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. In the hydroxy-imino tautomer this mechanism is driven by the repulsive 1πσ* excited state. The excited-state relaxation mechanisms occur by low-lying So-S1 conical intersections.