Temperature-dependent structural properties of the chlorocyclohexane/thiourea inclusion compound investigated by synchrotron X-ray powder diffraction

Abstract

The structural properties of the chlorocyclohexane/thiourea inclusion compound have been determined, using Reitveld refinement techniques, from synchrotron X-ray powder diffraction data recorded at temperatures above and below the phase transition temperature (ca. 190–194 K). The structural properties determined for the high-temperature phase (at 289 and 199 K) are in agreement with those reported previously from single-crystal X-ray diffraction data. The thiourea molecules form a rhombohedral ‘host’ tunnel structure, with the chlorocyclohexane ‘guest’ molecules substantially disordered within these tunnels. The host substructure in the low-temperature phase is sufficiently similar to that in the high-temperature phase that the high-temperature structure can be used as the initial structural model for Rietveld refinement of the low-temperature phase. The low-temperature phase (studied at 185 and 85 K) is monoclinic (P2₁/a; a≈ 9.9 Å, b≈ 16.0 Å, c≈ 12.5 Å, β≈ 113.9°), with both the host and guest substructures characterised by well defined atomic positions. The host substructure is a distorted form of the rhombohedral tunnel structure of the high-temperature phase. The guest molecules form an ordered head–head/tail–tail arrangement with an intermolecular chlorine–chlorine distance of 4.6 Å at 85 K. The chlorine atom is in an axial position with respect to the cyclohexane ring, giving direct confirmation of conclusion from previous spectroscopic studies. In the refined structure, the cyclohexane ring has an ‘envelope’ conformation, although further work is required to establish whether this deviation from the standard ‘chair’ conformation represents a genuine structural feature.

The strategy developed in this paper for structure determination of the low-temperature phase of chlorocyclohexane/thiourea will have wider application to other solid inclusion compounds exhibiting order–disorder phase transitions at low temperature.