Long-range σ-inductive interactions through saturated C–C bonds in polymethylene chains

Abstract

Analysis of the NMR shifts of unsaturated carbon in monounsaturated linear long-chain esters, alcohols and acids, including data from jojoba oil, shows a highly linear dependence of the logarithm of the shift separation in the CC bond upon the number of bonds separating this from the ester moiety. Both the linearity of this dependence, and the observation that it does not change sign upon reversal of the ester grouping, rule out the ‘electric field effect’ as an explanation, but are semi-quantitatively consistent with a remarkably simple, ‘σ-inductive’ through-bond mechanism. Semi-empirical MO calculations support this conclusion. The attenuation factor is 1.75 per saturated C–C bond in each class of compound, although the actual shift separations, for a given chain length, vary up to twofold with changes in the polarizing group and with (Z)–(E) isomerism at the CC bond. Other series of compounds show the same attenuation factor.