ABINITIO CONFIGURATION-INTERACTION AND RANDOM PHASE APPROXIMATION CALCULATIONS OF THE EXCITED SINGLET AND TRIPLET-STATES OF ADENINE AND GUANINE

The electronic absorption spectra of adenine and guanine have been calculated with ab initio multi-reference configuration interaction (MRCI) and random phase approximation (RPA) methods, using ground-state self-consistent field orbitals obtained with a double-ζ/polarization/diffuse Cartesian Gaussian basis set. Between 20 and 25 singlet-singlet transitions of varying intensity were found to contribute to the spectral region from 35000 to 55000 cm-1 11 of each molecule. Transition energy errors from both the MRCI and RPA studies were found on average to fall within a range of 12000 to 15000 cm-1, and a linear scaling of calculated transition energies was used to produce a qualitative resolution of the absorption spectra. In adenine, the lowest absorption band at 38 500 cm-1 (band I) was found to arise from a group of five electronic transitions, two σ ⟶ π⋆ and three π ⟶π⋆, but only one π⟶ π⋆ transition was of high intensity. The calculated polarization of the intense transition did not agree with the experimental value obtained for adenine crystals. However, it was shown that the polarization of band I is likely to vary considerably with molecular environment, as well as being sensitive to the level of calculation. In guanine, the absorption maximum at 36 300 cm-1 (band I) was found to consist of two π⟶π⋆ transitions, while one σ⟶π⋆ and three π ⟶π⋆ transitions were found to contribute to band II at 40000 cm-1. For each π⟶π⋆ transition, the excited state was found to contain a mixture of valence and Rydberg character. The polarizations of bands I and II could not be unequivocally determined, owing to the spectral congestion and sensitivity of the constituent excited states to the level of calculation. In the spectral region between 45000 and 55000 cm-1, the calculations for both molecules revealed a large number of transitions of varying intensity. Although the MRCI and RPA calculations give different detailed descriptions of the spectrum, the general features of the broad-banded absorption of this spectral region are provided by both methods. MRCI calculations of triplet states showed that the lowest triplet state, T1 in adenine lies approximately 24930 cm-1 above the ground state, and that there are four valence triplet (π,π⋆) states below the lowest excited singlet state. In guanine, valence states T1 and T2 were found at approximately 28 280 and 29 520 cm-1 above the ground state, while T3 and T4 were essentially isoenergetic with the lowest excited singlet state. © 1990, American Chemical Society. All rights reserved.