The Hydrate Inclusion Compounds

Although the topology of the hydrogen-bonded water host structure is closely related in the gas hydrates, the alkylamine hydrates, and the alkylammonium salt hydrates, with the pentagonal dodecahedron shown in Fig. 21.3 playing a prominent role, the interactions between the host and guest spedes are different. 

In the gas hydrates. which include some molecules which are liquids at room temperature, the host-guest interactions are solely van der Waals forces. These are true clathrates. 

In the alkylamine hydrates, with one exception, the functional group of the amine is hydrogen-bonded to or included with the water host lattice. These compounds are not true clathrates i.e., they, are sometimes referred to as semi-clathrates [433]. 

In the higher hydrates of the quaternary ammonium salts, the cations are the guest species and the water and anions form the anionic host lattice. In the recently discovered strong add hydrates such as HC104 6H20 [767], the anions are the guests in a cationic water lattice. 

The ability of water molecules to form dther neutral host lattices or to combine with ions4 to form ionic lattices implies that there is a much wider range of 

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Unfortunately, none of the hydrate inclusion compounds which have less regular structures, such as the tetrabutyl and isoamylammonium salt hydrates [432] and the amine semi-clathrates [433, 434], has been studied by neutron dif-... 

Hydrogen-bonding patterns in crystal structures of the cydodextrins and the simpler carbohydrates differ. The infinite, homodromic chains are common both in the low molecular-weight carbohydrates and in the cydodextrins. The principal difference lies in the frequency of occurrence of the homodromic and antidromic cycles, which are common in the cyclodextrin crystal structures and rare in the mono-, di-, and trisaccharides. The cyclic patterns are the rule in the clathrate hydrates and in the ices. From this point of view, the hydrogen-bonding patterns of the hydrated cydodextrins lie between those of the simpler hydrated carbohydrates and those of the hydrate inclusion compounds, discussed in Part IV, Chapter 21. 

Such three-dimensional networks of perfectly disordered hydrogen bonds are rare, but they are also found in the low-pressure cubic form of ice and in some of the high pressure forms [74], Proton disorder also occurs in some of the hydrate inclusion compounds in which the water molecules form regular three-dimensional arrangements. These structures have been reviewed by Jeffrey and Saenger [10]. 

The history of inclusion compounds (1,2) dates back to 1823 when Michael Faraday reported the preparation of the clathrate hydrate of chlorine. Other early observations include the preparation of graphite intercalates in 1841, the 3-hydroquinone H2S clathrate in 1849, the choleic acids in 1885, the cyclodextrin inclusion compounds in 1891, and the Hofmann s clathrate in 1897. Later milestones of the development of inclusion compounds refer to the tri-o-thymotide benzene inclusion compound in 1914, phenol clathrates in 1935, and urea adducts in 1940. 

The many polymorphs of ice [422, 423] and the wide variety of ice-like hydrate inclusion compounds provide a source of information about 0H---0 hydrogen bonds where the water molecules are the structure-determining molecular species... 

In only a few of the many publications on X-ray studies of the cyclodextrin inclusion compounds and of the cyclodextrin hydrates have the hydrogen atoms of... 

The alkylamine hydrate inclusion compounds form a large variety of structures. Phase diagram studies at the end of the last century [7651 (see Tkble 21.3), and an extensive series of phase diagram studies carried out between 1970 and 1981 [798], showed that aliphatic amines form a large variety of hydrates. Relatively few of these have been studied by X-ray crystal structure analysis and none by neutron diffraction [433, 799]. These hydrates differ from the gas hydrates and the alkyl ammonium salt hydrates in that there appear to be no definite structural types into which several hydrates can be classified. Hitherto, a different crystal structure has been observed for every alkylamine hydrate studied. In this respect, they resemble low hydrated crystals. 

Thus the blue inclusion complex becomes visible only when all the hydrogen sulfite has been consumed. According to studies by / . C. Teitelbaum, S. L. Ruby and T. ]. Marks the blue inclusion compound consists of the amylose component of the starch and the polyhalogenide anion Is", this was established by comparing the Raman and I Mofibauer spectra of the blue-black amylose-iodine complex with those of the adduct between trimesic acid hydrate and H Is , the structure of which is known (see figures). 

Hofmann-type clathrates Werner clathrates clathrate hydrates inclusion compounds of urea, thiourea, and selenourea cyclodextrins calixarenes gossypol hexa-hosts hydroquinone phenol and Dianin s compounds graphite intercalates natural and pillared clays and others. Such studies contributed to the birth of supramolecular chemistry, with relevance to a new understanding of the world of materials that was emphasized with a Nobel Prize. 

The template theory can be understood from the close structural analogy between the structures of ice and Si02 [27], hydrate inclusion compounds [28] and the various clathrasils [29] these analogies illustrate the fact that H2O and Si02 tetrahedra can both form three-dimensional four-connected nets. Figure 7. 

The addition of a liquid to a grinding mixture of two co-crystal components can sometimes enable co-crystallisation through the formation of a product that incorporates the molecules of the liquid as a constituent. For this reason, grinding in the presence of liquids is a versatile technique for searching for three-component co-crystals e.g. co-crystal hydrates, inclusion compounds ). In some... 

Salts, and more recently co-crystals, have attracted much interest in the pharmaceutical industry for their promise in tailoring the physical properties of an active pharmaceutical ingredient (API) to meet the needs of the drug product and ultimately the patient.Salt forms, produced by add (A)-base (B) reactions in the solid state, are multi-component solids comprising minimally an A B pair they may be crystalline or amorphous. The term co-crystal, on the other hand, refers specifically to crystalline molecular complexes, which may include an A B pair among the different components and by definition necessarily include solvates (hydrates), inclusion compounds, clathrates and solid solutions. 

The topic of hydrogen bonding of carbohydrates and hydrate inclusion compounds has been surveyed as have the topics of intramolecular hydrogen bonding and molecular association in monosaccharides and natural cellulose. Other reviews on physical aspects of carbohydrates have dealt with liquid crystal compounds and with molecular dynamics simulations in the field. ... 

In the next section we shall give a brief account of the crystal structure of the hydroquinone clathrates and of the gas hydrates, as far as is needed for a proper understanding of the subsequent parts. The reader who is interested in the phenomenology of other clathrate compounds should consult one of the many review articles7,8 39 on inclusion compounds. 

For instance, one of the various crystalline forms of polyoxacyclobutane is a hydrate [62], Syndiotactic polymethylmethacrylate also forms nonstoichiometric inclusion compounds with a variety of solvents [63,64]. 

Radon forms a series of clathrate compounds (inclusion compounds) similar to those of argon, krypton, and xenon. These can be prepared by mixing trace amounts of radon with macro amounts of host substances and allowing the mixtures to crystallize. No chemical bonds are formed the radon is merely trapped in the lattice of surrounding atoms it therefore escapes when the host crystal melts or dissolves. Compounds prepared in this manner include radon hydrate, Rn 6H20 (Nikitin, 1936) radon-phenol clathrate, Rn 3C H 0H (Nikitin and Kovalskaya, 1952) radon-p-chlorophenol clathrate, Rn 3p-ClC H 0H (Nikitin and Ioffe, 1952) and radon-p-cresol clathrate, Rn bp-CH C H OH (Trofimov and Kazankin, 1966). Radon has also been reported to co-crystallize with sulfur dioxide, carbon dioxide, hydrogen chloride, and hydrogen sulfide (Nikitin, 1939). 

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