Cryptand conductivity

For cryptands in which the molecular cavity is larger than in the case of the [l.l.l]-species [78], proton transfer in and out of the cavity can be observed more conveniently. Proton transfer from the inside-monoprotonated cryptands [2.1.1] [79], [2.2.1] [80], and [2.2.2] [81 ] to hydroxide ion in aqueous solution has been studied by the pressure-jump technique, using the conductance change accompanying the shift in equilibrium position after a pressure jump to follow the reaction (Cox et al., 1978). The temperature-jump technique has also been used to study the reactions. If an equilibrium, such as that given in equation (80), can be coupled with the faster acid-base equilibrium of an indicator, then proton transfer from the proton cryptate to hydroxide ion... 

Monoprotonation of the [2.1.1]-cryptand occurs rapidly but protonation of the monoprotonated species by hydronium ion and other acids can be followed kinetically in various solvents (Cox et al., 1982, 1983). In methanol, protonation of ii+ species by substituted acetic and benzoic acids to give i+i+ has been studied using the stopped flow technique with conductance detection. The values of the rate coefficients (kHA) for protonation (81) vary with the acidity of the donor acid from kHA = 563 dm3mol-1s-1 (for 4-hydroxy-benzoic acid) to kHA = 2.3 x 105 dm3mol 1s 1 (for dichloroacetic acid). 

The addition of a cryptand to some polyelectrolytes leads to significant increases in conductivity and in some cases IR and Raman spectroscopy demonstrate that the cryptand breaks up the ion-ion interactions (Chen, Doan, Ganapathiappan, Ratner and Shriver, 1991 Doan, Ratner and Shriver, 1991). Apparently the reduction of ion association more than offsets the reduction in mobility of the cation-crypt complex, which has a larger effective radius than the simple cation. It is also possible that the cryptand-ion complex is rendered more mobile by the reduction of polymer-cation complex formation, but this point has not been investigated in any detail. 

Conductivity monitoring is most valuable for studying reactions which have very small spectral changes but which are accompanied by pH changes. The interaction of group 1 and 2 metal ions with cryptands and diaza-crown ethers has been studied by flow/conductivity methods. Conductivity monitoring has been linked to reactions which may follow pulse radiolysis, for example, in examining the... 

Crown ethers and cryptands can solubilize certain normally insoluble reagents in solvents of low polarity. Problems of cosolubility between reagent and reactant can thus be overcome and the reaction conducted under milder conditions. 

K (26). The former value leads to an interlonlc distance i equal to 4.6 A according to the classical Fuoss equation. This value Is too small compared to the results obtained for cryptated living polypropylene sulfide (8) and for cryptated tetraphenyl-borides In THF (24). This might mean that either K is not located inside the cavity of the ligand or the oxanion can penetrate into the cavity of the cryptand. This last explanation is consistent with comparative conductivity data made on model compounds (17) as shown in Table III. 

Simultaneously, extensive investigations concerning the physico-chemical properties of cryptands have been conducted covering several sub-topics their complexing properties, X-ray analysis, molecular dynamics of inclusion complexes, etc. 

Pticyna et al. measured the electrical conductivity of polyoxirane terminated by OEtO K+ active centres in tetrahydrofuran. At concentrations of > 10 3 mol dm-3 these centres associate. The dissociation constant of low-molecular-weight alcoholates increases with growing chain length. Polyoxyethylene) (in a similarly way to crowns and cryptands) solvates the cation and thus promotes charge separation [259]. Littlejohn et al. quantified EtOCH SbCl dissociation in MeCl2 by specific conductivity and permittivity determinations [260]. 

Recent work has helped to refine the understanding of the physical and magnetic properties of these systems. The synthesis, structure, polymorphism, and electronic and magnetic properties of the electride Rb(cryptand[2.2.2])e have been described. Depending on the manner of preparation and the temperature, the antiferromagnetic electride can display a range of elecrical conductivity, from poor (<10 " Scm )—consistent with localized electrons— to near-metallic electrical conductivity. Studies of the phase transitions in Cs+(18-crown-6)2e with NMR, EPR, and variable-temperature powder X-ray diffraction indicates that it undergoes a slow irreversible... 

PT catalysts commonly used are quaternary onium salts (ammonium and phospho-nium), crown ethers, cryptands, and polyethylene glycols. The essential characteristics of a PT catalyst are that the catalyst must have the ability to transfer the reactive anion into the organic phase to conduct the nucleophilic attack on the organic substrate, and effect a cation-anion bonding loose enough to allow a high reaction rate in the organic phase. 

The function of solid-liquid phase transfer catalysis (SLPTC) is to conduct the reaction of a solid salt and the organic reactant using a PT catalyst that is easily dissolved in the organic phase in the absence of water. These catalysts can be tertiary amines, quaternary ammonium salts, diamines, crown ethers and cryptands, among which crown ethers, act as the catalysts because of their specific molecular structures [183-186]. Starks et al. [183] indicated that 100% of the yield of product benzyl acetate was obtained at 25°C in 2 h for... 

Kitano, H. Hasegawa, J. Iwai, S. Okubo, T. Kinetic study of the compiexation of cryptand 222 with alkaline earth ions by the conductance stopped-flow method. J. Phys. Chem. 1986, 90 (23). 6281-6284. 

The purpose of this monograph is to present some recent results involving the redox chemistry of some metal ion complexes, some of which contain very traditional and well established ligands, such as bipyridine (bipy), while others involve more esoteric ones, such as the cryptands. However, the main theme is not necessarily the electrochemistry of these complexes, nor their electron spin resonance (ESR) spectra, but rather the properties of the resulting redox products, whether in solution or in the solid state. Some of these redox products have been successfully crystallized, and some of their solid state properties have been measured, including X-ray diffraction and conductivities. These will be briefly presented. In addition, some of the properties of these reduced complexes in solution will also be presented and discussed. 

In US EPA Method 314.0, the determination of the matrix conductivity threshold (MCT) is required to assess the maximum concentration of common inorganic anions that the column should tolerate before observing significant loss of sensitivity for perchlorate. For samples that exceed the MCT, a dilution or sample pretreatment using OnGuard cartridges is required. EPA Method 314.1 eliminates these procedures by using a Cryptand concentrator column that... 

Poly(bis-methoxyethoxycthoxy phosphazene) (MEEP) and cryptand[2.2.2] (crypt) have b n intercalated separately into sodium exchanged montmorillonite (Na-mont), and impedance spectroscopy indicates that ionic conductivities of the resulting nanocomposites (log... 

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