Vicinal proton-proton coupling constants V. Couplings to methyl groups

Abstract

A study of the substituent effects upon the vicinal proton–proton coupling ³ J _Me to methyl groups has been carried out from data sets of experimental and calculated ³ J _Me couplings in ethane, denoted ³ J ⁰ _Me, monosubstituted ethanes, denoted ³ J ^x _Me, and 1, 1-disubstituted ethabes, denoted ³ J ^xy _Me, with substituents of the second (C,N O and F), third (Si, P, S and Cl), fourth (Ge, As, Se and BR) and fifth (Sn, Sb, Te, and I) rows of the periodic table. The calculations of ³ J _Me couplings were performed at the self-consistent field ab initio level using the 6-31G** basis set. Both the experimental and the calculated ³ J ^x _Me couplings of monosubstituted ethanes show a quadratic (or exponential) decay with the relative electronegativity Δ _χx of the substituents in each row. Different decays are found for each row suggesting a periodic behaviour for the effect of the alpha atom upon ³ J _Me in addition to the dependence on Δ χ _x. The ³ J _Me couplings have been analysed using different refined equations formulated from a substituent effect model. A data set of 33 experimental couplings is satisfactorily fitted by two different refined equations including a quadratic dependence on Δ _χx for the effects of the individual substituents and a cross-term on Δ _χx Delta chi y for the effects of interaction between substituents. In the first equa tion, with nine parameters, the periodic trends in the plots of ³ J _Me against Δ _χx are considered by using different coefficients for each row in the linear term in Δ _χx. In the second equation, with seven parameters, a unique coefficient is used for the term in Δ _χx and the periodicity is embodied into a linear term in the relative hardness of the substituent Δ _ηx.