N-15 NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY - CHANGES IN NUCLEAR OVERHAUSER EFFECTS AND T-1 WITH VISCOSITY

A procedure is described for probing changes in N-15 T-1 relaxation ra tes and nuclear Overhauser effects (NOE) with viscosity as a function of temperature. The large freezing-point depression and high viscositi es of 8.8 M dimethyl sulfoxide (DMSO)-water solutions allowed study of the molecular motions of several ammonium salts, amides, and heterocy cles on the pico- to nanosecond time scale. Dipole-dipole interactions provide the dominant form of relaxation for the ammonium salts, but c hemical-shift anisotropy (CSA) also plays a significant role in the re laxation of amides and heterocycles. For pyridine, CSA is a particular ly important mechanism and the shielding anisotropy of pyridine in 8.8 M DMSO-water is estimated to be on the order of 325 ppm. The N-15 NOE of NH4Cl is greater than the theoretical maximum for intramolecular d ipolar relaxation at high viscosities, and can be accounted for either by significant intermolecular contributions operating on a different time scale or by rapid motional averaging.