| Abstract
| - The prototypical thiourea inclusion compound, cyclohexane/thiourea, in which cyclohexane guest moleculesare located within tunnels in a hydrogen-bonded thiourea host structure, is known to exist in three phases(denoted I, II, and III). The stable phase at ambient temperature (phase I) undergoes a second-order transitionto phase II at ca. 148 K, and phase II undergoes a first-order transition to phase III at ca. 127 K. The structuralproperties of phases I−III have been determined in the present work from synchrotron X-ray powder diffractiondata, employing the Rietveld refinement technique. The structural properties determined for phase I are inagreement with those reported previously from single-crystal X-ray diffraction data. The structures of thetwo low-temperature phases (phases II and III) have not been reported previously, and we emphasize theadvantages of using X-ray powder diffraction techniques in these cases due to the occurrence of crystal twinningon entering these phases. In phase II, the observed distortion of the thiourea tunnel is consistent with anincrease of orientational ordering of the guest molecules in comparison with phase I, although a uniquedetermination of the disordered guest substructure could not be established. On entering phase III, however,the Rietveld refinement indicates a further distortion of the thiourea tunnel structure, which is associatedwith ordering of each guest molecule in a single discrete orientation. Importantly, structural aspects of theguest substructure in this phase (particularly concerning the tilt angle of the C3 axis of the guest moleculerelative to the tunnel axis) are in excellent agreement with results on the orientational characteristics of theguest molecules in phase III deduced from a previous single-crystal 2H NMR study.
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