A Theoretical Approach to Analytical Properties of 2,4-Diamino-5-phenylthiazole in Water Solution. Tautomerism and Dependence on pH

Abstract

Theoretical models are used to study pH-dependent equilibria of 2,4-diamino-5-phenylthiazole tautomer molecules in water. A complete screening of semiempirical SCF multiple minimums of hypersurfaces, corresponding to several solute−water supermolecules, has been made. Multiple minimum hypersurface searching confirms experimental NMR results indicating that the native diamine tautomer predominates in aqueous neutral and basic media. This tautomeric structure, protonated either in N3 and N4, also predominates in aqueous acid media with a minor presence of a protonated monoimine tautomer, in agreement with 1H NMR results in D2O. High-level ab initio SCF MO of the main structures, where solvent reaction field effects are taken into account with a dielectric constant equivalent to that of water, predict a nonconjugated protonated monoimine tautomer in nonprotic solvents, according to 1H NMR data in polar aprotic solvents. Calculated electron excitation patterns of hydrated species in water agreed with the experimental UV spectra at different pH values. The quantum chemical procedures for calculating total energies and frontier orbital eigenvalues in local minimum geometries of the relevant supermolecules provide an appropriate model for comparisons of theoretical results with experimental facts in the case of analytical voltametry. The frontier orbital eigenvalues of the most populated minimums discard the appearance of electroanalytic signals in the case of acidic samples because of the similarity of the predicted values for all protonated isomers and water. Experimental measurements confirm the oxidative character of electroanalytic signals.