Indirect Fluorine Coupling Anisotropies in p -Difluorobenzene:  Implications to Orientation and Structure Determination of Fluorinated Liquid Crystals

The spin−spin coupling tensors, J XF (X = H, C, F), in p -difluorobenzene (C 6 H 4 F 2 ) were determined using ab initio multiconfiguration self-consistent field linear response calculations and NMR experiments performed in nematic liquid crystal phase. The theoretical spin−spin coupling constants are in fair agreement with experimental results. By supplying the experimental data analysis with some of the results of the theoretical calculations, the remaining anisotropy and asymmetry parameters pertinent to the n J CF ( n = 1, 2, 3, 4) and 5 J FF tensors were obtained in good agreement with the ab initio data. The results indicate that the tensorial properties of the fluorine couplings typically contribute a few percent to the corresponding experimental anisotropic couplings. In D 2 h or lower point group symmetries, the indirect coupling can even dominate the experimental dipolar coupling because of occasional cancellation of the direct part. Consequently, the contribution of J XF aniso must be taken into account when using anisotropic couplings in accurate determinations of the geometry or orientation of fluorinated liquid crystals or other molecules containing fluorine-substituted phenyl rings dissolved in mesophases.