ANOTHER LOOK AT COUPLING CONSTANTS OF DIRECTLY BONDED NUCLEI.
ILLINOIS UNIV URBANA NOYES CHEMICAL LAB
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The indirect coupling constant JXN was observed for the magnetic nuclei in fifty different pairs of directly-bonded X-N atoms. A synopsis is given of the reported values along with the corresponding reduced constant KXN which depends only on the molecular electronic structure. There are three nuclei, N 1H, 13C and 19F for which KXN is now known for fifteen or more different nuclei X, enough that trends are visible in the dependence of KXN upon the position of X in the periodic table. The significance of these observed trends is considered. The Ramsey theory for the electron coupling of the nuclear spins includes orbital, spin-dipolar, and contact contributions. 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 employ ns orbitals in the bond, the direct contact interaction term, which is positive, dominates. 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 expect KNF to be negative, KFF positive and KOF probably 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. author
- Atomic and Molecular Physics and Spectroscopy