Phase transitions and molecular dynamics in the cyclohexane/thiourea inclusion compound - art. no. 054106

In thiourea inclusion compounds the host structure comprises a hydrogen bon ded arrangement of thiourea molecules and contains unidirectional, noninter secting channels within which guest molecules are located. Structural and d ynamic properties of the cyclohexane/thiourea inclusion compound have been studied previously by a wide range of experimental techniques, providing co ntradictory interpretations. This organic composite crystal displays three thermotropic structural phases on cooling from room temperature, denoted ph ase I (rhombohedral), phase II (monoclinic), and phase III (monoclinic). Ho wever, until now, there has been no attempt to understand the relationship between the dynamics of the guest molecules and the structural and symmetry properties of the composite inclusion compound in the three phases. In the first part of this paper, we determine the space group of each phase from powder x-ray diffraction data. From this knowledge of the symmetry properti es of each phase, the phase transitions are described in terms of order par ameters defined on the basis of the symmetry principles of Landau theory. T heoretical analysis of the crystal strain occurring at the phase transition s allows a direct comparison between the experimental temperature dependenc e of the lattice parameters and predictions derived from Landau theory. In the second part of this paper, we report results from powder and single-cry stal H-2 NMR spectroscopy on samples of cyclohexane/thiourea containing per deuterated cyclohexane guest molecules (C6D12). These H-2 NMR experiments h ave shown that in phase I, the motionally averaged quadrupole coupling tens or is axially symmetric. On passing from phase I to phase II, the threefold symmetry axis of the R=(3) over barc space group of phase I is lost, such that the motionally averaged quadrupole interaction tensor is not axially s ymmetric in phase Il. The single crystal H-2 NMR spectra probe very precise ly the relative orientations of the guest molecules, which are consistent w ith the site symmetry properties of the structure and proposed modes of cry stal twinning, and demonstrate that there is only one type of dynamic speci es of guest molecule in phase II. In phase III, a greater degree of orienta tional ordering of the cyclohexane molecules is evident. From a detailed co nsideration of the symmetry properties of the inclusion compound, the dynam ics of the guest molecules may be described using simple jump models (i.e., multidimensional pseudospin models) in all three phases. Using these model s, the temperature dependence of the order parameter components has been es tablished. These results, in conjunction with x-ray diffraction, provide im portant information needed to propose a microscopic model for the mechanism s of the phase transitions in the cyclohexane/thiourea inclusion compound.