Browsing by Author "Holz, Manfred"

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D2O-H2O Isotope Effect on Nuclear Magnetic Relaxation of Alkali Halide Nuclei and Preferential Solvation in Mixed Solvents
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1978, 74 (3), 1978) Holz, Manfred
The effect of substitution of H2O by D2O on the nuclear magnetic relaxation of the ionic nuclei 23Na, 87Rb and 81Br, which all relax by quadrupole interaction, has been investigated in several NaBr and RbBr solutions. The solvents were neat water and binary mixtures of water with methanol, formamide and dimethylsulphoxide. In the neat aqueous solutions it was found experimentally that the isotope effect is similar to that on the viscosity and that even at high salt concentrations, where all water is “hydration water”, this effect is still relatively strong. In the mixed solvents the composition dependence of the isotope effect on the relaxation rates has been studied in the water rich region, showing e.g. in methanol + water mixtures a quite different behaviour for cationic and anionic nuclei. A new method is presented, which, with certain assumptions, allows us to calculate the local mole fraction of water in the inner solvation sphere of the ions from the measured D2O–H2O isotope effect as a function of the macroscopic solvent composition. The analysis yielded the result that Na+ and Rb+ in methanol + water and to a smaller extent also Rb+ in formamide + water are preferentially hydrated. In the same way it was found that Br– is weakly preferentially solvated by methanol and Na+ by dimethylsulphoxide, whereas Br– is not selectively solvated in formamide + water. The results are discussed and compared with those obtained by other methods.
Nuclear Magnetic Relaxation of Alkali Halide Nuclei and Preferential Solvation in Methanol-J-Water Mixtures
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1977, 73 (1), 1977) Holz, Manfred; Weingartner, Hermann; Hertz, Hermann-Gerhard
Nuclear magnetic relaxation rates of the ionic nuclei 35Cl, 81Br, 127I, 23Na and 87Rb in methanol + water mixtures have been measured over the complete mixtures range and extrapolated to zero salt concentration. The relaxation of all these nuclei is controlled by quadrupole interaction; in the theoretical part of this paper a formula is given which describes their relaxation behaviour in the mixtures. In connection with these results a method is presented which uses the quadrupolar relaxation studies as a source of information regarding preferential solvation and which yielded the result that Na+ and Rb+ are preferentially hydrated whereas Cl– and Br– are preferentially solvated by methanol in the mixtures. These results are compared with those derived from chemical shift measurements by other authors and the discrepancy, so revealed, for Cl–(and Br–), is discussed.