Molecular Motion and Relaxation in Free-Radical Solutions of Benzene, Toluene, and Some Ethers as Studied by Dynamic Nuclear Polarization

Abstract

Magnetic double-resonance methods have been used to investigate the spectrum of motion, the proton and electron spin relaxation in organic solutions of free radicals. Studies of both frequency and temperature dependences of the dynamic nuclear polarization and the relaxation times yield information on the solvent-nucleus—free-radical interactions and on the molecular motion. Measurements in very weak magnetic fields (15 G) realize the conditions of ``extreme narrowing,'' data at 1070 and 3230 G fall into the main region of decay of the Overhauser effect. The results in radical solutions of benzene, toluene, dimethoxymethane, dimethoxyethane, and diethoxymethane can be explained by a pure dipole—dipole interaction governed by translational random motions of the spin-carrying molecules. Correlation times, activation energies, and other parameters are evaluated. An influence of different free radicals (molecules and ions) could obviously not be observed. Toluene at low temperatures displays an exceptional behavior. Some consequences for the phenomena of Overhauser polarization, proton, and electron spin—lattice relaxation in general are discussed. This especially includes the high-frequency limit of the dynamic nuclear polarization and the dependences of both leakage and saturation parameters important for all applications.