Substituent cross-interaction effects on the electronic character of the C=N bridging group in substituted benzylidene anilines--models for molecular cores of mesogenic compounds. A 13C NMR study and comparison with theoretical results.

13C NMR chemical shifts δC(CN) were measured in CDCl3 for a wide set of mesogenic molecule model compounds, viz. the substituted benzylidene anilines p-XC6H4CHNC6H4p-Y (X = NO2, CN, CF3, F, Cl, H, Me, MeO, or NMe2; Y = NO2, CN, F, Cl, H, Me, MeO, or NMe2). The substituent dependence of δC(CN) was used as a tool to study electronic substituent effects on the azomethine unit. The benzylidene substituents X have a reverse effect on δC(CN):  electron-withdrawing substituents cause shielding, while electron-donating ones behave oppositely, the inductive effects clearly predominating over the resonance effects. In contrast, the aniline substituents Y exert normal effects:  electron-withdrawing substituents cause deshielding, while electron-donating ones cause shielding of the CN carbon, the strengths of the inductive and resonance effects being closely similar. Additionally, the presence of a specific cross-interaction between X and Y could be verified. The electronic effects of the neighboring aromatic ring su...