Get access

Energetics of Cytosine Singlet Excited-State Decay Paths—A Difficult Case for CASSCF and CASPT2

Authors


  • This invited paper is part of the symposium-in-print: DNA Photodynamics.

*email: lluis.blancafort@udg.es (Lluís Blancafort)

Abstract

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 (nN,π*) state which could also be populated with a low energy excitation. The path associated with a conical intersection between the ground and (nO,π*) states is significantly higher in energy (>1 eV). The presence of minima on the potential energy surface for the (n,π*) states that could contribute to the biexponential decay found in the gas phase was investigated, but could not be established unequivocally.

Get access to the full text of this article

Ancillary