Modified π-SCF MO CI calculations for anionic forms of pyrimidinic nucleic acid bases

The π-scf mo ci method (within the Pariser-Parr-Pople scheme) is modifed to interpret the ultraviolet spectra of anionic forms of molecules and to obtain the π-electronic charge distribution in the ground state of the anions similar to the charges calculated within the cndo/2 (or indo) procedure. The calculated positions of the electronic absorption bands for anions of uracil, its thioanalogs, cytosine, and isocytosine are in good agreement with the experimental data. In particular, the calculations predict that the monoanions of uracil and thiouracils with the proton at N3 absorb at longer wavelengths than the corresponding monoanions with the proton at N1.