| Abstract
| - 13C NMR chemical shifts δC(CN) were measured in CDCl3 for a wide set of mesogenic moleculemodel 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 substituentdependence of δC(CN) was used as a tool to study electronic substituent effects on the azomethineunit. The benzylidene substituents X have a reverse effect on δC(CN): electron-withdrawing substituentscause shielding, while electron-donating ones behave oppositely, the inductive effects clearly predominatingover the resonance effects. In contrast, the aniline substituents Y exert normal effects: electron-withdrawingsubstituents cause deshielding, while electron-donating ones cause shielding of the CN carbon, thestrengths of the inductive and resonance effects being closely similar. Additionally, the presence of aspecific cross-interaction between X and Y could be verified. The electronic effects of the neighboringaromatic ring substituents systematically modify the sensitivity of the CN group to the electronic effectsof the benzylidene or aniline ring substituents. Electron-withdrawing substituents on the aniline ringdecrease the sensitivity of δC(CN) to the substitution on the benzylidine ring, while electron-donatingsubstituents have the opposite effect. In contrast, electron-withdrawing substituents on the benzylidenering increase the sensitivity of δC(CN) to the substituent on the aniline ring, while electron-donatingsubstituents act in the opposite way. These results can be rationalized in terms of the substituent-sensitivebalance of the electron delocalization (mesomeric effects). The present NMR characteristics are discussedas regards the computational literature data. Valuable information has been obtained on the effects of thesubstituents on the molecular core of the mesogenic model compounds.
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