THEORETICAL-STUDY OF THE ELECTRONIC-SPECTRA OF URACIL AND THYMINE

The complete active space (GAS) SCF method combined with multiconfigur ational second-order perturbation theory has been used to study the el ectronic spectra-i.e., vertical excitation energies, oscilator strengt hs, and transition moment directions-of the nucleic acid base monomers uracil and thymine. The wave functions and the transition properties are computed at the CASSCF level, while dynamic correlation contributi ons to the excitations energies are obtained through the perturbation treatment. The method yields energies, which are in agreement with exp eriment, while the determination of transition moment directions is mo re uncertain since they depend strongly on solvent effects. For uracil the following energies are obtained for pi --> pi transitions (exper imental data in parentheses): 5.0 (4.6-4.9), 5.8 (5.8-6.1), 6.5 (6.3-6 .6), and 7.0 (6.8-7.0) eV. Corresponding data for the four lowest n -- > pi transitions are 4.5, 6.0, 6.4, and 7.0 eV, respectively (no expe rimental data available). Computed (and experimental) pi --> pi trans ition energies for thymine are 4.9 (4.5-4.7), 5.9 (5.8-6.0), 6.1 (6.3- 6.6), and 7.1 (7.0) eV. n --> pi energies are 4.9, 5.9, 6.1, and 7.1 eV, respectively. It is proposed that the bands found around 5 eV in u racil and thymine are due to an n --> pi transition. The error limit of computed energies is +/-0.3 eV.