Frequency and temperature dependence of NMR relaxation for protic solvents with complexes of nitroxide radicals

Abstract

Spin-lattice relaxation times of various protons of methanol, propanol-2, and acetic acid have been measured between 100 kHz and 60 MHz over a wide temperature range for solutions containing nitroxide free radicals. Relaxation of the aprotic solvent protons (CH 3OD, CH 3COOD, C 3H 7OD) is governed by dipolar interactions with the unpaired electron, where the time dependence is given by elastic collisions with the free radical molecule (“translational motions”). Relaxation of the acid protons (CD 3OH, CD 3COOH, C 3D 7OH) is dominated by couplings via the hydrogen bond NȮ…HO. Scalar interaction contributes only at low frequencies and is modulated by lifetime effects of the complex of radicals with organic ligands. High relaxation rates by dipole-dipole interaction can be explained by anisotropic motions of the complexes. Spectral density functions for the various mechanisms and several parameters of the respective models have been determined and are discussed.