Cavity diameter

Cation Cation diameter, E Crown ether Cavity diameter, E... 

The match between crown cavity diameter and cation diameter is obvious from Table 3 showing that, eg, and 12-crown-4 (la) or, respectively and 18-crown-6 (Ic) correspond. Similar are the cryptands of gradually increasing cavity size [2.1.1], [2.2.1] and [2.2.2] for and... 

Various crown ethers (p. 96) with differing cavity diameters provide a range of coordination numbers and stoichiometries, although crystallographic data are sparse. An interesting series, illustrating the dependence of coordination number on cationic radius and ligand cavity diameter, is provided by the complexes formed by the lanthanide nitrates and the 18-crown-6 ether (i.e. 1,4,7,10,13,16-... 

The correlation between selectivity and intracrystalline free space can be readily accounted for in terms of the mechanisms of the reactions involved. The acid-catalyzed xylene isomerization occurs via 1,2-methyl shifts in protonated xylenes (Figure 3). A mechanism via two transalkylation steps as proposed for synthetic faujasite (8) can be ruled out in view of the strictly consecutive nature of the isomerization sequence o m p and the low activity for disproportionation. Disproportionation involves a large diphenylmethane-type intermediate (Figure 4). It is suggested that this intermediate can form readily in the large intracrystalline cavity (diameter. 1.3 nm) of faujasite, but is sterically inhibited in the smaller pores of mordenite and ZSM-4 (d -0.8 nm) and especially of ZSM-5 (d -0.6 nm). Thus, transition state selectivity rather than shape selective diffusion are responsible for the high xylene isomerization selectivity of ZSM-5. 

Figure 2. Effect of intracrystalline cavity diameter of several zeolites on selectivity in xylene isomerization.

Fig. 2. Space-filling models of a (6 units), P (7 units, and y (8 units) cyelodextrins. The cavity diameters range from 4,5 to 8.5 A.

The most important property of cyclodextrins is in their ability to accommodate guest molecules within their cavity, which has a volume of 262 per molecule or 157 mL per mol of [3-CD (cavity diameter 6.0-6.5 A). In aqueous solution, this cavity is filled with molecules of water the displacement of which by a less polar guest leads to an overall decrease in free energy. Stability constants and thermodynamic parameters for complexation of a vast number of guest molecules can be found in ref. [3]. 

Again, the potassinm cation acts as a coordination center, which ensures the template organization of anionic fragments during homogeneous reduction. The cavity diameter of the obtained mnltithiaheterocycle matches the doubled radius of potassium cation. 

Table 6. Approximate cavity diameters in macrocyclic, macrobicyclic and macrotricyclic ligands...

Ligand type Compounds Cavity Diameter (A) Number of Binding Sites Ref. 

In 1967, C. J. Pederson of DuPont deNemours Co. synthesized the cyclic polyethers ( ) These cyclic polyethers are commonly referred to as "crown ethers" (see Figure 3). In solution, crown ethers are extremely effective ligands for a wide range of metal ions. The size of the ring cavity and the ionic radius of the metal affect the stability of the complex. Tables I and II list the cavity diameters for the crown ethers and the ionic radii of a number of metal ions (6-11). 

Zeolite/zeotype Structure type code No. of tetrahedra in ring Window diameter (pm) Cavity diameter (pm)... 

Host n Molecular Weight Cavity Diameter (A) Aqueous Solution (mM)... 

One of the more spectacular examples of the development of novel interactions for nanostructuring of food systems is the self-assembly of partially hydrolysed molecules of a-lactalbumin at neutral pH in the presence of appropriate cations (Ca2+, Mn2+, Zn2+, Cu2+ or Al3+). These ordered nanostructures possess enhanced functionality for thickening, gelation and encapsulation, as compared to the individual protein molecules or their disordered aggregates. The molecules assemble into rather stiff nanotubes with a cavity diameter of 16 nm and a length of a few micrometres (Figure 1.1). The specific ion size and its preferred ligand coordination number seem to play a key mechanistic role. But hydrolysis is needed to make the a-lactalbumin prone to self-assembly. 

The barium cation (ionic radius 1.43 A) has a good compatibility with [2.2.2]cryptands (cavity diameter ca. 2.8 A). This cryptand will solubilize even BaS04 in aqueous solution,486 and this phenomenon has been investigated with a view to removal of BaS04 scale deposited by the use of sea water as an injection fluid in oil-producing formations. The scale deposition can lead to blockage of wells.487... 

A best-fit situation is required with regard to the cavity diameter and the diameter of the incoming cation, but ligand-ligand repulsions must be minimized. 

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