Vicinal proton-proton coupling constants

An assessment of the effect upon 3JHH of individual substituents with second row α-atoms has been carried out both theoretically, at ab initio and semi-empirical levels of approximation, and empirically, by parametrization of Karplus type equations. The Fermi-contact contributions calculated with a 6-31G** basis set to determine substituent effects ΔJx in monosubstituted ethanes CH3CH2X (X=CH3, NH2, OH and F), show a decay with the relative electronegativity of substituent ΔχX towards a constant value like that presented by the experimental data. Similar quadratic or exponential decays are calculated with the same basis set for the two largest parameters in the Karplus equation, i.e. for the angle independent term C0 and for the coefficient C2 of cos 2ϕ. By contrast, for the coefficient S2 of sin 2ϕ a proportional dependence on ΔχX is calculated. These calculated dependences upon ΔχX for C0, C2 and S2 have been confirmed by empirical parametrization of the corresponding Karplus type equations using an appropriate dataset of experimental 3JHH couplings.