Semi-empirical Self-consistent Field Molecular Orbital Calculation of the Electronic Structure of the Base Components of Nucleic Acids

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Abstract
SCF MO calculations have been carried out for adenine, guanine, thymine, cytosine, uracil, 9-methyladenine and 1-methylthymine. In evaluating the two-center repulsion integral, the Pariser-Parr and Mataga-Nishimoto approximations have both been used, and the results compared with each other and with the experimental data on the optical properties. In the calculation of thymine, 9-methyladenine, and 1-methylthymine, the hyperconjugation of the methyl group has been taken into account. The calculated wavelength and the oscillator strength of the singlet-singlet transition for the above-mentioned molecules explain the experimental values well except for thymine and 1-methylthymine. The theoretically determined directions of the polarization of the transition moments have been compared with the experimental results for adenine, 9-methyladenine and 1-methylthymine. With regard to the direction of the polarization of the transition moments of the first and the second absorptions of adenine and 9-methyladenine, the results are questionable. From the theoretical point of view, it is difficult to determine which assignment is correct, however, in view of the wavelength and the oscillator strength, the first absorption may reasonably be assigned to the long-axis polarized band. The calculated and the experimentally-determined direction of polarization for 1-methylthymine agree well with each other. The energies of the highest and lowest vacant orbitals and the π-electron densities for the molecules treated in the present paper have been calculated and compared with the result obtained by the Hückel MO method.