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 tw inning, and we focus on the implementation of methodology for handling twin ning in the structure determination process. Differential scanning calorime try indicates that fluorocydohexane/thiourea undergoes a solid state phase transition at about 107 K ton cooling). In the high-temperature phase (ambi ent temperature), fluorocyclohexane/thiourea has the conventional rhombohed ral (R (3) over barc) thiourea tunnel structure and the crystal is twinned through coexistence of domains of the obverse and reverse settings of the r hombohedral structure. The guest molecules are substantially disordered, al though there is evidence that they are located preferentially in certain re gions along the tunnel. In the low-temperature phase, the thiourea tunnel s tructure is monoclinic (P2(1)/n), based on a lattice that is close to the o rthohexagonal 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 wi th respect to the host structure. The strategy for structure determination of twinned crystals of inclusion compounds applied in this paper should fin d wider applications to other solid inclusion compounds. (C) 2001 Academic Press.