Magnetic relaxation dispersion of 7Li. II. Complex formation with nitroxides in the aqueous phase

Measurements of 7Li nuclear spin-lattice relaxation times are made at applied magnetic field strengths from 0.25 mT to 7.05 T, in order to determine directly the form of the frequency-dependent spectral densities that modulate relaxation. This magnetic resonance dispersion (MRD) technique provides detailed information regarding molecular dynamics down to the picosecond time scale. 7Li MRD measurements made on aqueous lithium ion in the presence of small concentrations of nitroxide free radicals give direct evidence supporting the formation of a coordination complex. The dipole-dipole electron-nuclear coupling is modulated by both translational and rotational diffusive motions, and both of these contributions are resolved. However, scalar coupling arising from the presence of paramagnetic electron spin density at the nucleus dominates the nuclear relaxation. Changes in the pH and free radical moiety are compared with dynamical variables, geometric constraints, and formation constants obtained from a model of nuclear relaxation. The calculated bimolecular formation constants are on the order of 2 x 10(-3) M-1, and the relative accuracy of this parameter is tested.