ELECTRONIC-SPECTRA OF THE NEUTRAL RADICAL AND H AND OH ADDUCTS OF URACIL

Theoretical assignments of the radicals of nucleic acid bases and the radical adducts are compared with experiment for the simplest base, uracil. The electronic excitation energies of H and OH adducts of uracil are calculated by ab initio methods to provide a catalogue of the electronic states and transitions for such radicals. The C5 and C6 adducts for both H and OH are found to support spectral transitions in entirely different regions. This result contradicts an experimental deduction that the C5 and C6 H adduct spectra have a similar shape. Continuum solvation reaction field shifts from the in vacuo transition energies are substantial and important in assigning transitions. The neutral radical spectrum is also analyzed since the addition of OH is suggested experimentally to result ultimately in the spectrum of this radical. Theoretical transitions are found to span the observed spectral region. Earlier studies of aromatic hydrocarbon radicals support the accuracy of the calculations. An additional test was made here by calculating the phosphorescent transition for the triplet state of uracil.