The effect of pi-stacking and H-bonding on ionization energies of a nucleobase: Uracil dimer cation
Electronic structure of the ionized pi-stacked and H-bonded uracil dimers, as well as the monomer, is characterized by 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 lowest vertical IE by 0.34 eV and 0.13 eV in the pi-stacked and H-bonded dimers, respectively. 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 is 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 the 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.