Relationship between proton—proton NMR coupling constants and substituent electronegativities. IV—An extended karplus equation accounting for interactions between substituents and its application to coupling constant data calculated by the Extended Hückel method

Some well known Karplus equations are discussed and their shortcomings are indicated. To remedy the problems associated with the experimentally observed non‐additive substituent effects on vicinal proton‐proton coupling constants, an extension given recently is discussed; this accounts for pairwise interactions between substituents through specific quadratic cross terms used to describe the electronegativity dependence of the coefficients in a truncated Fourier series. Since this formulation will be used in combination with simultaneous least‐squares optimization of both the Fourier coefficients and the electronegativity values, an invariance property of this formulation, and of similar ones, is discussed in order to elucidate problems associated with parameter redundancy. The procedure is then applied in a stepwise manner to the analysis of a set of 1404 coupling constants calculated by a repara‐meterized version of the Extended Hückel method for ethanes singly or multiply substituted with Cl, F, Me and OH, and ethane itself. This clearly reveals its essential features, and leads to a markedly improved description.