Temperature-dependent structural properties of a solid urea inclusion compound containing chiral guest molecules: 2-bromotetradecane/urea

Periodic structural properties of the 2-bromotetradecane/urea inclusion com pound have been investigated as a function of temperature. Differential sca nning calorimetry between 298 and 98 K identified three well-defined regime s, denoted the high-, intermediate-, and low-temperature phases. The struct ural properties of each phase (at 293, 207, and 142 K, respectively) have b een investigated by single crystal X-ray diffraction. In the high-temperatu re phase, the inclusion compound has the hexagonal urea tunnel structure (P 6(1)22) characteristic of the conventional urea inclusion compounds, with s ubstantial orientational disorder of the guest molecules. In the intermedia te-temperature phase, the symmetry is lowered to orthorhombic (C222(1)), al though the host structure remains close to the hexagonal tunnel structure o f the high-temperature phase and there is no clear evidence for increased o rientational ordering of the guest molecules. In the low-temperature phase, the urea tunnel structure is monoclinic (P2(1)), and is based on a 2 x 2 x 1 supercell of the hexagonal cell of the high-temperature structure. There are four independent types of tunnel, three of which are strongly distorte d from hexagonal geometry. Within these distorted tunnels, there is a compa ratively narrow distribution of guest molecule orientations, which correlat e well with the observed distortions of the tunnels. The 2-bromotetradecane /urea inclusion compound highlights several issues of wider relevance conce rning the structural properties of solid inclusion compounds.