¹⁹F and ¹H Magnetic Relaxation Dispersion Determination of the Translational Encounter between Ionic Salts and Nitroxide Free Radicals in Aqueous Solution

Abstract

Measurements of 19F and 1H nuclear spin−lattice relaxation times from aqueous solutions of PF6 - and (H3C)4N+ containing small concentrations of nitroxide 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-dependent spectral densities that modulate relaxation. This magnetic relaxation dispersion (MRD) technique may provide detailed information concerning molecular 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, and the treatment of the intermolecular potential as a reduction (increase) in the number of effective translational encounters between ions of like (unlike) charge is discussed. Represented by a mean field, the relatively long range Coulombic interaction does not impose spatial conditions on the diffusion equation through which the short-range magnetic dipole−dipole interactions are correlated. Calculated distances of closest approach are approximately 6.5 Å and are consistent with the dimensions expected for a sterically impeded encounter between the nuclear spin probes and the nitroxide centered paramagnetic spin density. Comparison of the (H3C)4N+ and PF6 - data provides a means of quantifying the Coulombic potential, which has a more dramatic effect on the 19F relaxation of the polarizable PF6 -.

Overhauser dynamic nuclear polarization and molecular dynamics simulations using pyrroline and piperidine ring nitroxide radicals

“…These results are remarkably close to each other, and provide further evidence that the dipolar coupling between the unpaired electron and 1 H of water is not affected by the properties of the 6-ring versus 5-ring structure, and that the key DNP parameters are the same for each radical. In agreement with previous FCR studies using nitroxide radicals [12,13,16,17], the translational model fits the data well; supporting the approximation that complex formation and rotational effects can be ignored in our DNP analysis. Our MD studies discussed below provide further support for this approximation.…”

“…(2) along with the self diffusion coefficient of water (2.3 Â 10 À9 m 2 s À1 [11]) gives d = 4.5 Å for hydrated spin label environments. The assumption that the self diffusion of bulk water can be used as the D I value for water directly interacting with the nitroxide radical is supported by our MD studies discussed below and by previous FCR experiments between nitroxide radicals and small charged solutes [12].…”

Molecular Oxygen Spin–Lattice Relaxation in Solutions Measured by Proton Magnetic Relaxation Dispersion

“…A second factor is the probability that a water molecule is in contact with the paramagnetic relaxation center. In neat solvents, this factor is suppressed, but included as a scaling factor that accounts quite well for the observations in dilute electrolyte solutions when the force-free conditions are relaxed (18). In the water-glycerol solution, the presence of glycerol excludes a certain volume from simultaneous occupancy by water adjacent to the oxygen.…”

Relaxometry of insensitive nuclei: Optimizing dissolution dynamic nuclear polarization

“…The theory was further developed by Ayant, Belorizki, Freed and coworkers, introducing minimum approach distances [35], as well as arbitrary pair correlation functions [36], rapidly relaxing spin, diffusion jumps and frequency dependent diffusion [37]. Bryant and coworkers have published numerous studies involving translational motions in the vicinity of paramagnetic nitroxides [38][39][40]. In their notation [41], the paramagnetic contribution to the spinlattice relaxation rate is…”