F-19 and H-1 magnetic relaxation dispersion determination of the translational encounter between ionic salts and nitroxide free radicals in aqueous solution

Measurements of F-19 and H-1 nuclear spin-lattice relaxation times from aqu eous solutions of PF6- and (H3C)(4)N+ containing small concentrations of ni troxide free radicals were made at applied magnetic field strengths ranging from 0.00025 to 7.05 T to directly determine the form of the frequency-dep endent spectral densities that modulate relaxation. This magnetic relaxatio n dispersion (MRD) technique may provide detailed information concerning mo lecular dynamics over the time scale range from milliseconds to picoseconds . The MRD data compare well to a theory for translational diffusion of hard spheres, one that accounts for the intermolecular electrostatic potential between ionic solutes. Theoretically accessible parameters are extracted, a nd the treatment of the intermolecular potential as a reduction (increase) in the number of effective translational encounters between ions of like (u nlike) charge is discussed. Represented by a mean field, the relatively lon g range Coulombic interaction does not impose spatial conditions on the dif fusion equation through which the short-range magnetic dipole-dipole intera ctions are correlated. Calculated distances of closest approach are approxi mately 6.5 Angstrom and are consistent with the dimensions expected for a s terically impeded encounter between the nuclear spin probes and the nitroxi de centered paramagnetic spin density. Comparison of the (H3C)(4)N+ and PF6 - data provides a means of quantifying the Coulombic potential, which has a more dramatic effect on the F-19 relaxation of the polarizable PF6-.