AB-INITIO MOLECULAR-ORBITAL CALCULATIONS OF DNA RADICAL IONS .5. SCALING OF CALCULATED ELECTRON-AFFINITIES AND IONIZATION-POTENTIALS TO EXPERIMENTAL VALUES

Ab initio molecular orbital calculations of the electron affinities (EAs) and ionization potentials (IPs) of the DNA bases are presented in this work. Comparisons of calculated and experimental values are made for a series of compounds of size and/or structure similar to the DNA bases. Excellent correlations between calculated and experimental values are found for both Koopmans EAs at the 6-31G* and D95v levels and calculated vertical EAs of the model compounds. Several basis sets are considered: 6-31G*, 6-31+G(d), and D95v. The best correlation overall is found for Koopmans D95v EAs and the worst for Koopmans 6-31+G(d) EAs; however, both 6-31G* and 6-31+G(d) vertical electron affinities also have good to excellent fits to experiment which allows for estimation of the vertical electron affinities of the DNA bases. Calculations at 6-31G* and 6-31+G(d) using both ROHF and ROMP2 theories show a consistent difference between calculated vertical and adiabatic EAs. This allows for a good estimate of DNA base adiabatic EAs, i.e:, -0.7, -0.3, 0.2, 0.3, and 0.4 eV; from the vertical EAs -1.23, -0.74, -0.40, -0.32, and -0.19 eV for G, A, C, T, and U respectively. While EAs must be scaled, we find that Koopmans IPs calculated at the simple 3-21G level predict vertical IPs of the DNA bases with only a 0.15 eV average absolute deviation from the experimentally reported values and calculations at MP2/6-31+G(d)//6-31G* for the adiabatic ionization potentials of the DNA bases are all within 0.1 eV of experiment.