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Abstract:
The crystal structure of cyclobutane was determined from X-ray diffraction data at 117 K. The crystal used was grown by sublimation. Since a rotationally-disordered high-temperature phase is present over a temperature range of 38 deg, no crystals could be obtained from the melt. The cell dimensions at 117 K are a=6.742(4)Å, b=4.204(4)Å, c=6.917(4)Å, β=90.17(4)°, space group I2/m, Z=2. The space group symmetry requires the space- or time-averaged molecules to have C2h(2/m) symmetry. The carbon atoms show initially large anisotropic displacement parameters perpendicular to the mirror plane. Two explanations are discussed. The first is based on earlier investigations and assumes the time-averaged molecules to undergo puckering motions, which is established for the gas phase and the solid state. The other explanation which is more compatible with the experimental data is a statistical disorder of bent molecules. The latter results in a dihedral angle of 31(2)°, which agrees with most other investigations.
