Energetics of cytosine singlet excited-state decay paths--a difficult case for CASSCF and CASPT2.

Three deactivation paths for singlet excited cytosine are calculated at the CASPT2//CASSCF (complete active space second-order perturbation//complete active space self-consistent field) level of theory, using extended active spaces that allow for a reliable characterization of the paths and their energies. The lowest energy path, with a barrier of approximately 0.1 eV, corresponds to torsion of the C5-C6 bond, and the decay takes place at a conical intersection analogous to the one found for ethylene and its derivatives. There is a further path with a low energy barrier of approximately 0.2 eV associated with the (n(N),pi*) state which could also be populated with a low energy excitation. The path associated with a conical intersection between the ground and (n(O),pi*) states is significantly higher in energy (> 1 eV). The presence of minima on the potential energy surface for the (n,pi*) states that could contribute to the biexponential decay found in the gas phase was investigated, but could not be established unequivocally.