Vibrational spectroscopy of neutral complexes of Fe and polycyclic aromatic hydrocarbons

Infrared absorption spectra of neutral complexes of iron with benzene, naphthalene, fluorene, pyrene, and coronene in solid Ar at 12 K have been obtained. Supporting calculations of the equilibrium geometries, stabilities, and harmonic vibrational frequencies of these complexes have been carried out using density functional theory (MPW1PW91/6-31+G(d,p) method) using a modified Perdew-Wang exchange and correlation functional/basis set. Our results indicate that the spin multiplicities of the complexes' electronic ground states are triplets. The calculations show that the iron atom is situated over the six-membered carbon ring of the polycyclic aromatic hydrocarbon (PAH) ligand. Calculated dissociation energies (D0) range from 0.52 eV for Fe(coronene) to 2.06 eV for Fe(fluorene). All are substantially less tightly bound than their cationic counterparts. Cationic Fe(PAH) complexes are therefore thought to be more plausible candidates for the carriers of the unidentified interstellar infrared (UIR) emission bands.