The availability of such experimental data has given us the opportunity to check the reliability of the quantum-chemical methods in predicting the position of the tautomeric equilibria in selected azonaphthols and related Schiff bases in solution. A major breakthrough in the UV-Vis spectroscopy was the development of chemometric methods to obtain the absorption spectra of the individual tautomers, even though these are never present in their pure form, and then to estimate the tautomeric constants as function of a number of external parameters (temperature, irradiation, solvents, pH, and concentration). On the other side, the exact estimation of the ΔG values (derived from the tautomeric constant K T, defined as keto/enol, / ratio, Scheme 2), requires isolated tautomeric forms, a condition that is impossible in most of the cases. On one side, the real equilibrium from the viewpoint of the detection limits of analytical chemistry means Gibb’s free energy in the range roughly from −2 to +2 kcal/mol, which is a challenge for quantum chemistry even now. While the potentially tautomeric structures can be easily distinguished either theoretically or experimentally, the study of real tautomeric equilibria in solution is a challenge.
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