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). 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 of the H-bond through proton transfer between the O and N atoms (Fig. 1, II): dCO and dCC become shorter and dCN more significantly. The d1 ~ d6 are also changed regularly. from the 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).