The excited state behavior of cytosine in the gas phase: A TD-DFT study

We report a TD-DFT study of the three lowest energy tautomers of Cytosine in the gas phase, focussing on the Franck–Condon region and on the excited state minima and comparing the performances of three different density functionals, namely CAM-B3LYP, M052X and PBE0. For what concerns the oxo-amino tautomer, the one present in water and within DNA, CAM-B3LYP and M052X provide a picture close to that of recent CCSD(T) calculations. In particular, in the Franck–Condon region the electronic transition involving the Lone Pair of the Nitrogen atom corresponds to S2 and is the lowest energy n π ∗ transition ( S N π ∗ ), being significantly more stable than that involving the Carbonyl Lone Pair ( S O π ∗ ), whose stability is instead overestimated by PBE0. CAM-B3LYP and M052X also provide minima fully consistent with those of the most recent MCSCF methods: different relative minima are present on the lowest energy adiabatic state and the most stable corresponds to the S N π ∗ diabatic state. PBE0 predicts instead a strong coupling of S O π ∗ with the lowest energy π π ∗ state. Calculations of the vibrationally resolved absorption spectra of the three lowest enegy tautomers, including Duschinsky effects, provide spectra in very good agreement with the experiments.