H_2 Ejection from Polycyclic Aromatic Hydrocarbons: Infrared Multiphoton Dissociation Study of Protonated 1,2-Dihydronaphthalene

摘要

1,2-Dihydronaphthalene (DHN) has been studied by matrix isolation infrared absorption spectroscopy, multiphoton infrared photodissociation (IRMPD) action spectroscopy, and density functional theory calculations. Formed by electrospray ionization, protonated 1,2-dihydronapthalene was injected into a Fourier transform ion cyclotron resonance mass spectrometer coupled to an infrared-tunable free electron laser and its IRMPD spectrum recorded. Multiphoton infrared irradiation of the protonated parent (mlz 131) yields two dissociation products, one with mlz 129 and the other with mlz 91. Results from density functional theory calculations (B3LYP/6-31 ++G(d,p)) were compared to the low-temperature matrix isolation infrared spectrum of neutral DHN, with excellent results. Calculations reveal that the most probable site of protonation is the 3-position, producing the trihydronaphthalene (THN) cation, 1,2,3-THN~+. The observed IRMPD spectrum of vapor-phase protonated parent matches well with that computed for 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 H_2 in the interstellar medium. This model involves hydrogenation of a PAH cation to produce one or more aliphatic hydrogen-bearing carbons on the PAH framework, followed by photolytic formation and ejection of H_2.