Phase transitions and molecular dynamics in the cyclohexane/thiourea inclusion compound

Abstract

In thiourea inclusion compounds the host structure comprises a hydrogen bonded arrangement of thiourea molecules and contains unidirectional, nonintersecting channels within which guest molecules are located. Structural and dynamic properties of the cyclohexane/thiourea inclusion compound have been studied previously by a wide range of experimental techniques, providing contradictory interpretations. This organic composite crystal displays three thermotropic structural phases on cooling from room temperature, denoted phase I (rhombohedral), phase II (monoclinic), and phase III (monoclinic). However, 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 properties of each phase, the phase transitions are described in terms of order parameters defined on the basis of the symmetry principles of Landau theory. Theoretical analysis of the crystal strain occurring at the phase transitions allows a direct comparison between the experimental temperature dependence of the lattice parameters and predictions derived from Landau theory. In the second part of this paper, we report results from powder and single-crystal ${}^{2}\mathrm{H}$ NMR spectroscopy on samples of cyclohexane/thiourea containing perdeuterated cyclohexane guest molecules $({\mathrm{C}}_{6}{\mathrm{D}}_{12}).$ These ${}^{2}\mathrm{H}$ NMR experiments have shown that in phase I, the motionally averaged quadrupole coupling tensor is axially symmetric. On passing from phase I to phase II, the threefold symmetry axis of the $R=3\ifmmode\bar\else\textasciimacron\fi{}c$ space group of phase I is lost, such that the motionally averaged quadrupole interaction tensor is not axially symmetric in phase II. The single crystal ${}^{2}\mathrm{H}$ NMR spectra probe very precisely the relative orientations of the guest molecules, which are consistent with the site symmetry properties of the structure and proposed modes of crystal twinning, and demonstrate that there is only one type of dynamic species of guest molecule in phase II. In phase III, a greater degree of orientational ordering of the cyclohexane molecules is evident. From a detailed consideration of the symmetry properties of the inclusion compound, the dynamics of the guest molecules may be described using simple jump models (i.e., multidimensional pseudospin models) in all three phases. Using these models, the temperature dependence of the order parameter components has been established. These results, in conjunction with x-ray diffraction, provide important information needed to propose a microscopic model for the mechanisms of the phase transitions in the cyclohexane/thiourea inclusion compound.