Inclusion compounds and clathrates

It has been recognized for some time that SiOj and H2O show a similar tendency to form tetrahedrally linked framework structures (Kamb, 1965). The mineral melanoph-logite is isostructural with cubic gas hydrate I, while the synthetic clathrasil, dodecasil- iC is isostructural with cubic gas hydrate II (Gies et ai, 1981,1982). A section of dodecasil-3C is shown in Fig. 1.41. 

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For two molecules to react, they must first collide with each other. Intermolecular reactions within nanoscopic reaction vessels are limited because reactants are mostly isolated from one another. Both inclusion compounds and clathrates are expected to inhibit bimolecular reactions. Moreover, shape selectivity, cf. Equation (4), and reaction site shielding,62,63 have been shown to preclude certain intermolecular... 

Organic inclusion compounds and clathrates have been known for a long time. While these compounds are still being investigated, there have been efforts to synthesise novel organic structures by supramolecular means. A recent example in this direction is the noncovalent synthesis of a novel channel structure formed by trithiocyanuric acid and bipyridine. The channels can accommodate benzene, xylenes and other molecules (Fig. 12) and the process is reversible. 

Herbstein s terminological proposals suggest that inclusion complexes would not be com-pounds, but one also finds expressions such as inclusion compounds and clathrate compounds in the literature. 

Some phenolic compounds can form flat hexagonal structures with the aromatic rings facing outward, and linked by hydrogen bonds. The internal space thus formed contains solvent. Such compounds are called inclusion compounds or clathrates. An example is dianin (2.14), which can form clathrates in more than 50 different solvents. 

In the Beginning Hoffman Inclusion Compounds and Werner Clathrates... 

The explosive ingredient in Sinoxyd-type primers is lead styphnate (lead trinitroresorcinate), which is very sensitive to static electricity, and fatalities have resulted from handling the dry salt. Preparation of the pure salt is difficult, and many patented preparations, including basic modifications, exist. Some claim special crystalline forms and/or reduced static electricity hazard. Explosive ingredient substitutes for lead styphnate were sought that would be easier to make and safer to use. These included lead azide, diazonitrophenol, lead salts of many organic compounds, complex hypophosphite salts, pic-rate-clathrate inclusion compounds, and pyrophoric metal alloys. 

The chemistry of inclusion compounds also looks back on a lively history There are many events of significance in the area of inclusion chemistry till the middle of the twentieth century including the discovery of new inclusion compounds and hosts (Fig. 1), among them the graphite intercalates, the P-quinol and cyclodextrin inclusion compounds, the Hofmann-type clathrates as well as the inclusion compounds of tri-o-thymotide, Dianin s compound, the choleic acids, of phenols, of urea and others specified in comprehensive monographs... 

Fig. 6. Hexa-host strategy Close connection between (a) a hydrogen-bonded hexamer unit typical of phenolic hosts (hydroquinone, Dianin s compound cf. Fig. 4) and (b) of a hexasubstituted benzene analogue (follow up the shaded hexagons) (c) characteristic constitution of hexa-host molecules and (d) host-guest packing of a representative clathrate inclusion compound (dioxane clathrate of I3a, 1 1 dioxane molecules hatched)...

It is essential that no covalent bonds are formed between host and guest molecules, as, e.g., when diastereomeric salts are used for separation. The differentiation of the enantiomers is effected only by the chiral (spatial) environment in the crystal or in the interior of the host molecule, respectively, whereby hydrogen bridges and dipol-dipol interactions may increase the stability of the inclusion compounds ( coordinato-clathrates ). The different energy constants of the diastereomeric host/guest inclusion compounds which result, e.g., in different solubilities, can be used to isolate one of the enantiomers. 

This article reviews the history and synthesis of tetraphenylene (tetrabenzo[n, c, e, g] cyclooctatetraene), its ability to enclathrate a wide range of guest species, the structural characteristics of the resulting class of clathrate inclusion compounds, and synthetic approaches to the preparation of tetraphenylene derivatives which may serve as potential host systems. 

MacNicol. D.D. Structure and Design of Inclusion Compounds The Clathrates of Hydroquinone. Phenol. Dianin s Compound and Related Systems. In Inclusion Compounds, Atwood. J.L.. Davies. J.E.D.. MacNicol. D.D.. Eds. Academic Press London, 1984 Vol. 2. 1-45. 

This article provides a brief overview of the theory of IR spectroscopy and Raman spectroscopy (for more in-depth descriptions of these methods see Refs. [1-3]). This is followed by a review of vibrational spectroscopic studies performed on clathrate hydrates (a class of inclusion compound) and macrocyclic suprainolecular compounds. Clathrate hydrates were highlighted in this article because of all the clathrate compounds, they are particularly amenable to vibrational spectroscopy and are of great industrial significance. Similar IR/Raman methods can be applied to other well-known clathrate compounds. including quinol and urea " clathrates. Finally, future directions on the use of vibrational spectroscopy in supramolecular compounds will be given. 

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