Temperature-Dependent Structural Properties and Crystal Twinning in the Fluorocyclohexane/Thiourea Inclusion Compound

Structural properties of the fluorocyclohexane/thiourea inclusion compound have been investigated as a function of temperature by single-crystal X-ray diffraction. The inclusion compound exhibits different forms of crystal twinning, and we focus on the implementation of methodology for handling twinning in the structure determination process. Differential scanning calorimetry indicates that fluorocyclohexane/thiourea undergoes a solid state phase transition at about 107 K (on cooling). In the high-temperature phase (ambient temperature), fluorocyclohexane/thiourea has the conventional rhombohedral (R c) thiourea tunnel structure and the crystal is twinned through coexistence of domains of the obverse and reverse settings of the rhombohedral structure. The guest molecules are substantially disordered, although there is evidence that they are located preferentially in certain regions along the tunnel. In the low-temperature phase, the thiourea tunnel structure is monoclinic (P21/n), based on a lattice that is close to the orthohexagonal cell of the structure in the high-temperature phase. The host structure is distorted from the rhombohedral tunnel structure of the high-temperature phase, and the guest molecules adopt a preferred orientation with respect to the host structure. The strategy for structure determination of twinned crystals of inclusion compounds applied in this paper should find wider applications to other solid inclusion compounds.