Electron Coupling of Nuclear Spins. II. Molecular Orbital Interpretation of Coupling Constants Observed in Fluorobenzenes

The H–H, H–F, and F–F coupling constants and relative signs determined previously in high resolution NMR experiments are considered in terms of molecular orbital theory. The data, including the dependence of the coupling upon the relative positions of the atoms in the benzene ring, are sufficient to indicate which of the various interactions contribute most to the coupling. In general, the largest contribution is that carried by s electrons through the Fermi contact interaction. However, there are significant one-electron terms. The H–F and F–F coupling constants suggest that there is about 5% fluorine 2s character in the C–F bond.