| Abstract
| - The low-lying excited singlet states of the keto, enol, and keto−imine tautomers of cytosine have beeninvestigated employing a combined density functional/multireference configuration interaction (DFT/MRCI)method. Unconstrained geometry optimizations have yielded out-of-plain distorted structures of the π → π*and n → π* excited states of all cytosine forms. For the keto tautomer, the DFT/MRCI adiabatic excitationenergy of the π → π* state (4.06 eV including zero-point vibrational energy corrections) supports the resonanttwo-photon ionization (R2PI) spectrum (Nir et al. Phys. Chem. Chem. Phys.2002, 5, 4780). On its S1 potentialenergy surface, a conical intersection between the 1ππ* state and the electronic ground state has been identified.The barrier height of the reaction along a constrained minimum energy path amounts to merely 0.2 eV abovethe origin and explains the break-off of the R2PI spectrum. The 1ππ* minimum of the enol tautomer is foundat considerably higher excitation energies (4.50 eV). Because of significant geometry shifts with respect tothe ground state, long vibrational progressions are expected, in accord with experimental observations. Forthe keto−imine tautomer, a crossing of the 1ππ* potential energy surface with the ground-state surface hasbeen found, too. Its n → π* minimum (3.27 eV) is located well below the conical intersection between theπ → π* and S0 states, but it will be difficult to observe because of its small transition moment. The identifiedconical intersections of the π → π* excited states of the keto cytosine tautomers are made responsible for theultrafast decay to the electronic ground states and thus may explain their subpicoseconds lifetimes.
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