Water effect on the excited-state decay paths of singlet excited cytosine

Two low-energy deactivation paths for singlet excited cytosine, one through a S1/S0 conical intersection of the ethylene type, and one through a conical intersection that involves the (nN, π*) state, are calculated in the presence of water. Water is included explicitly for several cytosine monohydrates, and as a bulk solvent, and the calculations are carried out at the complete active space self-consistent field (CASSCF) and complete active space second order perturbation (CASPT2) levels of theory. The effect of water on the lowest-energy path through the ethylenic conical intersection is a lowering of the energy barrier. This is explained by stabilization of the excited state, which has zwitterionic character in the vicinity of the conical intersection due to its similarity with the conical intersection of ethylene. In contrast to this, the path that involves the (nN, π*) state is destabilized by hydrogen bonding, although the bulk solvent effect reduces the destabilization. Overall, this path should remain energetically accessible.