Cavity size/diameter

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... 

Cavity size and cation diameter 283 Spatial arrangement of binding sites 289 Additional binding sites 293 Electron density at the binding sites 299 Nitrogen and sulfur donor sites 302 Solvent effects 304 Anion effects 308... 

The stability of crown-ether complexes depends on several factors these include cavity size of the ligand, cation diameter, spatial distribution of ring binding sites, the character of the hetero-atoms, the presence of additional binding sites and the type of solvent used. In apolar solutions it also depends on the nature of the anion. The effects of these parameters will be illustrated in the next sections. 

Fio. 1 Relation between log K and the ratio of cation diameter to cavity size for (cfa-syn-c r)-dicyclohexyl-18-crown-6 [20] in water at 2S°C. Ionic diameters taken from Pedersen (1967a), and log K values from Izatt et al. (1976a) and Frensforf (197 la)... 

Another way to increase the host cavity is by using exchange reactions in y-CD (diameter of the cavity from 7.5 to 8.3 A) instead of j8-CD (diameter of the cavity from 6.0 to 6.5 A)." " The larger cavity size of y-CD decreases (and inverts) the enantioselectivity of valine from krJkL = 0.32 to kolk = 1.41 and that of isoleucine from ko/k = 0.26 to k /kL = 2.28. This observation indicates that the three amino acids have optimal enantioselectivity with j8-CD. Conversely, phenylalanine increases in selectivity from k /kL =1.2 (with j8-CD) to kolk =1.8 (with y-CD). This observation suggests that the larger cavity of y-CD allows each enantiomer of the larger amino acid to find more distinct interactions with the larger host. 

Due to the larger size of sorbates and the lower diffusion rates in liquids, larger pore sizes are needed to treat liquid effluents, normally in the range 30 A (Noble and Terry, 2004). For gas-phase effluents, the pores sizes are in the range 10 to 25 A. For example, zeolite Y (Figure 4.3) exhibits the FAU (faujasite) structure. It has a three-dimensional pore structure with pore diameter 7.4 A and cavity of diameter 12 A. 

Figure 10 (a) Relation between log Ks and the ratio of cation diameter to cavity size for dicyclohexyl[18]crown-6 in water at 25 °C (b) log K, values of several crown ether-alkali cation complexes... 

The increase in Tc depended on metal ions, as shown in Fig. 5. A large temperature increase was observed for potassium chloride, while the increase was only 1.5 °C for sodium chloride. The temperature increase was not observed by the addition of lithium and cesium chlorides. The binding affinity of a crown ether with metal ions depends on the cavity size [16]. When the ion diameter fits in the cavity size, the ion is captured by the crown ether. The cavity size of benzo[18]crown-6 is known to accommodate the diameter of K+ [17]. The relative Tc increase correlates well with the binding affinity of benzo[18]crown-6 to metal ions. K+ which efficiently binds to pendant benzo[18]crown-6 most pronouncedly increased Tc, while Li+ and Cs+ which are hardly captured by the crown ether groups could not increase Tc. 

We can interpret the 42% decrease in equilibrium pressure, caused by a 1% change in composition, in terms of cavity size ratios shown in Table 2.4. Propane only fits into the 51264 cavity of structure II, C3H8 is too large to occupy any other cavity. For CH4, diameter ratios in the 512 cavities of si (0.86) and sll (0.87) differ by 1.5%. 

In principle, the negatively charged, presumably planar network I can be combined with one molar equivalent of tetraalkylammonium ion IGN"1" of the right size as interlayer template to yield a crystalline inclusion compound of stoichiometric formula (IGN+) I C(NIG ) ICO2 that is reminiscent of the graphite intercalates. Anionic network n, on the other hand, needs twice as many monovalent cations for charge balance, and furthermore possesses honeycomb-like host cavities of diameter 700 pm that must be filled by... 

Q-cyclodextrin an overall diameter of 14.6 A (1.46 nm) and a cavity size around... 

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