Indirect fluorine coupling anisotropies in p-difluorobenzene: Implicationsto orientation and structure determination of fluorinated liquid crystals

The spin-spin coupling tensors, J(XF) (X = H, C, F), in p-difluorobenzene ( C6H4F2) were deternined 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 dat a analysis with some of the results of the theoretical calculations, the re maining 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 a b initio data. The results indicate that the tensorial properties of the fl uorine couplings typically contribute a few percent to the corresponding ex perimental anisotropic couplings. In D-2h Or lower point group symmetries, the indirect coupling can even dominate the experimental dipolar coupling b ecause of occasional cancellation of the direct part. Consequently, the con tribution of J(XF)(aniso) must be taken into account when using anisotropic couplings in accurate determinations of the geometry or orientation of flu orinated liquid crystals or other molecules containing fluorine-substituted phenyl rings dissolved in mesophases.