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

Abstract

For nitroxide radicals in solution one can identify three frequency regimes in which (1)H spin-lattice relaxation rate of solvent molecules depend linearly on square root of the (1)H 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 (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 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 (1)H nuclear magnetic resonance (NMR) relaxometry. The approach has thoroughly been tested by applying to a large set of experimental data-(1)H 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 (1)H 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.

Reactivity of Fluoro‐Substituted Bis(thiocarbonyl) Donors with Diiodine: An XRD, FT–Raman, and DFT Investigation

“…Combining the expansion of ${J_{{\rm{trans}}} \left( \omega \right)}$ with these relaxation formulae, three ranges of linearity of ${R_1 \left( \omega \right)}$ versus ${\sqrt \omega }$ have been predicted and confirmed experimentally 48. 51 The linearity ranges are denoted as low frequency (LF) range, intermediate frequency (IF) range, and high frequency (HF) range. Figure 17 a shows an example for glycerol solution 51.…”

“…51 The linearity ranges are denoted as low frequency (LF) range, intermediate frequency (IF) range, and high frequency (HF) range. Figure 17 a shows an example for glycerol solution 51. Expressions linking the relaxation slopes in the different ranges with the diffusion coefficient ${D_{12} }$ have been provided 48.…”

“…Expressions linking the relaxation slopes in the different ranges with the diffusion coefficient ${D_{12} }$ have been provided 48. 51 In the low‐frequency range (denoted as I in Figure 17 a) the slopes (and in consequence, the expressions) depend on the nitrogen isotope ( 15 N or 14 N).…”

“…Solid black line: 4‐oxo‐TEMPO in glycerol from ref. 51; solid green line: pure glycerol, ref. 10; dashed black line: pure propylene glycol, ref.…”

Fast field cycling NMR relaxometry studies of molten and cooled cocoa butter

“…Callaghan et al [24] used the standard pulse gradient stimulated echo sequence (PGSTE) [28,29], and Rachocki and Tritt-Goc [27] used FFC-NMR. This technique, thanks to the advances in the area, has become more readily accessible, and has been used more frequently for the study of viscous liquids such as polyols and polymers [30][31][32][33].…”