DFT analysis of rotational barriers, ¹H and ¹³C NMR chemical shifts in neutral and protonated furfurylidenanilines

Rotational barriers were calculated for 4′-substituted neutral and protonated furfurylidenanilines (FA) by DFT methods and the corresponding equilibrium geometries were identified. Neutral FA have equilibrium dihedral angle values sensitive to the substituent. ¹H and ¹³C Substituent Induced Shifts in FA were calculated from shielding tensors and compared with experimental ones. The experimental trends in ¹H and ¹³C Substituent Induced Shifts of these compounds are well reproduced by the calculations. Differences in Mulliken charges for each position in neutral and protonated furfurylidenanilines were correlated with experimental ¹H and ¹³C Substituent Induced Shifts. The experimental trends in ¹H and ¹³C Substituent Induced Shifts were well reproduced by charge differences. Only the functional group ¹H shifts do not correlate with the charges reflecting the variable effect of the anisotropy term due to the benzene ring. Protonation of FA removes this variability.