Valence-Bond Studies of the Dependence upon Substituents of C13–H and Si29–H Coupling

Abstract

An interpretation is presented for the additivity of substituent effects on the C13–H coupling constant, which has been observed previously in the high-resolution NMR spectra of substituted methanes. Each atom or group X is assigned a characteristic ``affinity'' for s character in the carbon hybrid orbital of the C—X bond. Distribution of s character among the carbon orbitals in accord with the relative s affinities of the four substituents leads to the observed additivity relation provided that the total s character is conserved. The valence-bond approach used with this model gives a linear relation between the s character of the carbon hybrid orbital involved in the C–H bond (αH2) and the observed C13–H coupling constant (JCH=500 αH2). Also, it allows the determination of the s character of the other carbon orbitals. The dependence of the s character of the C—X bond on the electronegativity of X is discussed in terms of electron spin and charge correlation. It is noted that the hybridization changes should affect not only JCH but also JHH, which is consistent with the observed proportionality between JCH(CH3X) and the cis and trans H–H coupling constants in CH2=C13HX. The treatment developed for methanes has been extended to JCH in substituted ethylenes and to the Si29–H coupling in silanes. For the former, introduction of the s-electron affinities, obtained from the values of JCH(CH3X) observed for the substituted methanes, leads to the result that JCH(CH2=C13HX) = JCH(CH2=CH2)+43 [JCH(CH3 X) – JCH(CH4)]. Values predicted in this manner are systematically larger than those observed, which implies that there is a small, negative π-electron contribution of 5 to 10 cps to JCH(CH2=C13HX). Such a contribution is compatible with current theories for proton and C13 hyperfine splittings in ESR spectra of free radicals. The Si29–H coupling constants observed in substituted silanes exhibit large, systematic deviations from the simple additivity found in the methanes. These deviations are explicable qualitatively in terms of changes in the Si–H-bond polarity.