Effect of substituents on the chemical shift of benzylic protons: evidence for a conformational dependence

Abstract

The benzyl proton shifts in 11 series of para-substituted toluenes bearing different α substituents have been determined. In each series, a plot of the benzylic shift against the Hammett σ values of the para-substituent is linear. The slope of these plots varies from 0.2 to 0.0 p.p.m./σ. The variation is found to be conformationally dependent, a fact which is not explicable by current theory. A hyperconjugative interaction is proposed to account for these results and for several anomalies in the literature.