VALENCE-BOND BOND-ORDER FORMULATION FOR CONTACT NUCLEAR SPIN-SPIN COUPLING

The valence-bond theory of contact nuclear spin-spin coupling is formulated in terms of Penney-Dirac bond orders. For molecular systems in which there are only small deviations from a localized bond description, the coupling constants are related to the nonlocal bond orders of four-electron fragments. Terms in the coupling constant expression which are of first order in these bond orders are called “direct” contributions, whereas terms which are of second or higher order are called “indirect” contributions. It is suggested that this classification provides a clearer and more useful description of the coupling mechanism than others (e.g., through-bond vs. through-space) which have been used in the past. The bond-order formulation is shown to be useful for semiempirical estimates of coupling constants. Illustrations of the utility of esr hyperfine splittings and nmr coupling constants for the determination of fragment bond orders are given and the results are applied to a number of systems. © 1969, American Chemical Society. All rights reserved.