| Abstract
| - The complete active space (CAS) SCF method andmulticonfigurational second-order perturbation theory(CASPT2) have been used to study the electronic spectra of the nucleicacid base monomers guanine and two tautomersof adenine (the N(9)H and N(7)H forms). The calculations includevertical excitation energies, oscillator strengths,and transition moment directions in gas phase. For guanine solventeffects were included using a self-consistentreaction field model. The lowest π → π* excited valencestates of N(9)H-adenine are calculated at (experimentaldata 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 degeneratestates are characterized by small and medium intensities, respectively.The third and fifth transitions have largeoscillator strengths. Two less clear assignments can be performedto the transitions observed in experiment at 4.6and 6.3−6.4 eV. Presently they can be assigned to the21A‘ and 51A‘ states of the N(9)H tautomer ofadenine, butpossible contributions to the 31A‘ and 61A‘states of the N(7)H tautomer of adenine cannot be ruled out. Asbothtautomers appear to be present in experiment, the measured andcalculated polarization angles differ substantially.For guanine the following energies are obtained for the lowest π→ π* 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 twotransitions are almost perpendicular and point along the short and longaxes, respectively. The fourth and sixthtransitions are the most intense peaks of the spectrum. Theexperimental transition moment directions are reproducedwith an accuracy better than 6°. The fourth transition isstrongly shifted to lower energies in polar solvents.
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