The effect of pi-stacking, h-bonding, and electrostatic interactions on the ionization energies of nucleic acid bases: adenine-adenine, thymine-thymine and adenine-thymine dimers
A combined theoretical and experimental study of the ionized dimers of thymine and adenine, TT, AA, and AT, is presented. Adiabatic and vertical ionization energies (IEs) for monomers and dimers as well as thresholds for the appearance of the protonated species are reported and analyzed. Non-covalent interactions strongly affect the observed IEs. The magnitude and the nature of the effect is different for different isomers of the dimers. The computations reveal that for TT, the largest changes in vertical IEs (0.4 eV) occur in asymmetric h-bonded and symmetric pi-stacked isomers, whereas in the lowest-energy symmetric h-bonded dimer the shift in IEs is much smaller (0.1 eV). The origin of the shift and the character of the ionized states is different in asymmetric h-bonded and symmetric stacked isomers. In the former, the initial hole is localized on one of the fragments, and the shift is due to the electrostatic stabilization of the positive charge of the ionized fragment by the dipole moment of the neutral fragment. In the latter, the hole is delocalized, and the change in IE is proportional to the overlap of the fragments' MOs. The shifts in AA are much smaller due to a less efficient overlap and a smaller dipole moment. The ionization of the h-bonded dimers results in barrierless (or nearly barrierless) proton transfer, whereas the pi-stacked dimers relax to structures with the hole stabilized by the delocalization or electrostatic interactions.