Electronic Spectroscopy of [FePAH](+) Complexes in the Region of the Diffuse Interstellar Bands: Multireference Wave...

The low-energy states and electronic spectrum in the near-infrared-visible region of [FeC6H6](+) are studied by theoretical approaches. An exhaustive exploration of the potential energy surface of [FeC6H6](+) is performed using the density functional theory method. The ground state is found to be a (4)A1 state. The structures of the lowest energy states ((4)A2 and (4)A1) are used to perform multireference wave function calculations by means of the multistate complete active space with perturbation at the second order method. Contrary to the density functional theory results ((4)A1 ground state), multireference perturbative calculations show that the (4)A2 state is the ground state. The vertical electronic spectrum is computed and compared with the astronomical diffuse interstellar bands, a set of near-infrared-visible bands detected on the extinction curve in our and other galaxies. Many transitions are found in this domain, corresponding to d → d, d → 4s, or d → π* excitations, but few are allowed and, if they are, their oscillation strengths are small. Even though some band positions could match some of the observed bands, the relative intensities do not fit, making the contribution of the [Fe-C6H6](+) complexes to the diffuse interstellar bands questionable. This work, however, lays the foundation for the studies of polycyclic aromatic hydrocarbons (PAHs) complexed to Fe cations that are more likely to possess d → π* and π → π* transitions in the diffuse interstellar bands domain. PAH ligands indeed possess a larger number of π and π* orbitals, respectively, higher and lower in energy than those of C6H6, which are expected to lead to lower energy d → π* and π → π* transitions in [FePAH](+) than in [FeC6H6](+) complexes.