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

The complete active space (GAS) SCF method combined with multiconfigurational second-order perturbation theory has been used to study the electronic spectra-i.e., vertical excitation energies, oscilator strengths, 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 contributions to the excitations energies are obtained through the perturbation treatment. The method yields energies, which are in agreement with experiment, while the determination of transition moment directions is more uncertain since they depend strongly on solvent effects. For uracil the following energies are obtained for pi --> pi* transitions (experimental 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 experimental data available). Computed (and experimental) pi --> pi* transition 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 uracil and thymine are due to an n --> pi* transition. The error limit of computed energies is +/-0.3 eV.