Clathrate hydrates host water, structure

In an oligonucleotide-drug hydrate complex, the appearance of a clathrate hydrate-like water structure prompt a molecular dynamics simulation (40). Again the results were only partially successful, prompting the statement, "The predictive value of simulation for use in analysis and interpretation of crystal hydrates remains to be established." However, recent molecular dynamics calculations have been more successful in simulating the water structure in Ae host lattice of a-cyclodextrin and P-cyclodextrin in the crystal structures of these hydrates (41.42). 

Illustrative examples of substances which can behave as porous hosts in one of the above ways are also given. For instance, water readily forms open ice lattices which incorporate guests in clathrate hydrates of types I and II (see later text). Ordinary ice also possesses considerable porosity so that, as shown in Table I, He and Ne can readily diffuse through it. Ice below 0°C is zeolite-like in that it has a permanent, somewhat porous structure which (unlike the open-ice frameworks of the clathrate hydrates) does not require guest molecules for stabilization. 

In addition to the various forms of ice, there are a large number of ice-like structures with four-coordinated water molecules that constitute a host framework, which are only stable when voids in the host framework are occupied by other guest molecules. Such compounds are known as clathrate hydrates. 

Solids formed by gases and water are called gas hydrates when the molecules of the volatile component (guest) are trapped inside a lattice (host) formed by hydrogen bonded water molecules (Davidson, 1973 Jeffrey and McMullan, 1967). The structures of these clathrate hydrates are reviewed from the standpoint of idealised structures of the water host framework, recognizing that in real clathrates, this framework describes one component of an at least two component system. 

Because in the cyclodextrin hydrates the cavity is occupied by water molecules, they can be considered as the inverse of the clathrate hydrates discussed in Part IV, Chapter 21. In these, the water molecules form the host structure and the organic molecule is the guest. 

Clathrate hydrates (known also as gas hydrates) belong to a large class of crystalline, non-stoichiometric, inclusion-compound materials that are stable within a certain range of pressure and temperature. The host solid framework structure is made up of water molecules, connected through hydrogen bonds that form cavities (cages) . The cavities can be stabilized by the inclusion of small molecules such as CH4, CaHg, CO2, N2, Ar, etc. Over 100 different molecules are known to form hydrates. 

Clathrate hydrates are crystalline inclusion compounds composed of host water cages that trap guest molecule-. [-.The three most common types of clathrate hydrates are known as structure I, II, and H, which differ in the type of water cage they contain. The type of clathrate hydrate structure formed depends mainly on the size of the guest molecules present (for further details, see the article Clathrate Hydrates and Refs. [8,9]). The structures of these compounds were first determined from x-ray diffraction studieshowever, as mentioned previously, vibrational spectroscopy can provide important complementary information on the structures and dynamics of these compounds and can also detect the presence of any guest molecule-host lattice interactions. 

Pauling (1960) rejected the idea that liquid water contains a significant number of aggregates with quartzlike structures as proposed previously by Bernal and Fowler (1933). Instead, he proposed to view liquid water as a hydrate of itself. The idea is based on the well-known fact that molecules such as xenon, chlorine, and methane form clathrate compounds with water having a well-defined crystalline structure [for details, see Pauling (1960) and Frank and Quist (1961)]. Why not assume, then, that the same structure could host a water, instead of a nonelectrolyte, molecule Frank and Quist (1961) undertook a quantitative development of Pauling s model [the essential features of their treatment, as well as a similar one by Mikhailov (1967), are discussed in detail in the next section]. 

In the presence of small-nonpolar molecules, a different structure from ice has been observed, which is called clathrate hydrate and is a kind of guest host compounds [10,11]. The host structure is made of only water molecules. Clathrate hydrates are nonstoichiometric compounds whose stability depends significantly on temperature and gas pressure of guest [10,11]. 

With water, nitrogen trifluoride forms a clathrate hydrate when ice at -25 to -40°C is exposed to pressurized NF3 (9 to 23 atm). Hydrate formation was found complete within 24 h by the fall of gas pressure. X-ray analysis indicated the clathrate hydrate to be of structure 1, space group Pm3n. The dielectric behavior and the F NMR spectrum of that hydrate as well of others containing tetrahydrofuran, p-dioxane, or sulfur hexafluoride as additional hosts, were analyzed to study the site distribution and motional dynamics of NF3 [1 ]. A crystalline clathrate... 

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