Electronic spectra of nucleic acid bases: Variable electronegativity calculations and an analysis of existing results

Variable electronegativity Pariser–Parr–Pople (VE‐PPP) calculations have been carried out on the bases of nucleic acids and 5‐fluorouracil. Directions of polarization for the various electronic transitions in these molecules have been calculated using the wave functions obtained by this and the CNDO/S‐CI methods. The calculated excitation energies, oscillator strengths, and directions of polarization have been compared with the PPP results due to Pullman and co‐workers and Bailey as well as the experimental ones. It has been found that a comparative study of this type is much more useful for understanding the transitions in the molecules than the individual calculations. The VE‐PPP method is found to generally give the best results. The directions of polarization have been used to identify the transitions in the molecules and to compare the excitation energies obtained by different methods. It is shown that the transitions in these molecules do not have the characteristics of those of benzene derivatives. All the observed peaks in the molecules have been explained as π*→π.