Abstract
Nuclear magnetic relaxation rates of23Na+,87Rb+, and14N in acetonitrile-water mixtures have been measured over the complete mixture range. The interaction of the quadrupole moment of ionic nuclei with electric field gradients is an excellent short ranged probe for the direct neighborhood of ions in solution. Thus the23Na+ and87Rb+ relaxation contains information about dynamics composition and symmetry of the inner solvation sphere in the mixed solvent. It was found that the relaxation rate of both ionic nuclei has an unexpected marked maximum in the acetonitrile (AN) rich region. The14N and2H relaxation rates of the solvent molecules revealed that the maximum could not be explained by dynamic effects. Further experimental results showed that it is caused by local symmetry changes. By measuring 1/T1 of the cationic nuclei in AN-H2O and AN-D2O it was possible for the first time to separate quantitatively asymmetry effects from selective solvation effects. It turned out that both cations are strongly preferentially hydrated. Comparison of the results of two approaches for the evaluation of the D2O-H2O isotope effect led to interesting hints concerning the location of the electric point dipole in acetonitrile molecules having contact with cations.
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Braun, B.M., Holz, M. Separation of local asymmetry and selective solvation effects using the quadrupole relaxation of the23Na+ and87Rb+ ions in acetonitrile-water mixtures. J Solution Chem 12, 685–701 (1983). https://doi.org/10.1007/BF00647381
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DOI: https://doi.org/10.1007/BF00647381