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 (E As) 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 valu es are found for both Koopmans EAs at the 6-31G and D95v levels and c alculated 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) EA s; however, both 6-31G and 6-31+G(d) vertical electron affinities als o have good to excellent fits to experiment which allows for estimatio n of the vertical electron affinities of the DNA bases. Calculations a t 6-31G and 6-31+G(d) using both ROHF and ROMP2 theories show a consi stent difference between calculated vertical and adiabatic EAs. This a llows 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, -0.3 2, 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 abs olute deviation from the experimentally reported values and calculatio ns 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.