45719-AC6
Spectroscopy of Polycyclic Aromatic Hydrocarbon Cations and Complexes
The involvement of polycyclic aromatic hydrocarbons (PAHs) and carbon chain clusters in the astrochemistry of the interstellar medium (ISM) has been recognized for over twenty years now. While the presence of various metals in the ISM is also well known, the reaction of metals with PAHs or carbon clusters has only recently been proposed to account for the observed depletion of these metals in the ISM.
In the past year we have shown that iron may interact with PAHs to form stable complexes. Infrared absorption spectra of neutral complexes of iron with benzene, naphthalene, fluorene, pyrene, and coronene in solid Ar at 12K 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). 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). In previous work, we investigated complexes of the iron ion, Fe+, with various PAHs using a Fourier transform ion cyclotron resonance spectrometer couple with an infrared-tunable free electron laser. We have now shown that the neutral Fe-PAH compleses are substantially less tightly bound than their cationic counterparts.
We have also investigated the clusters formed between copper and carbon. Formed by dual Nd/YAG laser vaporization, copper/carbon clusters were trapped in a solid Ar matrix at 12K and investigated by infrared spectroscopy. Density functional calculations of a number of possible molecular structures for Cu/carbon clusters have been performed and their associated vibrational harmonic mode frequencies and dissociation energies determined with a 6-311++G(3df) basis set using both B3LYP and MPW1PW91 functionals. Both computations and 13C-isotopic substitution experiments indicate that new bands observed at 1830.0 and 1250.5 cm-1 are due to the asymmetric and symmetric C=C stretching modes in the near-linear CuC3 (X 2A′) cluster. Photo-induced 12/13C isotopic scrambling in Cu12/13C3 clusters has been observed. The mechanism for the photoscrambling is shown to involve the formation of a bicyclic CuC3 isomer. Evidence for larger copper carbide clusters has also been found.
