Clathrate-type structures

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-... 

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. 

Fig. 1.1 The characteristic polyhedrons of clathrate-type structures a the pentagonal dodecahedron, b the tetrakaidecahedron, c the pentakaidecahedron, d the hexakaidecahedron, e the 12-faced 4 5 6 polyhedron, f the 20-faced 5 6 polyhedron [5]...

In the classification proposed by Jeffrey [26], five other clathrate-type structures are indexed, IB, IV, V, VI, VII ... 

Some authors use other notations for clathrate-type structures CS-1, CS-2, CS-3 and CS-4 for the four cubic 1, 11, VI and VII types, respectively, TS-1 for the tetragonal type III and HS-1, HS-2 and HS-3 for the three hexagonal IV, V and H types. More details about the strucmre and the corresponding representative clathrate hydrates will be given further (see Sect. 1.10 and Tables 1.3 and 1.4). 

Clathrate-type structures also exist with Si02 host lattices, another well known tetrahedrally-based compound, which has several analogous structural polymorphs with H2O. These clathrates are called clathrasils and classified as a subgroup of porous tectosilicates. In addition to their pure silica frameworks, they differ from zeolites by the presence of almost spherical and medium large voids with small apertures limited to six atoms, instead of wide interconnected channels [46]. 

A third clathrate-type structure of silica, called dodecasil IH (or DOH) was reported by Gerke and Gies [52]. It is isostructural with the type H hydrates (see Sect. 1.4.2). The lattice constants of the hexagonal unit-cell of dodecasil IH enclosing piperidine as guest species in the large 5 c = 1.119 nm and the space group P6/mmm. 

Syndiotactic poly(styrene) displays a complex polymorphic behavior that reflects the specific role played by solvents. Four crystalline forms have been reported.(289,290) The a and p forms can be obtained from the melt (or glass), depending on the crystallization conditions.(291) Both structures comprise planar zigzag chains that have the same identity period of 5.1 A. The a form has a trigonal unit cell while the p form is orthorhombic. The P form can also be produced by crys-taflization from solution.(292,293) The y and 8 structures develop after interaction with solvent. In contrast to the all trans bond orientation of the a and p structures, the chains in the y and 8 crystals adopt a ttggttgg sequence of bond orientation. Thus a helical ordered structure evolves. This structure is similar to the crystalline chain conformation of syndiotactic poly(propylene).(294) The difference between the y and the 8 polymorphs is that in the former the sample is completely dried, while the solvent is included in the 8 form. It therefore represents a clathrate type structure. The formation of these structures is, thus, solvent specific.(292,293,295,296) The... 

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... 

A novel class of crystalline, microporous aluminophosphate phases has been discovered. It represents the first class of molecular sieves with framework oxide compositions free of silica. The new class of materials encompasses some fourteen reported three-dimensional microporous framework structures, and six two-dimensional layer-type structures. The three-dimensional structures include structural analogues of the zeolites sodalite and erionite-offre-tite. The novel phases can be synthesized hydro-thermally in the presence of organic amines and quaternary ammonium templates. The template is entrapped or clathrated within the crystallizing aluminophosphate network. After thermal decomposition of the template the three-dimensional molecular sieves have the general composition of Al303 1.0 ... 

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. 

Hydrates of Ar, Kr, and Xe were first synthesized by Villard in 1896 [141]. They were further studied, as well as hydrates of krypton and xenon, by de Forcrand [142]. Several structures for noble gas hydrates are known [143-146]. All the hydrate structures are different from that of ordinary hexagonal ice. In the two fundamental structures, the water molecules form pentagonal dodecahedra which are stacked with different degrees of distortion from their ideally regular forms [146]. The two types of structures are shown in Fig. 26a and 26b [140]. One structure contains 46 water molecules in the unit cell with 2 small and 6 larger cavities. The other structure has 136 water molecules in the unit cell with 16 small and 8 larger cavities. The formation of the two fundamental types of hydrates depends mainly on the size of the guest species. More detailed data for the two principal clathrate hydrate structures are available from the literature [147]. 

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. 

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. 

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