This invited paper is part of the symposium-in-print: DNA Photodynamics.
Energetics of Cytosine Singlet Excited-State Decay Paths—A Difficult Case for CASSCF and CASPT2†
Article first published online: 9 MAY 2007
Photochemistry and Photobiology
Volume 83, Issue 3, pages 603–610, May/June 2007
How to Cite
Blancafort, L. (2007), Energetics of Cytosine Singlet Excited-State Decay Paths—A Difficult Case for CASSCF and CASPT2. Photochemistry and Photobiology, 83: 603–610. doi: 10.1562/2006-05-29-RA-903
- Issue published online: 9 MAY 2007
- Article first published online: 9 MAY 2007
- Received 29 May 2006; accepted 28 September 2006; published online 3 October 2006
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.