NMR PROTON RELAXATION AND CHEMICAL-EXCHANGE IN THE SYSTEM (H2O)-O-16/(H2O)-O-17-[H-2(6)]DIMETHYLSULPHOXIDE

NMR proton relaxation rates of normal and O-17 enriched water in a mixture of 68 mol % water and 32 mol % [H-2(6)]dimethylsulphoxide were measured for temperatures between 298 K and 183 K. In the range between 240 K and 204 K the limit of fast proton-proton exchange between H-2-O-16 and H-2-O-17 is not obeyed, and relaxation curves deviate from mono-exponential behaviour. By fitting the relaxation curves to a model of NMR two-phase relaxation the proton-proton exchange rate within the aqueous component could be obtained. With decreasing temperature, proton-proton exchange slows down and a residence time of about 125 ms at 215 K is found, but it becomes faster again for still lower temperatures. From the phase-averaged relaxation rates of water in the O-17 enriched mixtures. the O-17 induced proton relaxation rate was derived as a function of temperature. This yields the rotational correlation times of the water molecule in the mixture and the dipolar spin-lattice coupling parameter. The latter is considerably lower than the one predicted from the geometry of water.