Translational diffusion in paramagnetic liquids by 1H NMR relaxometry: Nitroxide radicals in solution

For nitroxide radicals in solution one can identify three frequency regimes in which H-1 spin-lattice relaxation rate of solvent molecules depend linearly on square root of the H-1 resonance frequency. Combining a recently developed theory of nuclear (proton) spin-lattice relaxation in solutions of nitroxide radicals [D. Kruk et al., J. Chem. Phys. 137, 044512 (2012)] with properties of the spectral density function associated with translational dynamics, relationships between the corresponding linear changes of the relaxation rate (for N-14 spin probes) and relative translational diffusion coefficient of the solvent and solute molecules have been derived (in analogy to N-15 spin probes [E. Belorizky et al., J. Phys. Chem. A 102, 3674 (1998)]). This method allows a simple and straightforward determination of diffusion coefficients in spin-labeled systems, by means of H-1 nuclear magnetic resonance (NMR) relaxometry. The approach has thoroughly been tested by applying to a large set of experimental data-H-1 spin-lattice relaxation dispersion results for solutions of different viscosity (decalin, glycerol, propylene glycol) of N-14 and N-15 spin probes. The experiments have been performed versus temperature (to cover a broad range of translational diffusion coefficients) using field cycling spectrometer which covers three decades in H-1 resonance frequency, 10 kHz-20 MHz. The limitations of NMR relaxometry caused by the time scale of the translational dynamics as well as electron spin relaxation have been discussed. It has been shown that for spin-labeled systems NMR relaxometry gives access to considerably faster diffusion processes than for diamagnetic systems. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4772097]