (5.5 degrees at 373 K) and the probability function of orientations of C-60 inside the cage shows large values only at the refined positions, indicating that the energy barrier of reorientation is large. Refinement of an ordered twinned structure was unsuccessful; the orientations of neighboring C-60 appear to be uncorrelated.