Substituent Effects. VI. Theoretical Interpretation of Additivity Rules of NMR Using McWeeny Group Functions

McWeeny group functions, in conjunction with the popular Ramsey formulation, are used to theoretically account for the additive influence of substituents observed in NMR spectra. In this treatment the group wavefunctions are assumed to perturb one another only to first order. This approach is applied to chemical shifts and coupling constants involving various types of nuclei. In general, the substituent effect is found to be pairwise additive. If the substituent is far removed from the nuclei in question then the pairwise rule reduces to direct additivity. For a local operator, such as the Fermi contact term in spin coupling, the effects of substituents are directly additive even though the substituents are bonded to one of the nuclei in question. If the group wavefunctions act as a large perturbation on one another then the second-order terms cannot be neglected, and the simple direct and pairwise additivity rules fail. Existing data and new data are discussed in terms of the above theoretical treatment.