Substituent Effects in the 13C NMR Chemical Shifts of Para-(para-substituted benzylidene amino)benzonitrile and Para-(ortho-substituted benzylidene amino)benzonitrile

13C NMR chemical shifts were measured in CDCl3 for two series of substituted benzylidene anilines. The substituted benzylidene anilines p-X-C6H4CH=NC6H4-p-CN p-X-C6H4CH=NC6H4-o-CN (X = NO2, F, Cl, Br, H, Me, MeO, NMe2). The substituent dependence of δC(C=N) was used as a tool to study electronic substituent effects on the azomethine unit. The benzylidene substituents X have a reverse effect on δC(C=N): electron-withdrawing substituents cause shielding, while electrondonating ones do the reverse, the resonance effects clearly predominating over the inductive effects. Additionally, the presence of a specific cross-interaction between X and C=N could be verified. The electronic effects of the neighboring aromatic ring substituents systematically modify the sensitivity of the C=N group to the electronic effects of the benzylidene substituents. These results can be rationalized in terms of the substituent-sensitive balance of the electron delocalization (mesomeric effects).