A theoretical study of the electronic spectra of adenine and guanine

The complete active space (GAS) SCF method and multiconfigurational second-order perturbation theory (CASPT2) have been used to study the electronic spectra of the nucleic acid base monomers guanine and two tautomers of adenine (the N(9)H and N(7)H forms). The calculations include vertical excitation energies, oscillator strengths, and transition moment directions in gas phase. For guanine solvent effects were included using a self-consistent reaction field model. The lowest pi(-->)pi* excited valence states of N(9)H-adenine are calculated at (experimental data in parentheses) 5.1, 5.2 (4.9), 6.2 (5.7-6.1), 6.7, 7.0 (6.8), and 7.6 (7.7) eV. The first two almost degenerate states are characterized by small and medium intensities, respectively. The third and fifth transitions have large oscillator strengths. Two less clear assignments can be performed to the transitions observed in experiment at 4.6 and 6.3-6.4 eV. Presently they can be assigned to the 2(1)A' and 5(1)A' states of the N(9)H tautomer of adenine, but possible contributions to the 3(1)A' and 6(1)A' states of the N(7)H tautomer of adenine cannot be ruled out. As both tautomers appear to be present in experiment, the measured and calculated polarization angles differ substantially. For guanine the following energies are obtained for the lowest pi(-->)pi* valence excited states: 4.7 (4.5-4.8), 5.1 (4.9-5.0), 6.0 (5.5-5.8), 6.5 (6.0-6.4), 6.6, 6.7 (6.6-6.7), and 6.7 eV. The polarization vectors of the first two transitions are almost perpendicular and point along the shore rind long axes, respectively. The fourth and sixth transitions are the most intense peaks of the spectrum. The experimental transition moment directions are reproduced with an accuracy better than 6 degrees. The fourth transition is strongly shifted to lower energies in polar solvents.