Guest@host definition

In the context of supramolecular chemistry, the composition and structure of a non-stoichiometric compound can be considered in terms of the geometrical disposition of its constituents, which somehow display a guest-host structure. Under this broad definition, even a solution (interstitial or substitution crystals) can then be considered a non-stoichiometric compound or an adsorbed layer of gas on a solid surface can be considered as a separate phase [Mandelcorn, 1964]. This guest-host structure image can be extended to channel/tunnel and cage structures (see next section). 

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...

Checking the literature of the second half of the 1980s for applications of crown complexes of arenediazonium salts for synthetic purposes, one gets the impression that these complexes are not widely used. The two comprehensive reviews by Izatt et al. (1985, 1991) show a definite decline of interest in host —guest complexation of arenediazonium ions in the second half of the 1980s. The subject of arenediazonium... 

The question that emerges at the climax of this survey relates to the possibility of using crystalline inclusion phenomena for optical resolutions of molecular species. Can this be done effectively with suitably designed host compounds The definitely positive answer to this question has elegantly been demonstrated by Toda 20) as well as by other investigators (see Ch. 2 of Vol. 140). An optically active host compound will always form a chiral lattice. Therefore, when an inclusion type structure is induced, one enantiomer of the guest moiety should be included selectively within the asymmetric environment. 

With proper design and implementation, however, it is possible to construct a self-sorting system whose behavior is different from its components [33c]. For example, consider the simple system comprising two hosts (A and B) and two guests (M and N) that can form four possible host-guest complexes (AM, AN, BM, and BN). We fix the total concentrations of hosts A and B ([A J and [B J) at 1 mM and choose the four equilibrium constants such that host A (KT-fold) and host B (10-fold) both prefer guest M (Scheme 4.7). The various mole fraction definitions (Scheme 4.7c) are used to construct a plot (Scheme 4.7d) of the composition of the mixture as a function of total guest concentration ([M J = [Nj J). When [AjJ = [Bj J > = [Nj j], complexes AM and BN dominate because... 

Another way to achieve uniform illumination is to lower the optical density of the sample by increasing the effective molar volume. Solid solution of an absorbing guest in a transparent host in either a normal crystal or a molecular inclusion compound can achieve this end so can using a pure crystal of a much larger molecule which includes the chromophore of interest [27]. While it is harder to reach high dilution with the latter approach, it has the virtue that the initial compound is a pure crystal and thus better suited for definitive X-ray investigation. 

Supramolecular chemistry has been defined by one of its leading proponents, Jean-Marie Lehn, who won the Nobel Prize for his work in the area in 1987, as the chemistry of molecular assemblies and of the intermolecular bond . More colloquially this may be expressed as chemistry beyond the molecule . Other definitions include phrases such as the chemistry of the non-covalent bond and non-molecular chemistry . Originally supramolecular chemistry was defined in terms of the non-covalent interaction between a host and a guest molecule as highlighted in Figure 1.1, which illustrates the relationship between molecular and supramolecular chemistry in terms of both structures and function. 

These descriptions, while helpful, are by their nature noncomprehensive and there are many exceptions if such definitions are taken too literally. The problem may be linked to the definition of organometallic chemistry as the chemistry of compounds with metal-to-carbon bonds . This immediately rules out Wilkinson s compound, RhCl(PPh3)3, for example, which is one of the most important industrial catalysts for organometallic transformations known in the field. Indeed, it is often the objectives and thought processes of the chemist undertaking the work, as much as the work itself, which determine its field. Work in modern supramolecular chemistry encompasses not just host-guest systems but also molecular devices and machines, molecular recognition, so called self-processes ... 

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