A THEORETICAL-STUDY OF THE ELECTRONIC-SPECTRA OF ADENINE AND GUANINE

The complete active space (GAS) SCF method and multiconfigurational se cond-order perturbation theory (CASPT2) have been used to study the el ectronic spectra of the nucleic acid base monomers guanine and two tau tomers of adenine (the N(9)H and N(7)H forms). The calculations includ e vertical excitation energies, oscillator strengths, and transition m oment directions in gas phase. For guanine solvent effects were includ ed using a self-consistent reaction field model. The lowest pi(-->)pi excited valence states of N(9)H-adenine are calculated at (experiment al 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 characte rized by small and medium intensities, respectively. The third and fif th transitions have large oscillator strengths. Two less clear assignm ents 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 cann ot 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 ve ctors of the first two transitions are almost perpendicular and point along the shore rind long axes, respectively. The fourth and sixth tra nsitions are the most intense peaks of the spectrum. The experimental transition moment directions are reproduced with an accuracy better th an 6 degrees. The fourth transition is strongly shifted to lower energ ies in polar solvents.