NMR Relaxation in Solutions of Six Fluorinated Benzenes Containing a Free Radical

Abstract

H and I9F NMR spin-lattice relaxation times and dynamic nuclear polarization (DNP) enhancements have been measured at various temperatures in a magnetic field of 17 mT for solutions of six polyfluorinated benzenes containing free radicals. From the radical induced relaxation rates and DNP coupling factors, dipolar and scalar hyperfine coupling contributions have been separated. As a result, the dipolar fluorine and the total proton relaxation rates are more or less the same, and they are only controlled by the molecular motion. The scalar relaxation rates depend upon the chemical structure, but their temperature dependence is also given by the process of translational diffusion. The activation energies for molecular translation, and the scalar contact coupling intensity both increase as one goes from mono-to hexafluorobenzene. Coupling mechanisms and comparison with viscous flow and diffusion are discussed.

A comparative study of 1H and 19F Overhauser DNP in fluorinated benzenes

“…However, regarding the coupling factors for 19 F that were obtained from DNP experiments, a rather small difference between DFB and HFB is observed, which cannot be explained only by variations in t t , even if the large uncertainty interval for t t is taken into account. This is in qualitative agreement with observations from 19 F DNP experiments using the radicals BDPA 31 or DPPH, 29 where increasing scalar coupling strength was observed for an increasing number of fluorine atoms per molecule. However, the small extent of this effect that was observed for our system rather suggests a similarity to the behaviour of the GALV radical, which showed no significant dependence on the fluorination of the solvent.…”

“…In the past there has been extensive research on Overhauser DNP for several nuclei ( 1 H, 19 F, 13 C, 31 P, 7 Li), a broad range of magnetic fields from a few mT up to around 2 T and for a large variety of solvents and radicals, e.g. TTBP [24][25][26][27][28] (2,4,6-tri-tertbutylphenoxyl), GALV 24,29,30 (galvinoxyl), BDPA 30,31 (a,g-bisdiphenylene-b-phenylallyl) or DPPH 29,30 (2,2-diphenyl-1-picrylhydrazyl). The findings of the earlier work on Overhauser DNP have been summarized in a number of expert reviews.…”

Chemical‐Shift‐Resolved 19F NMR Spectroscopy between 13.5 and 135 MHz: Overhauser–DNP‐Enhanced Diagonal Suppressed Correlation Spectroscopy

“…In the early years, DNP enhanced 1D 19 F NMR was measured in the liquid state at low and moderate fields, [3][4][5][6][7] and chemical shift resolution was known especially at X and Q bands. Further, differential DNP enhancements at different 19 F sites in a molecule were also investigated.…”

“…Unlike 1 H, where dipolar interactions dominate almost invariably, resulting in negative signal enhancements under DNP, 19 F is known to be subject to both scalar and dipolar cross-relaxation. [3][4][5][6][7] Owing to these competing influences, the radical and field dependence [2a, 8a] of 19 F ODNP is striking. Figure 1 depicts the field dependence of scalar and dipolar spectral densities on ODNP.…”