A theoretical study of the electronic spectra of N-9 and N-7 purine tautomers

The complete active space (CAS) SCF method and multiconfigurational second- order perturbation theory (CASPT2) have been used to study electronic spect ra of the N(9)H and N(7)H tautomers of purine. The calculations include ver tical excitation energies, oscillator strengths, dipole moments, and transi tion moment directions in gas phase. In accord with experiment in nonpolar solvents, the two lowest pi --> pi* excited singlet valence states are pred icted to be located at 4.7 and 5.1 eV. The latter is expected to shift to t he red in aqueous solutions. A satisfactory interpretation of the electroni c 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 tautomer s, Two bands reported at 6.2 and 6.6 eV in nonpolar solvents match the corr esponding B-1(b) and B-1(a) states of the N(9)H purine, respectively. The a bsence 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.