Host-guest relationship

The relationship between crown ether and the ion that is transport is called a host-guest relationship. 

BLCKM1NS l ERP LLLERENE (Buckyballs). C60. Spherical aromatic molecule willi a hollow truncated-icosahedron structure, similar to a soccer ball. First reported in the mid 1980s. Capable of enclosing ions or atoms in a host-guest relationship. See also Carbon Compounds. 

Klarner et al. undertook detailed studies of the host/guest relationships between molecular tweezers formed from condensed hydrocarbon arenes and symmetrically and asymmetrically dendryl-substituted viologens - with a 4,4 -bi-pyridinium core acting as -electron acceptor and a Frechet-type dendron as 71-electron donor (Fig. 6.23). The strong fluorescence of the 1,3-dimethylenoxyben-zene units of the Frechet dendron is quenched as a result of the donor/acceptor interaction. Accordingly, in dichloromethane solution the 4,4 -bipyridinium core... 

The zeolite, porosil or A1P0 is the "host" and the zeolitic component the "guest". The host-guest complex is a solution, amenable at equilibrium to solution thermodynamics (14,15), and the host-guest relationship thereby described is one of the most important in the chemistry of porous crystals because, without the zeolitic guest,... 

The relation between SDAs and the zeotype structures they direct is not simply bijective. Usually, the formation of a certain zeotype structure can be directed by more than one SDA [5]. For example, the sodalite structure shown in Fig. 1 can also be directed by molecules other than trioxane, namely, 1,3-dioxolane [15], 1,3-dioxane [16], ethylene glycol [17], ethanolamine [18], and ethylenediamine [18]. Also, depending upon the reaction conditions, an organic molecule may direct the formation of different frameworks [8, 9]. It has therefore been proposed that the term "template", formerly used instead of SDA, should now be reserved for cases of strong structure direction and strong host-guest relationships, in line with its usage in other areas of the natural sciences [5]. A strict condition for a true "template" is that it should not exhibit disorder in the voids of the silica host. 

As the hydrocarbon chain is lengthened, more urea molecules are required to extend the channel but the ratios given in Table 1 show that the host-guest relationship is not stoichiometric. For preparation of a complex, a liquid hydrocarbon is... 

Molecular inclusion was demonstrated by the aqueous solubilization and UV and fluorescence analysis of the guest dyes phenol blue (8), pinacyanol chloride (9). chlortetracycline (10).and naphthalene (11).Fluorescence lifetime decay experiments using 1,6-diphenylhexatriene (12) as the probe further supported the host-guest relationship while electron microscopy (EM) coirobo-rated the single molecule, non-aggregated state. Calculated molecular modeling diameters o/48 A in a fully extended conformation were also substantiated via EM. 

As nanocasting allows us to obtain porous materials with desired pore sizes and shapes, these materials are ideal candidates to act as nanoreactors" or to study host-guest relationships. However, the latter is a topic in its own right and is not mentioned here in further detail. 

Levitskaia. T.G. Bryan. J.C. Sachlehen. R.A. Lamb. J.D. Moyer, B.A. A surprising host-guest relationship between 1.2-dichloroethane and the cesium complex of tetrabenzo-24-crown-8. J. Am. Chem. Soc. 2000, 122. 

A tris-ferrocenyl coronand recognizes the ammonium cation in the presence of potassium X-ray diffraction analysis of the ammonium complex has revealed a host-guest relationship [81],... 

Large-scale molecular dynamics simulations [108] of the polymerization of sihdc acid in aqueous solution using specially adapted potentials have yielded realistic activation energies, and extrapolations based on simulated reaction rates agree within one order of magnitude with observations. In view of the complexities involved, a simulation of host-guest relationships inside a sol-gel-encapsulated medium is still awaited, but theoretical adsorption-desorption studies have already been initiated [109]. 

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