Clathrates, definition

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...

Within this approach, clathrates (for a definition see Chapter 1 in Vol. 140 of this series) and related lattice-type aggregates may be considered as multi-supramolecular systems where guest molecules are included in a crystal matrix. They allow a great many applications which have been specified in Vol. 140, first of all the separation of enantiomers by enantioselective recognition and inclusion of racemic guest molecules. 

An enormous variety of solvates associated with many different kinds of compounds is reported in the literature. In most cases this aspect of the structure deserved little attention as it had no effect on other properties of the compound under investigation. Suitable examples include a dihydrate of a diphosphabieyclo[3.3.1]nonane derivative 29), benzene and chloroform solvates of crown ether complexes with alkyl-ammonium ions 30 54>, and acetonitrile (Fig. 4) and toluene (Fig. 5) solvates of organo-metallic derivatives of cyclotetraphosphazene 31. In most of these structures the solvent entities are rather loosely held in the lattice (as is reflected in relatively high thermal parameters of the corresponding atoms), and are classified as solvent of crystallization or a space filler 31a). However, if the geometric definition set at the outset is used to describe clathrates as crystalline solids in which guest molecules... 

Some teachers would question whether a clathrate can be regarded as a true compound. The answer depends on our definition of a compound. In the author s opinion, the classical definition (relaxed a little, as it must now be, to allow for second-order anomalies) certainly covers clathrates. 

If water will normally form ice in the absence of a solute molecule, the question arises about the mechanism for forming a clathrate with an exact structure, when the solubility of hydrocarbon molecules in liquid water is known to be small (or negligible in ice), relative to the amount of hydrocarbon needed for hydrates. Thus, along with the definition of what the hydrate structures are, comes the logical question of how these structures form. During the past two decades, sophisticated experimental and modeling tools have been applied to address this question. The microscopic mechanism and the macroscopic kinetics of hydrate formation are the major considerations of Chapter 3. 

In the ideal case, guest and host will occur in definite stoichiometric proportions thus, at least from a thermodynamic perspective, the clathrate is a specific substance (i.e., a chemical compound). In normal usage the distinction between such a clathrate and a chemical compound is presumably that in a clathrate the forces involved are van der Waals forces rather than transfer or sharing of electrons. 

More generally, clathrates often tend to be nonstoichiometric. The clathrate is a distinct phase but its composition is not definite (i.e., the degree to which the structural holes are filled depends on the gas pressure of the guest). In such cases, the... 

Centrichromatography, 74 Charge-transfer, 45 Chemisorption, 41, 42 Chiral separations, 261-268 Chiral stationary phases (CSP s), 265-268 Chromatogram, 8 Chromatography classification of, 5 definition of, 4-7 Clathrate, 46... 

With the exception of the clathrate framework model, all these hypotheses appear to be qualitatively consistent with the available X-ray diffraction data on liquid water. It is argued by some investigators, however, that there are still significant inconsistencies between the most sophisticated statistical thermodynamic models for liquid water and the most sophisticated X-ray and neutron diffraction measurements [734-736]. The interpretation of these data from different experiments, using the concept of pair-correlation functions, shows discrepancies that are considered significant in terms of the instrumental precision, and the definitive answer seems not yet available [737J. 

Classification and Nomenclature of Supramolecular Compounds, p. 267 Clathrate Hydrates, p. 274 Crystal Growth Mechanisms, p. 364 Self-Assembly Definition and Kinetic and Thermodynamic Considerations, p. 1248 Self-Assembly in Biochemistry , p. 1257 Supramolecular Polymers, p. 1443... 

Classical Descriptions of Inclusion Compounds, p. 253 Clathrate Hydrates, p. 274 Concepts in Crystal Engineering, p. 319 Crown Ethers, p. 326 Cryptands, p. 334 DNA Nanotechnology, p. 475 Enzyme Mimics, p. 546 The Lock and Key Principle, p. 809 Molecular-level Machines, p. 931 Selectivity Thermodynamic and Kinetic, p. 1225 Self-Assembly Definition and Kinetic and Thermodynamic Considerations, p. 1248 Self-Assembly Terminology, p. 1263 Soft and Smart Materials, p. 1302 Spherands, p. 1344... 

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