Clathrate compound

Well-defined addition compounds formed by inclusion of molecules in cavities existing in crystal lattices or present in large molecules. The constituents are bound in definite ratios, but these are not necessarily integral. The components are not held together by primary valence forces, but instead are the consequence of a tight fit which prevents the smaller partner, the guest, from escaping from the cavity of the host. Consequently, the geometry of the molecules is the decisive factor.

Inclusion compounds can be subdivided into (1) lattice inclusion compounds (inclusion within a lattice which, as such, is built up from smaller single molecules); (2) molecular inclusion compounds (inclusion into larger ring molecules with holes); and (3) inclusion compounds of macromolecules. The best-known lattice inclusion compounds are the urea and thiourea channel inclusion compounds, which are formed by mixing hydrocarbons, carboxylic acids, or long-chain fatty alcohols with solutions of urea. Other representatives of lattice inclusion compounds are the choleic acids, which are adducts of deoxycholic acid with fatty acids, and other lipoic substances. Some aromatic compounds form an open crystal lattice which can accommodate smaller gas and solvent molecules (clathrates in the stricter sense of the word). The gas hydrates are inclusion compounds of gases in a somewhat expanded ice lattice. The gas or solvent molecules are inserted into definite places within the ice lattice and are surrounded by water molecules on all sides.

Crown ether compounds are cyclic or polycyclic polyether compounds capable of including another atom in the center of the ring. In this way, sodium or potassium compounds can be solubilized in organic solvents. Similarly, a series of ionophore antibiotics can complex inorganic cations.

Some clay minerals are made up of distinct silicate layers. Between these layers some free space may exist in the shape of channels. Smaller hydrocarbon molecules can be accommodated reversibly within these channels. This phenomenon is used in some technical separation processes for separating hydrocarbons (molecular sieves). Furthermore, ion-exchange processes used for water deionization are based on similar minerals. See also Molecular sieve.

Enzymes are believed to accommodate their substrates in active sites, pockets, or clefts prior to the chemical reaction which then changes the chemical structures of the substrates. These binding processes are identical to those of low-molecular-weight inclusion compounds.

Examples of host molecules

A clathrate, clathrate compound or cage compound is a chemical substance consisting of a lattice of one type of molecule trapping and containing a second type of molecule. The name clathrate complex used to refer only to the inclusion complex of hydroquinone, but recently it has been adopted for many other weak composites which consist of a host molecule (forming the basic frame) and a guest molecule (held in the host molecule by inter-molecular interaction). Clathrates are also called host-guest complexes, inclusion compounds, and adducts (chiefly in the case of urea and thiourea). They used to be called molecular compounds.

A clathrate hydrate, in particular, is a special type of gas hydrate in which a lattice of water molecules encloses molecules of a trapped gas. Large amounts of methane naturally frozen in this form have been discovered both in permafrost formations and under the ocean sea-bed.^[1] Researchers have begun to investigate silicon and germanium clathrates for possible semiconducting and superconducting properties.

The word clathrate is derived from the Latin clatratus meaning with bars or a lattice.^[2]

Contents 1 History 2 Properties 3 Media References 4 See also 5 References

History

The history of clathrate compounds is relatively recent. Clathrate hydrates were discovered in 1810 by Humphry Davy^[3]. Clathrates were studied by P. Pfeiffer in 1927 and in 1930, E. Hertel defined "molecular compounds" as substances decomposed into individual components following the mass action law in solution or gas state. In 1945, H. M. Powell analyzed the crystal structure of these compounds and named them clathrates.

Urea- and thiourea-hosted clathrates were applied to the separation of paraffin. Thereafter, cyclodextrin, crown ether, and cryptand were found as host molecules (see figure). A much studied host molecule is Dianin's compound.

Properties

Clathrate complexes are various and include, for example, strong interaction via chemical bonds between host molecules and guest molecules, or guest molecules set in the geometrical space of host molecules by weak intermolecular force. Typical examples of host-guest complexes are inclusion compounds and intercalation compounds.

Clathrates can be isolated as chemically different species, and may have structural and positional isomers (enantiomers and diastereomers).

Media References

• In John Barnes' science-fiction novel Mother of Storms, the destruction of Arctic sea bed clathrates and the subsequent release of trapped CO₂ and methane is central to the plot development involving a vast supersonic hurricane.

See also

• Clathrate gun hypothesis, a theory regarding the sudden release of methane clathrate from ocean sediments.
• Weaire-Phelan structure, related to clathrate structure.
• Chelate

References

1. ^ Pearce, Fred (27 June 2009). "Ice on fire: The next fossil fuel". New Scientiest. pp. 30–33. http://www.newscientist.com/search?doSearch=true&query=clathrates. Retrieved 2009-07-05. 
2. ^ Latin dictionary
3. ^ Ellen Thomas (11 2004). "Clathrates: little known components of the global carbon cycle". Wesleyan University. http://ethomas.web.wesleyan.edu/ees123/clathrate.htm. Retrieved 13 December 2007. 

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