| Abstract
| - The spin−spin coupling of protons measured by the constant JHH‘ in NMR experiments is dominated by aterm proportional to the product of the electron spin densities at the two nuclei, the Fermi contact term. Theprobability of β electrons being in excess over α electrons at the position of nucleus n‘, given that there isan α electron at n, is proportional to the negative of the exchange density, the total Fermi correlation betweenn and n‘. Thus the delocalization of the Fermi hole between n and n‘ is the mechanism whereby the spinperturbation caused by the magnetic interaction of an electron with nucleus n is transmitted to n‘. The densityand exchange density within the basin of a hydrogen atom are described primarily in terms of s-type basisfunctions, and one may approximate the exchange density between two protons at n and n‘ by the exchangebetween the two associated atomic basins determined by the delocalization index δ(A,B) within the quantumtheory of atoms in molecules. It is shown that this model yields good linear regression equations relating JHH‘to δ(H,H‘) for hydrogen atoms bonded to different carbons in alkanes, alkenes, their cyclic congeners, andpolybenzenoid hydrocarbons.
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