Studies of the solvent effect observed in the absorption spectra of certain types of Schiff bases

The regularities of the solvent effect manifested in the absorption spectra of certain types of Schiff bases were investigated. In solvents capable of hydrogen-bonding, this solvent effect is observed for those Schiff bases in which there is an OH group in an ortho or para position on the aldehyde ring, and the electron density of the azomethine nitrogen is appropriately high. Studies in solvent mixtures demonstrated that the phenomenon can be interpreted in terms of an enol reversible arrow keto tautomer equilibrium. It was found that, in response to salts (particularly CaCl2) dissolved in ethanol, the equilibrium is shifted extensively in the direction of the keto form, as a function of the salt concentration. This experience was utilized to determine the molar extinction of the keto form at the maximum of the band at around 400 nm. In the knowledge of this, the equilibrium constant and the concentration of the keto form in various solvents were calculated. Calculation methods were described for determination of the full UV and visible absorption spectra of the keto and the enol forms. A correlation was found between the magnitude of the solvent effect and the acidity and basicity parameters of the solvents used. It was confirmed that both the acidity and the basicity play roles in the solvent effect. The results of the investigation further permit the assumption that the keto form develops via an enol reversible arrow zwitterions equilibrium.