π-Electronic Structures of DNA-Bases with Normal, Tautomeric and Ionized Forms

ASMO-SCF-CI calculations on the DNA-bases with both the normal and the tautomeric forms are carried out within the framework of the π-electron approximation and in terms of the Pariser-Parr-Pople approximation. The comparison between the electronic structures of the two forms is systematically made. The electronic structures for the anion, the cation and the first-triplet states of the bases are also calculated by means of the unrestricted Hartree-Fock method. It is shown that there is a close resemblance between the electronic states of the normal bases and those of the tautomeric ones: A↔G * , U↔C * , G↔A * and C↔U * . It is also shown for each of the bases that the excitation to triplet states and the ionization lead to large redistributions of π-electron densities whereas the excitation to singlet states does not. The calculated excitation energies for singlet states are in close agreement with observed ultra-violet absorption date on the lowest three absorption bands.