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

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 (m/z 131) yields two dissociation products, one with m/z 129 and the other with m/z 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.