Singly Excited States

A numerical study is reported concerning the first and second singlet excited-states of 5-benzyluracil using the multireference self-consistent field (state-averaged CASSCF) method. The vertical excitation energies of low-lying excited-states were characterized using the SA-CASSCF method, as well as using higher-level methods, such as CASPT2, MRCI and EOM-CCSD. The local minima and conical intersections found on the potential energy surfaces (PESs) were characterized in terms of molecular geometry and natural population analysis. Different relaxation pathways on the PESs are identified and discussed by comparing with the similar pathways found for the individual monomers of uracil and benzene. The molecule can be thought of as a model system for the study of crosslink reaction between DNA and proteins induced by UV light. state) that could compete with the crosslink. The system being amenable to simulation by the use of modern computational methods, we set out to determine the molecular structure, the energy of electronic excited states and the potential energy surfaces (PESs) of states potentially involved in the interaction of 5BU with a UV femtosecond laser pulse. In particular, this will be done for the ground state and the first two singlet excited electronic states of 5BU using calculations based on geometry optimization at the multireference self-consistent field (state-averaged CASSCF) level of theory. Special care should be taken on the search of noticeable points on the investigated PESs, i.e. local minima and conical intersections that can lead to ultrafast non-radiative decay processes. In the proximity of such points the dynamics of the process would involve more electronic states with consequent cross-talk and quantum interference between different decay paths. In addition, since the saturated methylene bridge hinders the electronic communication between components of 5BU via the p systems, the description of the excited states of the uracil base is expected to contribute to the growing knowledge of the photophysical properties of substituted nucleic acid bases. 10