Translational and rotational molecular motion in solutions of alkali-metal halides in dimethyl sulphoxide

Abstract

The relative proton relaxation rates 1/T1 and the translational diffusion coefficients of dimethyl sulphoxide (DMSO) in solutions of NaBr, KBr, Nal, KI, RBl and CsI have been measured at 25 °C. The n.m.r. B′ coefficients and the self-diffusion BD coefficients have been determined for all the salts. For a given salt the B′ and BD values are equal within the experimental error limits. A comparison of these data with literature values for the viscosity B coefficients of the same systems revealed that in DMSO (and in other organic liquids) the viscosity B coefficients always have higher values than the corresponding B′ and BD coefficients. Thus, the viscosity is more strongly influenced by the presence of the ions than the reorientational and translational molecular motion of the solvent. The n.m.r. B′ coefficients have been split into intra- and inter-molecular contributions (B′intra and B′inter) and into single-ion values. Thus the ratio τ±c/τ0c of the reorientational correlation time in the first solvation sphere of the ions to the reorientationl correlation time in the bulk could be determind. These values reflect the relatively strong ion–solvent interaction; however, the Na+–DMSO interaction turns out to be weaker than expected. The equality of B′intra and BD found for all systems leads to the conclusion that in the aprotic solvent DMSO strong coupling exists between translational and rotational molecular motion.