An assessment of the effect upon 3J(HH) of individual substituents with second row alpha-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 DELTAJ(X) in monosubstituted ethanes CH3CH2X(X = CH3, NH2, OH and F), show a decay with the relative electronegativity of substituent DELTA(chi)x 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 2phi. By contrast, for the coefficient S2 of sin 2phi a proportional dependence on DELTA(chi)x is calculated. These calculated dependences upon DELTA(chi)x for C0, C2 and S2 have been confirmed by empirical parametrization of the corresponding Karplus type equations using an appropriate dataset of experimental 3J(HH) couplings.
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