Crystal structure of cyclohexane I and II

Cyclohexane C₆H₁₂ (m.p. 279.8°K) undergoes an isothermal transition at 186°K. The so-called phase I is stable between 279.8 and 186°K, phase II below 186°K. Starting from a single crystal of I grown by a zone melting technique at 187°K, suitable single crystals of II were obtained through a careful control of the phase transformation and annealing at 185°K for two days. X-ray diffraction data have been collected at 195 and 115°K by the low-temperature precession technique. At 115°K, the crystal data are: monoclinic cell, a = 11.23 (2), b = 6.44 (2), c = 8.20 (2) Å, β = 108.8  (17)°, Z = 4; space group Cc or C2/c (the analysis confirmed the centrosymmetric group). The crystal and molecular structures were refined to an R index of 0.061; the `chair'-shaped molecule departs slightly but significantly from D_3d symmetry. The rigid-body model is a good approximation for the thermal motion. At 195°K, crystal data for the plastic phase I are: cubic cell, a = 8.61 (2) Å, Z = 4, space group Fm3m. Using the molecular shape previously found, two types of orientational disorder have been investigated: isotropic reorientations of the molecules about their centre of gravity (Pauling–Fowler model) and step reorientations between 24 equivalent positions (Frenkel model). In this case, the packing was found by a Monte–Carlo method coupled with a rigid-group least-squares refinement. The Frenkel model gives a better agreement with experiment. A tentative interpretation of the transformation is given. The same general procedure can probably be applied to other plastic crystals.