Electronic structure of the benzene dimer cation
The benzene and benzene dimer cations are studied using the equation-of-motion coupled-cluster model with single and double substitutions for ionized systems (EOM-IP-CCSD). 10 lowest electronic states of the dimer at t-shaped, sandwich and displaced sandwich configurations are described and catalogued based on the character of the constituent fragment molecular orbitals (FMOs). The character of the states, bonding patterns, and important features of the electronic spectrum are explained using qualitative dimer molecular orbitals - linear combination of fragment molecular orbitals (DMO-LCFMO) framework. Relaxed ground state geometries are obtained for all isomers. Calculations reveal that the lowest energy structure of the cation has a displaced sandwich structure and a binding energy (BE) of 20 kcal/mol, while the t-shaped isomer is 6 kcal/mol higher. The calculated electronic spectra agree well with experimental gas phase action spectra and femtosecond transient absorption in liquid benzene. Both sandwich and t-shaped structures feature intense charge resonance (CR) bands, whose location is very sensitive to the interfragment distance. Change in the electronic state ordering was observed between sigma and pi states, which correlate to the B and C bands of the monomer, suggesting a reassignment of the local excitation (LE) peaks in the gas phase experimental spectrum.