A theoretical study of the electronic spectra of N9 and N7 purine tautomers

The complete active space (CAS) SCF method and multiconfigurational second-order perturbation theory (CASPT2) have been used to study electronic spectra of the N(9)H and N(7)H tautomers of purine. The calculations include vertical excitation energies, oscillator strengths, dipole moments, and transition moment directions in gas phase. In accord with experiment in nonpolar solvents, the two lowest pi --> pi* excited singlet valence states are predicted to be located at 4.7 and 5.1 eV. The latter is expected to shift to the red in aqueous solutions. A satisfactory interpretation of the electronic spectra above 5.5 eV is obtained if the experimental data are assumed to consist of the superposition of the spectra of the N(9)H and N(7)H tautomers, Two bands reported at 6.2 and 6.6 eV in nonpolar solvents match the corresponding B-1(b) and B-1(a) states of the N(9)H purine, respectively. The absence of the 6.2 eV-band in water can be explained by the predominance in aqueous solution of the N(7)H form, which has a weak B-1(b) transition at 6.4 eV overlapped by a strong B-1(a) transition at 6.6 eV.