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

摘要

For nitroxide radicals in solution one can identify three frequency regimes in which ~1H spin-lattice relaxation rate of solvent molecules depend linearly on square root of the ~1H resonance frequency. Combining a recently developed theory of nuclear (proton) spin-lattice relaxation in solutions of nitroxide radicals [D. Kruk, J. Chem. Phys. 137, 044512 (2012)]10.1063/1.4736854 with properties of the spectral density function associated with translational dynamics, relationships between the corresponding linear changes of the relaxation rate (for ~(14)N spin probes) and relative translational diffusion coefficient of the solvent and solute molecules have been derived (in analogy to ~(15)N spin probes [E. Belorizky, 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 ~1H nuclear magnetic resonance (NMR) relaxometry. The approach has thoroughly been tested by applying to a large set of experimental data - ~1H spin-lattice relaxation dispersion results for solutions of different viscosity (decalin, glycerol, propylene glycol) of ~(14)N and ~(15)N 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 ~1H 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.