Quantum chemical investigation of the electronic spectra of the keto, enol, and keto-imine tautomers of cytosine

The low-lying excited singlet states of the keto, enol, and keto-imine tautomers of cytosine have been investigated employing a combined density functional/multireference configuration interaction (DFT/MRCI) method. Unconstrained geometry optimizations have yielded out-of-plain distorted structures of the pi -> pi* and n -> pi* excited states of all cytosine forms. For the keto tautomer, the DFT/MRCI adiabatic excitation energy of the pi -> pi* state (4.06 eV including zero-point vibrational energy corrections) supports the resonant two-photon ionization (R2PI) spectrum (Nir et al. Phys. Chem. Chem. Phys. 2002, 5, 4780). On its S, potential energy surface, a conical intersection between the (1)pi pi* 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 above the origin and explains the break-off of the R2PI spectrum. The (1)pi pi* minimum of the enol tautomer is found at considerably higher excitation energies (4.50 eV). Because of significant geometry shifts with respect to the ground state, long vibrational progressions are expected, in accord with experimental observations. For the keto-imine tautomer, a crossing of the 1 pi pi* potential energy surface with the ground-state surface has been found, too. Its n ->pi* minimum (3.27 eV) is located well below the conical intersection between the pi -> pi* and So states, but it will be difficult to observe because of its small transition moment. The identified conical intersections of the pi -> pi* excited states of the keto cytosine tautomers are made responsible for the ultrafast decay to the electronic ground states and thus may explain their subpicoseconds lifetimes.