The effect of pi-stacking and H-bonding on ionization energies of a nucleobase: uracil dimer cation

The electronic structure of the ionized pi-stacked and H-bonded uracil dimers as well as that of the monomer, is characterized by the equation-of-motion coupled-cluster method for ionization potentials (EOM-IP). Vertical ionization energies (IEs) for the 5 lowest states of the monomer and the 10 lowest states of the dimers are presented. Non-covalent interactions in the dimers reduce the lowest vertical IE by 0.34 and 0.13 eV in the pi-stacked and H-bonded dimers, respectively. The electronic spectra of the cations are also presented. The two dimer cations feature intense charge resonance bands at 0.52 and 0.11 eV, respectively. The position and intensity of the bands are sensitive to the dimer structures, for example, the geometric relaxation in the pi-stacked dimer blue-shifts the CR band by more than 1 eV and results in a threefold intensity increase. The results of the calculations are rationalized within the dimer molecular orbital-linear combination of fragment molecular orbitals (DMO-LCFMO) framework. The basis set effects and energy additivity schemes are also discussed.