Structural aspects of several urea inclusion compounds (differing in the identity of the ‘guest’ species) have been investigated, principally by single-crystal X-ray diffraction. This paper discusses the general structural features that are common to these different systems. Since the structural periodicities of the host (urea) and guest molecules are generally incommensurate, it is convenient to consider the urea inclusion compound to be composed of distinguishable, although not independent, host and guest substructures. The host substructure, for example, is represented in terms of an incommensurately modulated ‘basic host structure’. The basic structure possesses three-dimensional periodicity, and can be treated using conventional crystallographic principles. The incommensurate modulation describes structural perturbations to the basic host structure which arise from host–guest interaction. Similarly, the guest substructure is considered in terms of a ‘basic guest structure’ which experiences an incommensurate modulation mediated by the host substructure. These fundamental structural concepts are developed and used as a basis for rationalizing the X-ray diffraction patterns of urea inclusion compounds.The average basic host structure has been determined from single-crystal X-ray diffraction data measured, at room temperature, via conventional diffractometric techniques. This structure is essentially the same in each compound studied, and contains linear, parallel, non-intersecting channels (tunnels) within which the guest molecules are located. The crystallographic characteristics of the basic guest structure, on the other hand, depend critically upon the identity of the guest. Full structural details of the guest substructures in the different urea inclusion compounds will be given in future publications.

Structural aspects of urea inclusion compounds and their investigation by X-ray diffraction: a general discussion

Much of the intrinsic appeal of structural science arises from the fact that structural behaviour at the molecular level often resembles macroscopic structures that we can see in the world around us. In the same way that we perceive beauty in the symmetries and forms of macroscopic objects, there is an equally enthralling beauty in the way that nature fashions symmetry and diversity within the architectures of crystalline solids. In the field of inclusion chemistry, for example, many direct analogies can be drawn between the concepts of inclusion in the microscopic and macroscopic worlds, but the scientific interest and importance of inclusion chemistry extends far beyond such structural comparisons. As this article demonstrates, solid organic inclusion compounds can exhibit a diversity of interesting and important fundamental properties, which can form the basis of a range of important applications.

Meldola Lecture: understanding the properties of urea and thiourea inclusion compounds

The asymmetric CH stretching bands of the methyl groups of a number of alkane systems have been examined as a function of temperature. At low temperatures, two bands are seen for a CH3 group. These bands collapse into one as the temperature is raised. Examination of crystals of alkanes with both odd and even numbers of carbon atoms, alkanes in urea clathrates, and deuterium substituted alkanes allows the separation of intramolecular and intermolecular contributions to the band splittings and widths. Model Hamiltonians that express the stretching energy levels as a function of both the methyl torsional coordinates and the external coordinates of the alkanes are set up. These are used in the Redfield equations to derive expressions for the splittings and widths in terms of parameters which give the coupling of the various types of motion. The splittings are found to be proportional to the average of cos 5ϑ, where ϑ is the torsional angle, and a contribution to the width is found to be due to fluctuations in this quantity. Comparison of the equations with the observed widths shows that the relaxation times of the torsional motion, which are greater than 0.2 but less than about 20 ps, are fast enough to yield the motionally narrowed limit for the spectrum as required in the derivation of the Redfield equations.

The motional collapse of the methyl C–H stretching vibration bands

Effects of chain packing and chain mobility on the raman spectra of biomembranes

The phase transition temperatures of urea adducts with many n-paraffins and several paraffin-type compounds were measured by differential thermal analysis and the general structure change causing the phase transitions was revealed by X-ray structure analysis. The ordinary hexagonal adducts (high-temperature form) transform commonly into orthorhombic adducts (low-temperature form) except for a few adducts with guest molecules possessing cylindrical shapes. The phase transition can be regarded as an order-disorder transition with respect to the orientation of the guest molecules about the urea channel axis with a cooperative deformation of the urea channels resulting in the orderly orientation of guest molecules below the transition temperatures. The changes in orientation and motion of the guest molecules through the phase transitions are discussed on the basis of X-ray analysis, potential energy and broad-line NMR.

Y. Taki

Papers

Low-temperature form of urea adducts with n-paraffins