TEMPERATURE, ORIENTATION, AND SOLVENT DEPENDENCE OF ELECTRON SPIN-LATTICE RELAXATION RATES FOR NITROXYL RADICALS IN GLASSY SOLVENTS AND DOPED SOLIDS

T-1 was measured by saturation recovery for nitroxyl radicals as a function of temperature, solvent, and position in the CW spectrum. Long saturating pulses were used such that contributions to the saturation recovery curve were dominated by T-1 rather than by the tumbling correlation time or the nitroxyl nitrogen nuclear spin relaxation time. Data between about 20 and 150 K were fitted to log (1/T-1) = (2.34 +/- 0.15) x log (T) + (7.0 +/- 0.3). This temperature dependence is consistent with a two-phonon Raman process, although the relevant vibrations are likely to be intramolecular rather than lattice modes. At higher temperatures and in solvents with weak solute-solvent and or solvent/solvent interactions, there is increasing mobility of the nitroxyl, which causes T-1 to be more strongly temperature dependent. T-1 values are orientation dependent: T-1 is longer when the external magnetic field is along the molecular z axis and shorter when the magnetic field is in the perpendicular plane. This orientation dependence is consistent with the orientation dependence of spin-orbit coupling and with symmetry arguments concerning the orientation dependence of vibrations. Deuteration of the nitroxyl, deuteration of solvent, or replacement of natural-abundance nitrogen by nitrogen-15 in the nitroxyl caused less than 10% change in T-1, which indicates that inter- or intramolecular electron-nuclear couplings are not the dominant factors in nitroxyl spin-lattice relaxation in the temperature regimes examined. (C) 1995 Academic Press, Inc.