Clathrate compound

Overview
A clathrate or 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 word clathrate is derived from the Latin clatratus meaning with bars or a lattice. An example of a clathrate is clathrate hydrate, a special type of gas hydrate in which a lattice of water molecules encloses molecules of a trapped gas. Scientists believe that compounds on the sea bed have trapped large amounts of methane in similar configurations. Researchers have begun to investigate silicon and germanium clathrates for possible semiconducting and superconducting properties.

A clathrate therefore is a material which is a weak composite, with molecules of suitable size captured in spaces which are left by the other compounds. They 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".

In 1945, H. M. Powell named these compounds clathrate. Clathrate complex used to refer only to the inclusion complex of hydroquinone, but recently it has been adopted for many complexes which consist of a host molecule (forming the basic frame) and a guest molecule (set in the host molecule by interaction). The 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 compound.

The history of clathrate compounds is relatively new. Clathrate hydrates were discovered in 1810 by Sir Humphrey Davy. 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. Therefore, basic information about clathrate compounds (then called molecular compounds) was known in those days. The development of clathrate compounds was attained by analyzing crystal structure by Powell (mentioned above), and they were applied to the separation of paraffin using a urea or thiourea host. Thereafter, cyclodextrin, crown ether, and cryptand were found as host molecules (see figure). A much studied host molecule is Dianin's compound.

In analyzing and separating techniques, the concept of inclusion compounds has taken root and many applications of them are known. It is possible to isolate not only chemically different species but also structural isomers, positional isomers (enantiomers and diastereomers), and radioactive isomers using clathrate compounds.

The structure of Clathrates is also related to the structure of foams (see Weaire-Phelan structure).

For a theory regarding the sudden release of methane clathrate from ocean sediments see clathrate gun hypothesis.