Clathrate type

Figure 13.4 Low-level 18-cluster QCE model (RHF/3-21G level) of the water phase diagram, showing (above) the dominant W24 clathrate-type cluster of the ice-like solid phase, and (below) the overall phase diagram near the triple point (with a triangle marking the actual triple point). Note that numerous other clusters in the W2o-W26 range were included in the mixture, but only that shown (with optimal proton ordering) acquired a significant population.

An inclusion compound is composed of two or more distinct molecules held together by noncovalent forces in a definable structural relationship. Hosts can contain cavities that are rigid or that are developed by reorganization of the hosts during the process of complexation. Inclusion compounds may be subclassified as (1) the true clathrate type in which the guest molecules are... 

We have already discussed the technological importance of hydrogen storage materials in Section 7.9, where we looked at clathrate-type organic frameworks in this context. We now return to the subject... 

Figure 12 The clathrate-type structure of dodecasil-3C (MTN) (see Table 23)...

Our recent studies [74-76,85-93] show that almost all prepared polycrystalline adducts of zinc(II)-dithiocarbamates (see previous section) may form solvates, i.e. incorporating small guesf molecules such as benzene, various chlorohy-drocarbons and N-donor bases, which are held by van-der-Waals forces between molecules of the host compound. X-ray diffraction studies [85-91,93] have revealed an ordered system of molecular channels occupied by outer-sphere guest molecules in the crystal lattice of solvated forms of the adducts (see Fig. 23), i.e., clathrate type structures. 

Compared to the original adduct 35, single-crystal X-ray diffraction studies of its solvated forms 44 and 45, revealed clathrate type structures, i.e., the pres-... 

No evidence of polymorphism has been seen from microscopic and thermal analysis of various lots of indapamide (12). Studies have indicated that the compound is capable of forming non-stoichiometric clathrate-type solvates with such solvents as dichloromethane and methanol. Such clathrates will form a glassy (amorphous) state following solidification from the molten state. Material in the amorphous state exhibits an enhanced initial solubility, a behavior typical of an amorphous form. 

Co-condensed EtOH-water mixtures reveal the formation of distinct EtOH hydrate phases in different temperature domains. A hydrate 1 appears in the 130 K - 163 K range depending on the EtOH content. It is proposed to have a cubic lattice similar to that of the clathrate type I. Hydrate 2 is found to crystallize at 158 K or 188 K-193 K in correlation with the absence or the presence of ice Ic and EtOH content. Its composition seems to correspond to the monohydrate. The deposited solids undergo crystallization 10 K lower in comparison to frozen aqueous solutions. This reflects the remarkable ease with which water molecules initiate molecular rearrangement at low temperature. This seems most likely due to EtOH generating defects that facilitate the water reorientation . This may also reflect the generation of clusters (in the vapour phase before deposition) having a different nature relative to those encountered in the liquid solutions. These unusual structures may have implications in atmospheric chemistry or astrophysics. 

Vandenheede et al. (25) first pointed out possible roles for the hydrophobic methyl groups. They suggested that the AFGP might form clathrate-type inclusion bodies with developing ice crystals and stated... 

Of particular interest is the effect of noble gases in biological systems. For example, xenon has an anesthetic effect. This is somewhat surprising in that the conditions present in biological systems are obviously not sufficiently severe to effect chemical combination of the noble gas (in the ordinary sense of that word). It has been proposed that the structure of water might be altered via a clathrate-type interaction. 

A large number of substances form clathrate-type hydrates with water host lattices. The guest species are organic or inorganic molecules, or more rarely atoms which are insoluble or slightly soluble in water . Four kinds of clathrate hydrates are known. The water networks are formed by close packing of large polyhedra whose vertices are... 

Silicon and germanium have been found to form clathrate-type host lattices in which guest species are alkali atoms. The host lattices are exactly the same as those of type 1 and II hydrates "" (see 16.2.2). They are formed by atoms of only one kind, which are bonded together by strongly covalent forces, as in the Si or Ge diamond structures. The Si—Si or Ge—Ge bond lengths are of the same order of magnitude as in classical Si or Ge with the bond angles 109°28 which characterizes the tetrahedral sp hybridization of the carbon family. 

Figure 1. Comparison of the covalent radii of alkali-metal atoms with the free radius of the available voids in clathrate-type silicon host lattices (from ref. 3).

Three-dimensional linked polyhedra are frequently found in solids that are rich in E elements. The clathrate-type structures NasSi46 (clathrate-I) and Na cSii36 (clath-rate-II, 3 < x < have been known for many years. Further examples of the... 

Dehydrated hydrates may in principle belong to any of the classes just discussed, but the cases with which the author is familiar (findings not yet published) have all been either channel hydrates or clathrate type structures where water is the guest instead of the host in a cavity and in a nonstoichiometric amount. This subclass deals with crystals that dehydrate even at relatively high partial pressures of water. Therefore, the hydrate that forms in solution dehydrates almost immediately on removal from the mother liquor. When dehydration leaves an intact anhydrous structure that is very similar to the hydrated structure but with lower density, it is classified as a dehydrated hydrate. If there already exists an anhydrous crystalline form of the molecule, the dehydrated hydrate is classified as a polymorph. 

There are two ways to approach the hydration nature the thermodynamic and kinetic. The thermodynamic approach treats hydration as a reversible process of joining H O dipoles with the formation of peculiar aquatic complexes with a set coordination number. Such an approach is handy when studying the thermodynamics of chemical processes, in particular complex formation, and will be used in the sections dealing with these processes. The kinetic approach was introduced by O.Ya. Samoylov (1921-1980), who proposed a first model of clathrate type of water structure as early as 1946. 

The versatility of TOT in clathrate formation is further illustrated by the discovery of several other clathrate types belonging to various space groups PT (/raw-stilbene, cw-stilbene a-bromobutyric acid benzene ) Pl(PT) (methyl tran -cinnamate, methyl cw-cinnamate) P2 (weso-2,3-butanediol carbonate) P2j/c (weTO-2,3-di-bromobutane) C2jc (3-bromooctane ethyl a-bromobuty-rate ° ) Pbca (l,l,l-trifluoro-2-chloro-2-bromoethane) Pbcn (dl-2,3-dibromobu-tane) hexagonal R (a-chlorotetrahydropyran) The clathrates for which X-ray crystal structure determinations have been completed are indicated in Table 1. 

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