15N Nuclear Magnetic Resonance Spectroscopy. Changes in Nuclear Overhauser Effects and T 1 with Viscosity

A procedure is described for probing changes in 15N T 1 relaxation rates and nuclear Overhauser effects (NOE) with viscosity as a function of temperature. The large freezing-point depression and high viscosities of 8.8 M dimethyl sulfoxide (DMSO)−water solutions allowed study of the molecular motions of several ammonium salts, amides, and heterocycles on the pico- to nanosecond time scale. Dipole−dipole interactions provide the dominant form of relaxation for the ammonium salts, but chemical-shift anisotropy (CSA) also plays a significant role in the relaxation of amides and heterocycles. For pyridine, CSA is a particularly 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 15N NOE of NH4Cl is greater than the theoretical maximum for intramolecular dipolar 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.