Molecular Dynamics in the System Methanol-Dimethylsulphoxide

NMR proton relaxation rates of normal and oxygen-17 enriched m ethanol-d3 in a mixture of 71 mol% methanol and 29 mol% dimethylsulphoxide-d6 (DMSO) were measured as a function of temperature between 298 and 158 K. From these data rotational correlation times of the methanol molecule in the mixture and the oxygen-proton dipolar spin-lattice coupling parameter were ob­tained. The latter parameter is considerably smaller than the one obtained from neutron diffraction studies of the molecular geometry of methanol. Additionally, deuteron and oxygen-17 relaxation rates were measured over the same temperature range. Using the rotational correlation times obtained from the oxygen-17 induced proton relaxation rates, the quadrupole coupling constant for deuterium was derived; it shows a temperature dependence. Application of the Poplett relation yielded the oxygen-17 quadrupole coupling constants, so that oxygen correlation times could be obtained. Comparison of deuteron and oxygen correlation times indicates that the reorientation of the methanol molecule in the methanol/DMSO mixture is anisotropic.