^(15)N Nuclear Magnetic Resonance Spectroscopy. Changes in Nuclear Overhauser Effects and T_1 with Viscosity

Abstract

A procedure is described for probing changes in ^(15)N 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 ^(15)N NOE of NH_4Cl 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.