Aryl carbon chemical shifts of methylbenzoic acids and methylbenzoate anions

The aryl carbon chemical shifts of methylbenzoic acids in CDCl3 and methylbenzoate anions in aqueous NaOH solutions have been assigned. The shifts have been interpreted by means of a model based on the additivity of substituents which is similar to that previously developed for the analysis of the methyl chemical shifts. Corrective parameters included to take into account the interactions of adjacent substituents are found to be related to both the twist angle, θ, of the carboxyl or carboxylate substituents and the conformation of the methyl groups. The derived values of the twist angle are in closer agreement with x‐ray data than those estimated from UV spectra or from Cl‐PPP computations.