Ionization-induced structural changes in uracil dimers and their spectroscopic signatures
Electronic structure of the three representative isomers of the ionized uracil dimers is characterized by high-level electronic structure calculations. Non-covalent interactions between the fragments lower vertical ionization energies by 0.35-0.13 eV, the largest drop being observed for the stacked and the t-shaped isomers. The initial hole is delocalized in the stacked and the h-bonded isomers, and is localized in the t-shaped one. The ionization induces significant structural relaxation and increases binding energies. The stacked dimer cation relaxes to the symmetric structure bound by 22.7 kcal/mol. The t-shaped dimer cation has binding energy of 25.1 kcal/mol. Thus, the relative order of the stacked and t-shaped isomers is reversed upon ionization. Finally, the h-bonded isomer, which relaxes to the proton-transferred structure, is bound by 37.0 kcal/mol. The electronic spectra of all three isomers characterized at the vertical and the relaxed geometries show different patterns, which may be exploited in spectroscopic probing of ionization-induced dynamics in these species.