Crown ethers cavity size

The combination of crown ethers with mesogenic groups is interesting because the resulting hybrid materials possess the properties of both constructive subunits, i.e., liquid crystalline order in the mesophase and the ability to complex specific cations selectively according to the crown ether cavity size. 

Adenosine triphosphate alkali metal complexes, 34 vanadyl complexes, 568 Alane, 123 amine adducts, 107 phosphine adducts, 111 Alane, alkoxy-, 124 Alane, amino-, 109 Alane, imino-, 109 Alkali metal complexes, 1-70 acid anions, 30 acid salts, 30 bipyridyl, 13 crown ethers cavity size, 38 cryptates... 

Table 2 Crown ether cavity sizes and cation diameters.

Examples where the metal cation is small compared to the size of the macrocycle fall into two categories. The most conunon result is a constrictiou of the crown ether cavity, so that the macrocycle shrinks to match the cation radius. Au example is the Na+ complex of 18-crown-6 (91) the couformatiou of the crown is quite distorted compared to that observed in (86), with five of the O-donor atoms lying in a plane about the metal ion, and the sixth folded out of the plane. 

The preference of the SIPE for the trivalent actinides is due to the stabilization of the crown ether/ bonds due to a size-fitting effect where the diameter of the metal ion matches that of the crown ether cavity, leading to significantly greater extraction efficiency due to this synergistic effect. Another widely studied /3-diketone for the extraction of trivalent actinides has been l-phenyl-3-methyl-4-benzoylpyrazolone-5 (HPMBP), illustrated in Figure 93. The extraction of Am , Cm , Bk , and Cf by HPMBP may be expressed in two ways. First, the extraction of Cm has been observed to follow the extraction equilibrium shown in Equation (67) ... 

Separation of cations can be influenced by their interaction with crown ethers, which depends on the sizes of the cation and the crown ether cavity. The concentration of a crown ether in the running buffer also plays a role. The best results have been obtained with 18-crown-6-ether where the selectivity changes were largest. The use of crown ethers makes it possible to separate, e.g., potassium from ammonium. Electrolyte containing 4mmoll 18-crown-6, 4 mmol 1 copper sulfate, and 4 mmol 1 formic acid was successfully applied to complete separation of all alkali and alkaline earth cations including ammonium (Figure 2). 

The cavity size of the crown ether unit. Conformational positioning of the proton-ionizable side arm with respect to the crown ether cavity. Capping of residual silanol surface with trimethyl-silyl was found to influence the selectivity and efficiency properties incorporated into these... 

Crown ethers are cyclic polyethers that form complexes with cations. Ligand specificity is highly dependent on the size of the crown ether cavity for example, 12-crown-4 binds Li but not K dicyclohexano-18-crown-6 bindsi K but not Li. ... 

Chu et al. later reported the influence of various metal cations on the LCST of poly(NIPAM-co-BCAm). ° The additions of K+, Sr +, Ba +, Hg +, and Pb induced increases in the LCST. These ions possessed higher association constants than the others because their ionic radii fit the cavity size of the benzo[18]-crown-6-ether. However, while Cs is too large to fit into the crown ether cavity of this macrocycle, it could form a stable 2 1 complex with the crown ethers. 

Macrocyclic ligands such as crown ethers have been widely used for metal ion extraction, the basis for metal ion selectivity being the structure and cavity size of the crown ether. The hydrophobicity of the ligand can be adjusted by attachment of alkyl or aromatic ligands to the crown. Impressive results have been obtained with dicyclohexano-18-crown-6 as an extractant for Sr in [RMIM][(CF3S02)2N] IL/aque-... 

On the other hand, Bartsch et al. have studied cation transports using crown ether carboxylic acids, which are ascertained to be effective and selective extractants for alkali metal and alkaline earth metal cations 33-42>. In a proton-driven passive transport system (HC1) using a chloroform liquid membrane, ionophore 31 selectively transports Li+, whereas 32-36 and 37 are effective for selective transport of Na+ and K+, respectively, corresponding to the compatible sizes of the ring cavity and the cation. By increasing the lipophilicity from 33 to 36, the transport rate is gradually... 

Problem 18.15 l 15-Crown-5 and 12-crown-4 ethers complex Na+ and Li+, respectively. Make models of these crown ethers, and compare the sizes of the cavities. 

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