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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 beta = 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.
