Experimental and theoretical study of the cytosine tautomerism through excited states

The irradiation of water solution of cytosine with UV light (λ_max = 254 nm) shows oxo-hydroxy tautomerism with a rate constant of 6.297 × 10^-3 min^-1. The order of the reaction implies a tautomeric conversion. After removing the UV light source, we observed a dark reaction with a rate constant of 1.473 × 10^-3 min^-1 which leads to a restoration of the initial tautomer as before the irradiation. The mechanism of oxo-hydroxy tautomerism of cytosine in water solution was studied in the excited state. It was found that the transformations occur along the ¹πσ* excited-state reaction paths which link the Franck-Condon geometries of the tautomers and the conical intersections S₀/S₁ connected with the H-detachment processes of the corresponding bonds. Furthermore, we established that the conical intersections S₀/S₁ are also mutually accessible along the ¹πσ* excited-state reaction paths.

The ground-state equilibrium geometries were optimized at the B3LYP/aug-cc-pVDZ level of theory in water environment according to PCM as well as at the CC2/aug-cc-pVDZ level in the gas phase. The TD B3LYP and CC2 methods were applied for the study of the excited states. The tautomerization mechanisms were studied with the use of the linear interpolation in internal coordinates approach using the optimized geometries of tautomers minima and conical intersections S₀/S₁ at the CASSCF(6,6)/6-31G* level. All calculations were performed with the GAUSSIAN 16 commercial software.