VICINAL PROTON-PROTON COUPLING-CONSTANTS .5. COUPLINGS TO METHYL-GROUPS

A study of the substituent effects upon the vicinal proton-proton coup ling (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 (Me)(0), monosubstituted ethanes, denoted (3)J(Me)(X), and 1,1-disubst ituted ethanes, denoted (3)J(Me)(XY), With substituents of the second (C, N, O and Fl, third (Si, P, S and Cl), fourth (Ge, As, Se and Br) a nd fifth (Sn, Sb, Te and I) rows of the periodic table. The calculatio ns of (3)J(Me) couplings were performed at the self-consistent field a b initio level using the 6-31G* basis set. Both the experimental and the calculated (3)J(Me)(X) couplings of monosubstituted ethanes show a quadratic (or exponential) decay with the relative electronegativity Delta chi(X) of the substituents in each row. Different decays are fou nd for each row suggesting a periodic behaviour for the effect of the alpha atom upon (3)J(Me)(X) in addition to the dependence on Delta chi (X). The (3)J(Me)(X) couplings have been analysed using different refi ned equations formulated from a substituent effect model. A data set o f 33 experimental couplings is satisfactorily fitted by two different refined equations including a quadratic dependence on Delta chi(X) far the effects of the individual substituents and a cross-term on Delta chi(X) Delta chi(Y) for the effects of interaction between substituent s. In the first equation, with nine parameters, the periodic trends in the plots of (3)J(Me)(X) against Delta chi(X) are considered by using different coefficients for each row in the linear term in Delta chi(X ). In the second equation, with seven parameters, a unique coefficient is used for the term in Delta chi(X) and the periodicity is embodied into a linear term in the relative hardness of the substituent Delta e ta(X).