Macroscopic Evidence of Inclusion Phenomena in Urea and Thiourea Matrices

Abstract

Inclusion compounds actually represent suitable systems for investigating the nature of molecular guest-host interactions. Among such materials urea and thiourea inclusion compounds (i.e., clathrates), have been particularly investigated. X-ray diffraction studies have shown that this type of compound is built up with a hexagonal channel structure. The urea or thiourea matrix is formed by hydrogen bonding and a variety of guest molecules of appropriate size and shape can be included [1–4]. The nature of the vacancies in the host structure and the van der Waals host-guest interactions, as well as the “self assembling” capacity of the system may be considered as a real model for biological systems, e.g., those occurring in cavities containing protein or DNA structures [5]. From such a point of view it is interesting to investigate the conditions determining this type of molecular self-organization. The smallest urea channels are appropriate for example to storing non-branched hydrocarbon chains, while the thiourea channel diameters are large enough to accommodate bulky molecules such as substituted or non-substituted cyclic hydrocarbons [1]. This kind of inclusion phenomenon is attracting much interest in recent papers dealing for instance with guest molecule dynamics [6, 7]. We have personally studied the influence of the surrounding medium on the configuration of amine molecules in various environments and reported recently the synthesis of a series of urea, thiourea and cyclodextrin inclusion compounds. 13C-CP-MAS-NMR and 1H-NMR spectroscopy, and powder-X-Ray diffraction analysis confirmed the inclusion process [8–11]. We describe the visual phenomenon of decomposition of a crystal of ureadialkylamine (dibutyl-, dipentyl-, dihexyl-, dioctyl-) and thiourea-quinuclidine inclusion compounds by dissolution into a solvent such as water or dimethylsulfoxide, with phenolphthalein as indicator.