Reports: AC6
45719-AC6 Spectroscopy of Polycyclic Aromatic Hydrocarbon Cations and Complexes
The most abundant molecule in the interstellar medium, H2, has long been thought to form by the combination of hydrogen atoms on the surface of small dust grains. Recently the involvement of polycyclic aromatic hydrocarbons (PAHs) in the astrochemistry of the interstellar medium (ISM) has been recognized. Indeed, several groups have suggested that PAHs could act as reaction centers (i.e., catalysts) for the formation of H2.
In the past year, we have studied a prototypical PAH, 1,2-dihydronaphthalene (DHN), using matrix isolation infrared absorption spectroscopy, multiphoton infrared photodissociation (IRMPD) action spectroscopy, and density functional theory calculations. Using a Fourier transform ion cyclotron resonance mass spectrometer coupled to an infrared-tunable free electron laser, the IRMPD spectrum of protonated 1,2-dihydronapthalene was recorded. Multiphoton infrared irradiation of the protonated parent (m/z 131) yielded two dissociation products with m/z 129 and m/z 91.
Calculations reveal that the most probable site of protonation is the 3-position, producing the trihydronaphthalene (THN) cation, 1,2,3-THN+ . Extensive B3LYP/6-31G(d,p) calculations of the potential energy surface of 1,2,3-THN+ have been performed and provide insight into the mechanism of the two-channel photodissociation. These results provide support for a new model of the formation of H2 in the interstellar medium. The model involves hydrogenation of a PAH cation to produce one or more aliphatic hydrogen-bearing carbons on the PAH framework followed by photolytic excitation and eventual ejection of H2.
In other work, we have shown that electrospray ionization of a pyrene solution results in the protonation of the impurity azapyrene, almost to the exclusion of any protonated pyrene. This is due to the enhanced proton affinity and greater solubility of aza-PAHs compared to their PAH counterparts. The spectroscopic features observed allow for the identification of the particular azapyrene and the site of its protonation. The protonated azapyrene has features that are consistent with some of the emission bands observed from the interstellar medium.
