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