Intramolecular Resonance-assisted Hydrogen Bonds in N-Salicylidene-anilines

In the crystal structures of N-salicylidene-anilines, the ortho-OH group with the central N atom forms a planar six-membered ring through intramolecular hydrogen bonds (Fig. 1). The pseudo-aromatic ring is coplanar with its attached phenyl ring. The resonance strengthens the H-bond through proton transfer between the O and N atoms (Fig. 1, I II): dCO and dCC become shorter and dCN longer significantly. The d1～d6 are also changed regularly. According to the geometries obtained from X-ray analysis and retrieved from the Cambridge Structural Database (CSD, version 5.21)[1], we believe that the main resonance, at least in solid state, is the molecular keto form (II, Fig. 1) instead of the zwitterionic one (III, Fig. 1).