First single-crystal X-ray structural refinement of the rhombohedral C60 polymer

The single crystals of the rhombohedral C60 polymer were prepared by the polymerization of C60 single crystals under a high pressure of 5 GPa at 773 K. The X-ray structure analysis revealed that it crystallized in a rhombohedral space group R3̄m, Z=3 with a=9.175(1) Å, c=24.568(3) Å. The refinement of 49 parameters with 442 data converged to R1=0.0679 (for I>2σ(I)) and wR2=0.1586 (all data). The crystal structure is composed of hexagonal 2D layers formed by the polymerization of C60 molecules via [2+2] cycloaddition. Three of those layers are stacked rhombohedrally in the unit cell along the c-axis and held together by C…C van der Waals forces. The present structural model differs from a well-known proposed one, in which each C60 unit rotates 60° about the crystallographic 3-fold axis. This is the first single-crystal X-ray structural refinement of polymerized C60.