Experimental and theoretical study of the cytosine tautomerism through excited states.

Context: The irradiation of water solution of cytosine with UV light (λ = 254 nm) shows oxo-hydroxy tautomerism with a rate constant of 6.297 × 10 min. The order of the reaction implies a tautomeric conversion.

Comparative study of the relaxation mechanisms of the excited states of cytosine and isocytosine.

An experimental and theoretical investigation was performed to study the photostability of cytosine and isocytosine. The experimental UV irradiation of acetonitrile solutions of the two compounds showed that the amino-oxo tautomer of cytosine is photostable while the amino-oxo tautomer of isocytosine tautomerizes to the amino-hydroxy form. The theoretical investigations were carried out at the CC2 level of theory.

Photoinduced tautomerism of 2-thiobarbituric Acid studied by theoretical and experimental methods.

Combined, theoretical and experimental, investigation was performed to study the mechanism of the photoinduced tautomerism of 2-thiobarbituric acid (TBA). The irradiation of the solution of TBA in polar aprotic solvent with UV light (maximum at 366 nm) showed oxo-hydroxy photoisomerization of the triketo form of TBA to the hydroxy-imino tautomer. The studied mechanisms (TD DFT) of the photoinduced NH and OH dissociations in the keto and enol tautomer revealed that the proton detachment in the triketo tautomer occurs in the bright 1nSσ* excited state.