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).