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 molecule

Abstract The conformations of N -benzylideneanilines p -X–C 6 H 4 –CH N–C 6 H 4 – p -Y (X, Y = NO 2 , CN, CF 3 , F, Cl, Br, H, Me, OMe, NMe 2 ) 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 ( τ 2 ) is systematically controlled by substituents X and Y, the effect of Y being larger. The value of the dihedral angle τ 2 correlates nicely with equation τ 2 = ρ F ( Y ) X σ F ( Y ) + ρ R + ( Y ) X σ R + ( Y ) + k X or τ 2 = ρ F ( X ) Y σ F ( X ) + ρ R - ( X ) Y σ R - ( X ) + k Y , respectively, when aniline or benzylidene substituent is varied. ED substituents X diminish the sensitivity of τ 2 to the aniline substituent Y [ ρ F ( Y ) X and ρ R + ( Y ) X ] while ED substituents Y increase the sensitivity of τ 2 to the benzylidene substituent X [ ρ F ( X ) Y and ρ R - ( 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 C N 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 τ 2 and the experimental 13 C NMR chemical shift of the C N carbon of N -benzylideneanilines in CDCl 3 .