Contact Electron-Spin Coupling of Nuclear Magnetic Moments

The valence-bond theory for the contact electron-spin coupling of nuclear magnetic moments is used to calculate the proton-proton, proton-fluorine, and fluorine-fluorine coupling constants in ethanic and ethylenic molecules. A considerable simplification is introduced into the theory by approximations which reduce the problem to one involving only a small number of electrons and canonical structures. The agreement between calculated and experimental values is such as to demonstrate that the mechanism considered is the one of primary importance for the nuclear coupling in the compounds studied. Of particular interest is the theoretical confirmation of the observation that in ethylenic compounds the trans coupling between nuclei (HH, HF, FF) is considerably larger than cis coupling.