Zooming into pi-stacked manifolds of nucleobases: Ionized states of dimethylated uracil dimers
Electronic structure of 1,3-dimethyluracil and its dimer is characterized by electronic structure calculations. The methylation eliminates the h-bonded isomers and allows one to focus on the $\pi$-stacked manifold. In the neutral species, methylation increases binding energy by 3-4 kcal/mol, and reduces the lowest ionization energy (IE) by 0.6 eV. The effect of stacking interactions on IEs is similar in methylated and non-methylated dimers: the lowest IE is blue-shifted by 0.37 and 0.35 eV relative to the respective monomers. The ionized stacked isomers show two different relaxation patterns --- several isomers form structures with the delocalized hole stabilized by the orbital overlap, whereas others relax to the structures with the localized hole stabilized by electrostatic interactions. Electronic spectra of the ionized species at the neutral and the cation geometries are presented.