Ab Initio Ionization Energy Thresholds of DNA and RNA Bases in Gas Phase and in Aqueous Solution

Ionization energy thresholds have been calculated for the canonical DNA and RNA bases both in the gas phase and in aqueous solution at HF and MP2 levels of theory using standard 6-31++G(d,p) basis set. It is shown that the use of the spin projection procedure to correct the open-shell systems for contamination by higher spin states significantly improves the calculated ionization energies. This correction provides practically experimental accuracy to the calculated ionization energies in gas and in aqueous phase. The stabilization of the vertical and adiabatic radical cation energies by water solvation range from 2.15 to 2.58 eV, and from 2.12 to 2.79 eV, relative to the gas-phase results, respectively. The ab initio calculations show that long-range bulk polarization interactions have a significant role in the lowering of the first ionization energy of the DNA and RNA bases. Taking into account the stabilization of the free electron by the solvent, the adiabatic ionization energies in aqueous solution are estimated to be 5.27, 5.05, 4.91, 4.81, and 4.42 eV for uracil, thymine, cytosine, adenine, and guanine, respectively.