Saturation-recovery measurements of the spin-lattice relaxation times of some nitroxides in solution

Abstract Spin-lattice relaxation times ( T 1e ) of the simple nitroxides 1-oxyl-2,2,6,6-tetra-methyl-4-hydroxypiperidine (tanol), perdeutero-l-oxyl-2,2,6,6-tetramethyl-4-piperidone (tanone), and 15 N-substituted tanone were measured from +30 to -85dg. The solvent was sec-butyl benzene. Although η T (where η is the viscosity) varies by two orders of magnitude over this temperature range, T 1e varies by only one. At highest temperature spin-rotation may be the dominant relaxation mechanism, but at all other temperatures relaxation times are substantially shorter than predicted by known mechanisms. Intermolecular motions are thought to be playing an important role in the strong relaxation. Intermolecular relaxation caused by electron-electron dipolar interactions at high concentrations also exhibits a weak dependence on η T .