Anisotropies of the 19F Chemical Shifts in Fluorobenzene Compounds from NMR in Liquid Crystals

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Abstract
Fluorine-19 shift anisotropies, which have been determined from NMR spectra of nematic solutions, are reported for 1,2-, 1,3-, 1,4-difluorobenzene, 1,3,5-trifluorobenzene, 1,2,4,5-tetrafluorobenzene, and hexafluorobenzene. Combination of the anisotropies of different molecules leads to the following results for the principal values of the anisotropic part σ̂a = σ̂ − 13Trσ̂ of the screening tensor σ̂ for 19F nuclei without fluorine neighbors in ortho or para position: σ11a = −94 ± 12, σ22a = 27 ± 9, σ33a = 67 ± 3 in parts per million (2 axis in direction of C–F bond, 3 axis perpendicular to molecular plane). Fluorine substitution in ortho position changes the shielding tensor by Δσ11 = (+6 ± 3) × 10−5, Δσ22 = (−3 ± 3) × 10−5, Δσ33 = (+4 ± 1) × 10−5. There are difficulties in explaining the experimental results by the theory of Karplus and Das since a very high ionicity in the C–F bond has to be assumed that increases by ortho fluorine substitution.