Compounds, Clathrates

Clathrates are crystalline-addition compounds of at least two species of molecules. They are bound mainly by van der Waals forces. One compound, the host, makes up the structure. The other partner, the guest, is placed in free 

The size of the free space varies slightly as a result of the size and the shape of the molecule to be included. This fact is used in the separation of molecules. A relevant example in petroleum refinement is the separation of paraffins from other compounds with urea. In this case, a channel-like lattice is formed by urea. In the free space linear alkanes (n-octane) find space, whereas branched alkanes (i-octane) cannot be included. 

An example for a host molecule with a layerlike structure is graphite. Various types of both organic and inorganic inclusion compounds, as well as stoichiometric and nonstoichiometric compounds, are known. 

Gas hydrates are a special form of clathrates. Here water is the host molecule. The first gas hydrate (with chlorine) was described in 1818 by Sir Humphrey Davy. Naturally-occurring gas hydrates in Siberia are methane hydrates. 

Inspection of Table 13-1 shows that the number of water molecules required to form the hydrate increases with the size of the guest molecule to be captured. Hydrates are classified into simple and mixed hydrates depending on whether one or more host or guest molecules compose the compound. They also exhibit different structures. 

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Fig. 2. Classification/nomenclature of host—guest type inclusion compounds, definitions and relations (/) coordinative interaction, (2) lattice barrier interaction, (J) monomolecular shielding interaction (I) coordination-type inclusion compound (inclusion complex), (II) lattice-type inclusion compound (multimolecular/extramolecular inclusion compound, clathrate), (III) cavitate-type inclusion compound (monomolecular/intramolecular inclusion...

Extramolecular Cavity Inclusions Lattice-Type Inclusion Compounds, Clathrates... 

Another separation mechanism that depends primarily on the size of the solute molecules involves the formation of inclusion compounds, clathrates or adducts.7 Stable complexes are formed whereby one (called the guest) is trapped inside the other (called the host). Van de Waals forces are present and help stabilize the complexes, and in some cases hydrogen bonds are involved in forming the cages. These methods are not widely used, but they can produce some exceptional separations. A general discussion can be found in references 8 and 9, and some typical applications are in references 10 (GC), 11 (TLC), and 12 (LC). 

This study focuses on the potential use of clathrates as a novel hydrogen storage technique. A clathrate, also called a clathrate compound or a cage compound, is a material consisting of a lattice of one type of molecule trapping and containing a second type of molecule. Powell [8] was the first to call such compounds clathrates. In a paper published in the Journal of the Chemical Society. 

In this article, we briefly recall the principles of the technique and the state of the art of the technique as it is currently used in laboratories. Then we present some results obtained on supramolecuiar compounds charge-transfer crystals and inclusion compounds (clathrates and channel-like composites). These results were chosen to give a broad scope of what the interest of Brillouin scattering is for supramolecuiar scientists. More details on... 

This article provides a brief overview of the theory of IR spectroscopy and Raman spectroscopy (for more in-depth descriptions of these methods see Refs. [1-3]). This is followed by a review of vibrational spectroscopic studies performed on clathrate hydrates (a class of inclusion compound) and macrocyclic suprainolecular compounds. Clathrate hydrates were highlighted in this article because of all the clathrate compounds, they are particularly amenable to vibrational spectroscopy and are of great industrial significance. Similar IR/Raman methods can be applied to other well-known clathrate compounds. including quinol and urea " clathrates. Finally, future directions on the use of vibrational spectroscopy in supramolecular compounds will be given. 

Salts, and more recently co-crystals, have attracted much interest in the pharmaceutical industry for their promise in tailoring the physical properties of an active pharmaceutical ingredient (API) to meet the needs of the drug product and ultimately the patient.Salt forms, produced by add (A)-base (B) reactions in the solid state, are multi-component solids comprising minimally an A B pair they may be crystalline or amorphous. The term co-crystal, on the other hand, refers specifically to crystalline molecular complexes, which may include an A B pair among the different components and by definition necessarily include solvates (hydrates), inclusion compounds, clathrates and solid solutions. 

Polymerization in confined space usually includes polymerization of inclusion compounds (clathrates), polymerization of monomer crystals, polymerization in nanolayers, or in dispersed systems. 

Of those compounds (clathrates) which during crystallization occlude other compounds in the cavities of their crystals, those with urea and thiourea (8, 9) have a certain importance for the identification of hydrocarbons. 

The definition of a co-crystal is still matter of debate [74,75]. The definition initially put forward by Aakeroy focused on the aggregation state co-crystals are made from reactants that are solids at ambient conditions [76] and has also been taken up by others [71,77]. This definition, however, is not without ambiguity (see below). We prefer to take up Dunitz more liberal view of co-crystals as encompassing molecular compounds, molecular complexes, solvates, inclusion compounds, channel compounds, clathrates, and other types of multi-component crystals. This view has been echoed recently by Stahly [78] who wrote that co-crystals consist of two or more components that form a unique crystalline structure having unique properties. At the bottom line, these multicomponent systems ought to be looked at as crystals of supermolecules whereby the component units interacting via non-covalent interactions generate collective physico-chemical properties that are different from those of the homo-molecular crystals formed by the components. 

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