NMR T1 study of dipolar and scalar spectral densities for intermolecular relaxation in solutions of free radicals over a large frequency and temperature range

Abstract

Abstract Spin-lattice relaxation rates of 1H and 19F NMR have been measured at various temperatures and frequencies between 0.1 and 60 MHz in solutions of polyf uorinated benzenes containing free radicals. Analysis of the low-frequency data shows that the radical-induced dipolar relaxation of both nuclei is governed by the relative translational diffusion of the pairwise interacting molecules. Correlation times, relative diffusion coefficients, and activation energies have been determined. Dipolar relaxation over the whole frequency range cannot be explained by existing models based on a completely isotropic relative diffusion of of solvent and solute molecules; at high frequencies it decays more rapidly than expected. In addition to the dipolar interaction the relaxation rate of `9F contains diffusion-controlled scalar couplings characteristic of electron spin-density transfer during molecular collisions. Both the strength and the time dependence of this interaction have been deduced from the experiments.