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

Chandrakumar, N. ; Narasimhan, P. T. (1982) 1H and 19F dynamic nuclear polarization studies at X-band: 19F differential enhancements Molecular Physics, 45 (1). pp. 179-187. ISSN 0026-8976

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Related URL: http://dx.doi.org/10.1080/00268978200100141


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 spσ 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 spσ polarizability approach.

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