DFT study of the substituent cross-interaction effects on the conformation of substituted N-benzylideneanilines - Models of liquid crystal forming compounds: Use of 13C NMR chemical shift of the CN Carbon as a tool to predict the conformation of the molec

The conformations of N-benzylideneanilines p-X-C6H4-CHN-C6H4-p-Y (X, Y = NO2, CN, CF3, F, Cl, Br, H, Me, OMe, NMe2) have been studied by B3LYP density functional (DFT) hybrid method in combination with the 6-31G∗ or 6-311G∗ split valence basis set. The twist of the plane of the aniline ring with respect to the other part of the molecule (τ2) is systematically controlled by substituents X and Y, the effect of Y being larger. The value of the dihedral angle τ2 correlates nicely with equation τ2=ρF(Y)XσF(Y)+ρR+(Y)XσR+(Y)+kXorτ2=ρF(X)YσF(X)+ρR-(X)YσR-(X)+kY, respectively, when aniline or benzylidene substituent is varied. ED substituents X diminish the sensitivity of τ2 to the aniline substituent Y[ρF(Y)XandρR+(Y)X] while ED substituents Y increase the sensitivity of τ2 to the benzylidene substituent X[ρF(X)YandρR-(X)Y]. There seems to be two competitive conjugative interactions for the aniline ring π electrons: one with the nitrogen lone pair and one with the CN unit. Substituents X and Y adjust the extent of these interactions and therefore the conformation of the molecule. A good correlation is observed between the dihedral angle τ2 and the experimental 13C NMR chemical shift of the CN carbon of N-benzylideneanilines in CDCl3.