Hydrogen-fluorine spin coupling constants

Fluorine, like hydrogen, has a spin of 1/2 and couples with the vicinal proton. Fluorine-hydrogen coupling constants are significantly larger than those for vicinal hydrogens. 

A simpler case than benzene, which has chemical equivalence (due to symmetry) but not magnetic equivalence, is 1,1-difluoroethene. Both hydrogens couple to the fluorines ( F, spin = ) however, the two hydrogens are not magnetically equivalent, because Ha and Hb do not couple to Fa with the same coupling constant (Vhf)- One of these couplings is cis CJds) and the other is trans CJtrans)- In Table 5.4 it was shown that cis and trans coupling constants in alkenes were different in... 

This analysis is based on the fact that the a values are determined exclusively by s orbital spin density. The coupling constants for unit spin densities are 508 and 17,000 G, respectively, for the hydrogen and fluorine nuclei. Hence, = a"/508 and p = a /17,000 (62). 

FLF)- (L = H or D) anion in low temperature solutions of (C4H9)4N+ (FL)nF . The authors were able to determine zero-, one-, and two-bond, H/D isotope effects on hydrogen and fluorine NMR chemical shifts for the series n = 1 to n = 3, and to relate the observed spectra to H/D isotope effects on the hydrogen bond geometries. Isotope effects on spin-spin L-F and F-F coupling 13C constants were reported. 

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