Dynamic nuclear polarization in some aliphatic and aromatic solutions as studied by fluorine-electron double resonance

Dynamic nuclear polarization experiments were performed to study the solutions of the stable free radical alpha,gamma-Bisdiphenylene-beta-phenyl allyl complex with benzene (1:1) in some highly fluorinated aliphatic and aromatic solvents. In solutions examined in this study, the Overhauser effect, which normally arises due to both dipolar and scalar interactions between the unpaired electrons of the free radical molecules and fluorine nuclei of solvent molecules occurs mainly. 1-Iodotridecafluorohexane, 2,2,3,4,4,4-Hexafluoro-1-butanol, N-methyl-bis-trifluoroacetamide, hexafluoroacetylacetone, octafluorotoluene, and hexafluorobenzene were used as the solvents. The experiments were performed at a low field double resonance NMR spectrometer, which operates at 1.53 mT. The NMR enhancements depend on competition between intermolecular magnetic interactions. The measurements were performed at four different temperatures to test the dipolar and the scalar part of the coupling between the fluorine nucleus (F-19) and the unpaired electron. It was found that the dipolar interactions are more effective for the aliphatic solvents, while the scalar interactions are more effective for the aromatics. The nuclear-electron coupling parameter varies between 0.018 and 0.157 in all aliphatic solvents and between -0.063 and -0.035 in aromatic solvents