Experimental and theoretical investigations of spectral, tautomerism and acid-base properties of Schiff bases derived from some amino acids

The electronic absorption spectra of five Schiff bases derived from 2-hydroxy-1-naphthaldehyde with glycine, alanine, leucine, valine and phenylalanine have been measured in various solvents. The observed bands were assigned to the proper electronic transitions and compared with the predicted transitions at the semiempirical level of theory The calcula equilibrium constants are in agreement with the experimental results, predicting the existence of all studied compounds predominantly or completely in keto-imine tautomerism. On the other hand, a correlation between v (cm^-1) (main frequency of each compound) and the well known solvent parameters E_T (30), ∈_r, π^* has been made. Furthermore, the acid dissociation constants, pK_a, were determined by using three different spectrophotometric methods.