The indirect coupling constantJXNhas been observed for the magnetic nuclei in 50 different pairs of directly bonded Xsngbnd;N atoms. A synopsis is given of the reported values along with the corresponding reduced constantKXNthinsp;equals;thinsp;lpar;2pgr;thinsp;sol;thinsp;planck;ggr;Xggr;Nrpar;JXNwhich depends only on the molecular electronic structure. There are three nuclei, Ndblbnd;1H,13C, and19F for whichKXNis now known for 15 or more different nuclei X, enough that trends are visible in the dependence of KXNupon the position of X in the periodic table. The sign of KXN(positive for H2) changes across the table somewhat between Groups V and VI, the sense of the change for Ndblbnd;19F being the reverse of that for Ndblbnd;1H and13C. Furthermore, there is a marked increase in the magnitude ofKXNwith increasing nuclear charge of atom X in each Group,for negative as well as positive coupling constants. The significance of these observed trends is considered. The Ramsey theory for the electron coupling of the nuclear spins includes orbital, spinhyphen;dipolar, and contact contributions. For directly bonded atoms, the orbital contribution is zero unless there is multiple bonding, the tendency for which decreases with increasingZin a given Group. The spinhyphen;dipolar contribution increases with increasingZ; however, it is positive, and the values calculated are an order of magnitude smaller than those found experimentally forKXN. A model is presented attributing the observed trends to the contact contribution, which depends upon the nature of the bonding orbitals employed by each atom in the bond. If both atoms employnsorbitals in the bond, the direct contact interaction term, which is positive, dominates. The bonding of Group VII and, to a lesser degree, Group VI atoms employs primarilyporbitals. In such atoms the contact interaction is indirect, involving polarization of the coreselectrons and a change in sign of the term. The contact term with inclusion of such core polarization effects provides a model consistent with the data available. The model is used to predict the signs, in some cases also the magnitudes, of several coupling constants not yet observed. For example, in NF3, OF2, and F2, we expectKNFto be negative,KFFto be positive, andKOFprobably to be positive. A number of features are discussed including the coupling in highly ionic bonds such as the Rb, Cs, and H fluorides, and the relationship of the model to nuclear hyperfine interactions in atoms and ions with unpaired spins and in organic free radicals.
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