Electronic structure of the benzene dimer cation

Piotr A. Pieniazek, Anna I. Krylov, and S. E. Bradforth

J. Chem. Phys., 127, 044317 (2007).

π-stacked aromatic systems commonly occur in molecular engineering and DNA/RNA. Exposed to light or oxidative agents, they can undergo ionization, and ensuing electron/hole transfer processes lead to the formation of reactive intermediates. The ionized benzene dimer is a model system for these types of processes.

Theoretical description of open-shell dimer cations is difficult due to near-degeneracies, spin contamination, and symmetry breaking, which adversely impact the calculated excitation energies and properties. EOM-IP-CCSD, an analog of the Koopmans theorem descibes the ionized dimer system using the well-behaved closed-shell reference wave function, including correlation effects.

We characterized the ten low-lying excited states of the t-shaped, sandwich, and displaced sandwich isomers of the benzene dimer cation, and determined the important relaxation coordinates. These states are derived from the single ionization of the neutral and their character is defined by the doubly occupied (in the neutral) orbital from which the electron is removed. This orbital hosts the unpaired electron of the cation. For example, ionization from the bonding orbital results in the repulsive state, and vice versa.

We found that ionization changes the bonding from van der Waals (2.6 kcal/mol) to covalent (20 kcal/mol) type resulting in a significant structural change relative to the neutral species: the two rings move closer together as the system rearranges from a t-shaped to a displaced sandwich structure.

An important feature in the electronic spectrum of the dimer cation, both in the t-shaped and sandwich geometries, is the appearance of strong charge-resonance (CR) bands, which are not present in the monomer and are very sensitive to the geometry. Therefore, these transitions can be used to monitor the dynamics of the system.

The characters of the excited states, the bonding patterns, and important features of the electronic spectrum can be explained in terms of a qualitative Dimer Molecular Orbital Linear Combination of Fragment Molecular Orbital (DMO-LCFMO) framework. See the above reference for details.