¹H and ¹⁹F dynamic nuclear polarization studies at X-band: ¹⁹F differential enhancements

Abstract

X-band dynamic nuclear polarization (DNP) measurements on ¹H and ¹⁹F nuclei in 1,4-bis(trifluoromethyl)benzene, 1,2,4-trifluorobenzene and ¹⁹F nuclei in octafluorotoluene are reported. The free radical used in all these cases is tri-t-butylphenoxyl. Chemically shifted fluorines have been resolved and clear differential ¹⁹F DNP trends have been established in these systems. Q-enhancement corrections to ultimate-ultimate enhancement values are discussed. Three theoretical models of the radical-solvent interaction involving, respectively, transfer of an electron spin, M.O. treatment of solvent-radical interaction at separations around the van der Waals distance, and exchange polarization of the solvent molecule are reported. The latter, treated as a closed-shell perturbation in the molecular orbital framework, leads to an sp_σ polarizability parameter which reflects the present experimentally observed ¹⁹F DNP trends reasonably well. Apart from the present work, the only other experimental results on ¹⁹F differential DNP are those of Dwek et al. at Q-band and their results have also been satisfactorily interpreted in terms of our sp_σ polarizability approach.