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  • 1
    ISSN: 1572-8927
    Keywords: N-methylformamide ; nonaqueous solvents ; preferential solvation ; isosolvation point ; sodium-23 NMR
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Sodium-23 NMR chemical shifts and linewidths have been measured for 0.1M NaClO4 in binary mixtures of N-methylformamide (NMF) with a series of other solvents, as a function of the solvent mole fraction. The relative solvent composition at the isosolvation point, the mid-value of the Na-23 chemical shift between those measured in the respective pure solvents, reveals preferential solvation of the sodium cation in many cases. The isosolvation composition correlates well with the relative solvating abilities of the two solvents-as characterized by their donicities-provided that the cation-solvent interactions are of the ‘hard-hard’ type and that they are not complicated by interionic interactions. The variation in the electric field gradient around the sodium nucleus, as the composition of the solvent changes, results in broadening of the resonance line. Maximum broadening occurs close to the solvent mole fraction corresponding to the isosolvation point.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1572-8927
    Keywords: Lithium-7 NMR ; sodium-23 NMR ; cesium-133 NMR ; far infrared ; alkali complexes ; cryptates ; formation constants ; nonaqueous solvents ; membranes
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Nuclear magnetic resonances of alkali nuclei,7Li,23Na, and133Cs, as well as far infrared measurements are used to study alkali complexes of a bicyclic diazapolyoxa ligand—the dilactam of cryptand C222. Measurements were carried out in pyridine, tetrahydrofuran, acetonitrile, nitromethane, dimethylformamide, and aqueous solutions. The complexing ability of the dilactam is similar to, but weaker than, that of the cryptand C222. The limiting chemical shifts of the complexed cations were solvent-dependent, indicating incomplete enclosure of the cation by the ligand. Formation constants of Li+ and Cs+ complexes were calculated from the chemical-shift dependence on the ligand/metal ion mole ratio.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1572-8927
    Keywords: Crown ethers ; sodium-23 NMR ; cesium-133 NMR ; thallium-205 NMR ; nonaqueous solvents ; enthalpy of complexation ; entropy of complexation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Formation constants for complexes of dibenzo-21-crown-7, dibenzo-24-crown-8 and dibenzo-27-crown-9 with the Na+, Cs+ and Tl+ ions have been determined by multinuclear NMR measurements in several nonaqueous solvents. The measurements of the cesium complexes were carried out at different temperatures and the enthalpy and entropy of complexation were determined from the temperature dependence of the formation constants. The stabilities of these complexes in solvents of low to medium donicity, —nitromethane, acetonitrile, acetone, methanol, and propylene carbonate, vary in the order Tl+〉Cs+〉Na+. Depending on the relative sizes of the cation and of the ligand cavity, either a three-dimensional ‘wrap-around’ complex or a two-dimensional ‘crown complex’ are formed. For the cesium complexes, the values of ΔH c o and ΔS c o are definitely solvent-dependent and in all cases the complexes are enthalpy stabilized but entropy destabilized. A compensating effect is observed in the variation of the enthalpy and entropy of complexation so that the overall influence of the above solvents on the stability of the complexes is rather limited.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1572-8927
    Keywords: Solvation ; ion pairs ; hexafluorophosphate ion ; fluorine-19 NMR ; sodium-23 NMR ; nonaqueous solvents
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Fluorine-19 and sodium-23 NMR measurements were carried out on sodium hexafluorophosphate solutions in a number of solvents. In solvents of medium polarity and donicity (e.g., propylene carbonate, acetone, acetonitrile) the 19 F chemical shift moved upfield with increasing concentration of the salt. This behavior is indicative of anion-cation interactions which may be of long-range type, i.e., formation of solvent-separated ion pairs; the possibility of contact ion pair formation, however, cannot be excluded. In solvents of low polarity and donicity (acetic acid, tetrahydrofuran), the salt is essentially completely associated in the 0.1–1.0M concentration range. On the other hand, in solvating solvents with high dielectric constants, such as dimethyl-formamide, dimethylsulfoxide, and formamide, there is very little ionic association in the same concentration range. The above conclusions are supported by 23 Na chemical shift measurements. Potassium hexafluorophosphate solutions do not show any concentration dependence of the 19 F chemical shifts, while for tetra-n-butylammonium solutions the 19 F resonance moves downfield with increasing concentration of the salt.
    Type of Medium: Electronic Resource
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