ASMO-SCF-CI calculation on the electronic structures of the bases of nucleic acids with special emphasis on the configuration interaction.

Abstract

Abstract The calculation was carried out on the electronic structures of the bases of nucleic acids by means of semi-empirical self-consistent field configuration interaction (SCF-CI) method taking into account of the CI calculation. The effects of mixed configurations, e.g., doubly excited ones, on the transition energies and oscillator strengths, were remarkable. On the other hand, the effects on the direction of the transition moments and the π–electron densities were small. The more configurations were involved, the better the calculated data agreed with the experimental result, but the change of the calculated values with increasing number of configurations from twenty to thirty was small. The agreement between the calculated singlet transition energies and the observed ones obtained by the vapor spectra of the bases was satisfactory, when twenty or thirty configurations including doubly excited ones were considered. The direction of the transition moment calculated by the present authors for thymine agreed well with the experimental result by Stewart and Davidson. For adenine, the calculated direction of transition moment of the main band was polarized parallel to long-axis and it coincided not with the result obtained by Stewart and Davidson, but with that proposed by Mason. Variation of the core resonance integral exerted the influence on the electronic structures, and K in Eq. (1), viz., −0.49 was better than −0.431 when doubly excited configurations were taken into calculation.