Quantum-chemical study of the keto-enol equilibrium in the series of Schiff bases

The results of ab initio calculations show that the state of a wide series of keto-enol (amino-imino) prototropic equilibria in the gas phase or in no npolar medium can be predicted on the basis of the length (order) of the C- C bond adjacent to the hydroxy group of the enol form: shorter C-C bonds fa vor the keto form, whereas the enol isomer is stabilized at longer C-C bond s. A simple quantitative structural criterion of the stability of Schiff ba se derivatives as a particular case of such equilibria has been found.