Crystal structure of cyclohexane I and II

Abstract

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 I grown by a zone melting technique at 187 °K, suitable single crystals 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, crystal data are: monoclinic cell, a= 11.23 (2), b = 6.44 (2), c= 8.20 (2) A, fl= 108.83 (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 Dan 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)/~, 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 rigidgroup 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.

High-pressure phases of cyclohexane-d 12

“…Notwithstanding the relatively poor resolution on short-length scales (_< 1.5 A) resulting from the large wavelength, it is apparent from the bond parameters that the molecule exhibits a significant deviation from D~a symmetry and tetrahedral bonding. The size of this deviation is, however, not inconsistent with that observed by Kahn et al (1973) for phase II. A carbon skeletal representation of the refined molecular arrangement relative to the unit cell is shown in Fig.…”

“…With as few as 30 distinct peaks in our data, the deviation from the true solution over which the refinement will successfully converge is not great, even with a constrained refinement. This problem is potentially aggravated if the refinement is sensitive to deviations of the molecular geometry from the ideal symmetry, as is seen in the present work for phase III and by Kahn et al (1973) for phase II. It is, therefore, not altogether surprising that a satisfactory determination of the molecular parameters for phase IV was not forthcoming.…”

Local structure correlations in plastic cyclohexane—a reverse Monte Carlo study

“…(see Figure 5 in [20]). The transition involves alignment of each molecular three-fold axis with the monoclinic c-axis (parallel to the [112] direction in the cubic phase).…”

“…Rietveld refinement of phase I utilised a Frenkel model, where the cyclohexane molecule was modelled over 24 orientations, effectively rotating about its centre of mass. Kahn et al found this model gave a marginally better fit to X-ray single crystal data than a completely isotropically disordered model [20]. The refinement was carried out against data collected on the 2θ = 50-74…”

“…Rietveld refinement of phase II used the atomic coordinates of the cyclohexane structure deposited in the Cambridge Structural Database with the Refcode 'CYCHEX' as an initial starting model [20,32]. Refinement was carried out against data collected on all detector banks with exception of the 2θ = 5-12…”