Hydrogen bonding interactions of benzylidene type Schiff bases studied by vibrational spectroscopic and computational methods

The structural features of four benzylidene type Schiff bases [(E)-benzaldehyde-N-phenyl imine, (A) (E)-2-hydroxybenzaldehyde-N-phenyl imine (B) (E)-benzaldehyde-N-2-hydroxyphenyl imine (C) (E)-2-hydroxybenzaldehyde-N-2-hydroxyphenyl imine (D)] were studied by FT-IR spectroscopy in solution, photoacoustic and Raman spectroscopies in the solid state and quantum chemical calculations. It was found that molecule D dimerised in the solid state with concomitant loss of aromaticity in the benzylidene ring. Beside the intermolecular C=O...HO hydrogen bonds, intramolecular N-H...C=O hydrogen bonds could be found experimentally as well as computationally. Spectra taken in solution and ab initio quantum chemical calculation helped to identify hydrogen bonding interactions occurring for compounds B and C. Intramolecular OH...N hydrogen bond predominated in molecule B, while this interaction, although it existed, was weaker.