Abstract
Dynamic nuclear polarization has been examined in liquid solutions of 16 different radicals. Hexafluorobenzene was used to gauge the intermolecular hyperfine interaction. The scalar coupling reveals transient complexation between radical and receptor and various bond-forming tendencies. Perhalocarbon radicals show very weak scalar coupling with fluorine nuclei. Verdazyls show strong scalar coupling, with negligible effects from substituents on the outer phenyl rings. Phenoxyl and pyrylyl radicals show a wide range of coupling which is only partly explained by planar complexation. Semiquinones show increasing scalar coupling with increasing chlorination. DNP with aliphatic nitroxide radicals is dominated by the NO group, but aromatic nitroxides show a strong ring substituent effect, exceeding intramolecular hyperfine variation in ESR. It is suggested that DNP can provide unique signatures of substituents on phenyl rings.
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