Clathrates examples

Inorganic clathrates Examples No example No exanq>le Noexanq>le... 

Under high pressure, many gases, including the noble gases, form aqueous solutions in relatively fixed concentrations that can take on solid form. Such solutions are called clathrates. Example argon clathrate < 7.99 Ar 46H20. 

For physical processes, two examples are the elimination of normal paraffins from a mixture by their adsorption on 5 A molecular sieves or by their selective formation of solids with urea (clathrates)... 

Examples of the hydroquinone inclusion compounds (91,93) are those formed with HCl, H2S, SO2, CH OH, HCOOH, CH CN (but not with C2H 0H, CH COOH or any other nitrile), benzene, thiophene, CH, noble gases, and other substances that can fit and remain inside the 0.4 nm cavities of the host crystals. That is, clathration of hydroquinone is essentially physical in nature, not chemical. A less than stoichiometric ratio of the guest may result, indicating that not all void spaces are occupied during formation of the framework. Hydroquinone clathrates are very stable at atmospheric pressure and room temperature. Thermodynamic studies suggest them to be entropic in nature (88). 

An example is the complex with argon which can be kept indefinitely in an ordinary bottle, although the equilibrium pressure of argon over the crystal amounts to several atmospheres at room temperature. Powell31 named these complexes "clathrate compounds, which according to him are those compounds "in which two or more components are associated without ordinary chemical union but through complete enclosure of one set of molecules in a suitable structure formed by another. ... 

In the present review a description is given of the phase behavior of clathrates on the basis of a solution theory. The treatment is restricted to those cases where the empty host lattice ( solvent") is indeed unstable, although many of the present considerations also apply to the few cases known where the host lattice is stable. An example of the latter is the chroman complex first discovered by Dianin9 and recently examined by Baker and McOmie and Powell and Wett ers.34... 

These are characteristic examples of clathrates in which only one type of cavity occurs. The left-hand side of Eq. 25 then contains a single term only and the composition along the three-phase line is completely determined by... 

Another example of such a behavior is provided by the interesting polymerization of butadiene molecules imprisoned in tubes of clathrates of urea.9 Of course, the configuration of the resulting polymer is strongly influenced by the order introduced in the assembly of monomers and thus all trans polybutadiene is formed. 

Table 1 Types of Cd(CN)2 and related clathrates, space groups and examples.

Clathrate Host lattice Space group Examples for G... 

An enormous variety of solvates associated with many different kinds of compounds is reported in the literature. In most cases this aspect of the structure deserved little attention as it had no effect on other properties of the compound under investigation. Suitable examples include a dihydrate of a diphosphabieyclo[3.3.1]nonane derivative 29), benzene and chloroform solvates of crown ether complexes with alkyl-ammonium ions 30 54>, and acetonitrile (Fig. 4) and toluene (Fig. 5) solvates of organo-metallic derivatives of cyclotetraphosphazene 31. In most of these structures the solvent entities are rather loosely held in the lattice (as is reflected in relatively high thermal parameters of the corresponding atoms), and are classified as solvent of crystallization or a space filler 31a). However, if the geometric definition set at the outset is used to describe clathrates as crystalline solids in which guest molecules... 

Aromatic derivatives of cyclotriphosphazenes, rigid six-membered ring systems built on a framework of alternating P and N atoms, provide one of the more beautiful early examples of hosts that form channel-type clathrates and can be useful for molecular separations 32,42>, Although these clathrate systems were discovered by accident, the conclusions that emerged from their investigation have been extremely helpful for the molecular design of other potential host molecules. 

Fig. 21 a and b. Stereoviews of a the 1 1 16 (R = Ph3C) toluene clathrate (the disordered guest methyl is not shown), and b the crystal structure of the uncomplexed host (it represents one of a very few examples of a guest-free lattice which could be obtained from this type of compounds) 21)... 

The detailed structures of several clathrates have been characterized, and a certain degree of selectivity on complexation with different isomers has been detected 21). Most of these complexes are of the channel type, but some of them have structures which simultaneously qualify for channel and cage type descriptors representative examples are illustrated in Figs. 19-21. The crystal data of the complexes are summarized in Table 1. 

Selectivity studies with DTU indicated marked discrimination in the clathrate formation 23,45). As in other types of clathrates, the steric factor is important in differentiation between compounds of similar functionality but different shape. For example, DTU forms crystalline complexes with some alcohols (methanol, ethanol, propanol, 1-butanol) but not with others (2-butanol). It complexes the ethyl esters of N-acetyl derivatives of glycine, alanine, methionine and aspartic acid, but not of proline, serine, phenylalanine and glutamic acid. 

Furthermore, all the examples shown above in Sects. 4.1-4.3 emphasize the significance of both steric and functional features in selective crystallizations. The latter are needed not only for binding between the individual host and guest constituents, but also for effecting a continuous and relatively unflexible pattern of intermolecular arrangement in the crystal lattice. This observation appears to be a very useful one in the systematic design of novel clathrate-based synthetic receptors. 

A further example of the steric fit and thus the conditions of the second rank interactions between host and guest is illustrated by the channel structure of the acid inclusions of 26 (see inclusion compound with acetic acid, Fig. 32a). The tunnel has a mostly hydrophobic character being made up mainly from the aromatic portions of the roof-shaped host molecule. We must note that this arrangement applies possibly for the acetic acid clathrate of 1 as well. 

Studies carried out on Earth, for example, by the NASA infrared telescope on Mauna Kea (Hawaii), showed albedo variations which indicated the presence of holes in the Titanian cloud formations (Griffith, 1993). It is, however, still unclear as to whether these inhomogeneities result from differences in the surface composition. Lorenz et al. (1997) reported large variations in Titan s atmosphere due to photochemical processes. The methane contained in the dense nitrogen atmosphere is decomposed by solar and thermal radiation, and its content may be replenished from methane lakes or from clathrates. 

Details of Cs3Hg2o, which is a beautiful new example of an amalgam with high coordination number polyhedra, have been published recently. It can be described simply as being built up by linked octa-capped icosahedra of mercury or as a framework structure related to the clathrate-I type [6]. 

Systems that react in this manner fall into two classes. In the first of these the framework that dominates the crystal structure scarcely participates in the reaction. This is the case, for example, in the reaction of an organic molecule intercalated in graphite or a clay, or of a guest molecule held in a clathrate of urea or thiourea. Some cases of this sort will be treated in the next section. 

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