Keto forms of salicylaldehyde Schiff bases: structural and theoretical aspects.

Twelve Schiff bases of methoxy-substituted salicylaldehyde have been examined by crystallographic and spectroscopic methods, as well as by DFT theoretical calculations in order to investigate the effect of the substituent's position on the keto-enol equilibrium in the crystalline state. Four out of the 10 structurally characterized compounds with methoxy substitution on the para and/or ortho positions with respect to the aldimine bridge and deriving from aliphatic amines or alkylarylamines are found as cis-keto tautomers and form dimers. In contrast, the five pure enol tautomers derive either from aliphatic or alkylarylamines and are meta substituted or from aniline or benzylamine and are para and/or ortho methoxy substituted. The DFT calculations support the crystallographic results and, moreover, they have shown that keto and enol tautomers are affected differently by the relative arrangement of the monomers. Overall, the DFT calculations point to a plausible hypothesis for the stabilization of the keto form in the crystalline state: In cases with a sufficiently low enol-keto energy difference of the isolated monomers, as when the methoxy group is at ortho and/or para positions with respect to the aldimino group, extra stabilization of the keto form is derived from molecular association, thus leading to its crystallization.