Intermolecular nuclear relaxation in paramagnetic solutions: from free radicals to rare earths

The principles of the intermolecular relaxation of a nuclear spin by its fluctuating magnetic dipolar interactions with the electronic spins of the paramagnetic surrounding species in solution are briefly recalled. It is shown that a very high dynamic nuclear polarization (DNP) of solvent protons is obtained by saturating allowed transitions of free radicals with a hyperfine structure, and that this effect can be used in efficient Earth field magnetometers. Recent work on trivalent lanthanide Ln³⁺ aqua complexes in heavy water solutions is discussed, including paramagnetic shift and relaxation rate measurements of the ¹H NMR lines of probe solutes. This allows a determination of the effective electronic magnetic moments of the various Ln³⁺ ions in these complexes, and an estimation of their longitudinal and transverse electronic relaxation times T_1e and T_2e. Particular attention is given to Gd(III) hydrated chelates which can serve as contrast agents in magnetic resonance imaging (MRI). The full experimental electronic paramagnetic resonance (EPR) spectra of these complexes can be interpreted within the Redfield relaxation theory. Monte-Carlo simulations are used to explore situations beyond the validity of the Redfield approximation. For each Gd(III) complex, the EPR study leads to an accurate prediction of T_1e, which can be also derived from an independent relaxation dispersion study of the protons of the probe solutes.