Effect of substituents on the 13C chemical shifts of the azomethine carbon atom of N-(substituted phenylmethylene)-3- and -4-aminobenzoic acids

Abstract

13C chemical shifts of the azomethine carbon atom for N-(substituted phenylmethylene)-3- and -4-aminobenzoic acids having a wide range of substituent effects, were determined in deuterated DMSO solution. Good Hammett correlations of the 13C NMR chemical shifts of azomethine carbons with electrophilic substituent constants σ p + for electron-donor substituents for both series of acids indicate an important resonance interaction of the substituents on the benzylidene ring with the azomethine carbon atom. On the other hand, good correlations of the 13C NMR chemical shifts of azomethine carbon atom of both series of acids with inductive substituent constants for electron-acceptor substituents in benzylidene ring indicates that the chemical shifts are influenced only by inductive effect of the substituents. The demand for electrons by the azomethine carbon atom in both investigated series have been compared, discussing the mode of transmission of substituent effects, both inductive and resonance, in relation to the geometry of investigated imines.