Solvent effects in nuclear magnetic resonance spectroscopy. Part XIV. Solvent shifts induced by trifluoroacetic acid in methoxybenzenes

Abstract

Trifluoroacetic acid causes downfield shifts of the methoxy-group resonances (relative to their positions in carbon tetrachloride as solvent) in the spectra of several methoxybenzenes. In para-substituted anisoles a correlation exists between the magnitude of the solvent shifts and the Hammett σ_p values for the various substituents. This suggests that protonation of the methoxy-group in trifluoroacetic acid is an important factor in determining the solvent shifts. The results for 1,2,3-trimethoxybenzene and other compounds in which a methoxy-group is flanked by two bulky ortho-substituents illustrate the influence of steric factors on the basicity of methoxy-groups.