Rotational molecular motion and solvation of Na+ and Cs+ in water-dimethylacetamide. A nuclear magnetic study

Nuclear magnetic relaxation by intra- and intermolecular quadrupoleelectric field gradient interaction has been used for the study of the systems DMA-water-NaI and DMA-water-CsI at 25°C.14N relaxation of DMA and2H relaxation of D2O measured over the complete mixture range reveal the behavior of the rotational molecular motion of the two solvent components. For both solvent components a marked maximum of the reorientational correlation time has been found, reflecting hydrophobic effects and strong DMA-water interaction. The quadrupolar relaxation rates of23Na+ and133Cs+ in pure DMA were evaluated giving an indication that the electric solvent dipoles in the solvation shell are not located on positions of cubic symmetry. A quantitative study of preferential solvation of the cations in the mixed solvent has been performed by using the H2O-D2O isotope effect on23Na+ and133Cs+ relaxation. For both cations an obviously typical change in the selectivity occurs. In the range l>xH2O>0.7 we find weak preferential hydration, but in the range 0.7>x H2O>0 strong preferential solvation by DMA is reflected.