Crystal and molecular structures of norbornene

The ordered, low-temperature phase of the globular organic molecule norborn ene, (bicyclo[2.2.1]hept- 2-ene, C7H10), has been determined by high resolu tion powder X-ray diffraction using synchrotron radiation, by auto-indexing , then by intensity extraction and direct methods using the EXPO package. T he structure is monoclinic, space group P2(1)/c, Z = 4, with a = 7.6063(9) Angstrom, b = 8.6220(1) Angstrom, c = 8.749(1) Angstrom, beta = 97.24(1)deg rees at 85 K. The structure was refined whilst restraining the molecule to m (C-s) symmetry as found in the gas phase. The molecular structure agrees with that obtained from ab-initio molecular orbital calculations, and is le ss distorted than that obtained from a previous solution of the structure i nvolving an unrestrained refinement from conventional powder X-ray diffract ion data (Min, Benet-Buchholz and Boese, 1998). Norbornene starts to transf orm to the hexagonal close-packed, orientationally-disordered, room-tempera ture form at around 116 K on warming.