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

Abstract

Periodic structural properties of the 2-bromotetradecane/urea inclusion compound have been investigated as a function of temperature. Differential scanning calorimetry between 298 and 98 K identified three well-defined regimes, denoted the high-, intermediate-, and low-temperature phases. The structural properties of each phase (at 293, 207, and 142 K, respectively) have been investigated by single crystal X-ray diffraction. In the high-temperature phase, the inclusion compound has the hexagonal urea tunnel structure (P6122) characteristic of the conventional urea inclusion compounds, with substantial orientational disorder of the guest molecules. In the intermediate-temperature phase, the symmetry is lowered to orthorhombic (C2221), although the host structure remains close to the hexagonal tunnel structure of the high-temperature phase and there is no clear evidence for increased orientational ordering of the guest molecules. In the low-temperature phase, the urea tunnel structure is monoclinic (P21), and is based on a 2 × 2 × 1 supercell of the hexagonal cell of the high-temperature structure. There are four independent types of tunnel, three of which are strongly distorted from hexagonal geometry. Within these distorted tunnels, there is a comparatively narrow distribution of guest molecule orientations, which correlate well with the observed distortions of the tunnels. The 2-bromotetradecane/urea inclusion compound highlights several issues of wider relevance concerning the structural properties of solid inclusion compounds.Key words: urea inclusion compounds, X-ray diffraction, phase transitions, chiral recognition, incommensurate solid, 2-bromotetradecane/urea.