Spectrophotometric investigations and computational calculations of prototropic tautomerism and acid–base properties of some new azo dyes

Abstract

The prototropic tautomerism in four novel azo compounds derived from pyrazolo[1,5- a] pyrimidin-7(4H)-one was intensively examined. Tautomeric structures which result from annular and azo-hydrazone tautomerism were exposed to semiempirical and density functional theory (DFT) calculations, allowing the recording of structural parameters, physicochemical properties and equilibrium constants to be recorded. The values of the equilibrium constants determined among the most stable forms clearly showed that compounds 1 and 2 co-exist in the NH and hydrazone forms. However, NH tautomers were strongly preferred to other forms in compounds 3 and 4. The observed electronic absorption bands were assigned and compared with the predicted transitions using a time-dependent DFT method (TDDFT). In all solvents employed, except for DMF and acetonitrile, compounds 1 and 2 exhibited azo-hydrazone tautomerism. However, the ionized species were predominant in highly polar solvents for compounds 3 and 4. In DMF, all the investigated dyes exist either in acid–base equilibrium or in the ionized form depending on the molecular structure. Hence, the values of the ionization constant ( K ion) and Gibbs free energy (Δ G) of the equilibrium existing in solution were calculated. In addition, the pK a values of the investigated dyes were determined spectrophotometrically.