Photo-Induced Tautomerism of Isocytosine in Aqueous Solution when Irradiated with UVC Light

Authors : Tsvetina Cherneva, Mina Todorova, Rumyana Bakalska, Vassil Delchev

Pages : 321-330

Doi:10.18596/jotcsa.1325480

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Publication Date : 2024-02-04

Article Type : Research Paper

Abstract :It was found that the irradiation of aqueous solution of isocytosine with UVC light provokes an oxo-hydroxy phototautomerism of the compound with a rate constant of 5.29× 10-3 min-1. It was observed a backward reaction, after removing the UV light source, with a rate constant of 0.12×10-3 min-1. Two mechanisms of the phototautomerism were investigated at the B3LYP/aug-cc-pVDZ theoretical level in water surroundings (PCM). The first one showed a consecutive dissociation and association of a proton through conical intersections S0/S1 whose structures were located at the same theoretical level in the gas phase. It occurs along the 1πσ* excited-state reaction pathway. The more probable mechanism includes an excited-state H-transfer supported by a water molecule as a catalyst. It occurs along the 1ππ* excited-state reaction pathway which we found over the IRC ground-state energy curve. The water molecule drastically reduces the energy barrier in the ground state as well in the excited state.
Keywords : Conical intersections S0 S1, Excited states, Isocytosine, TD DFT methods, UV irradiation