H-2 Ejection from Polycyclic Aromatic Hydrocarbons: Infrared Multiphoton Dissociation Study of Protonated Acenaphthene and 9,10-Dihydrophenanthrene

The infrared multiple-photon dissociation ( IRMPD) spectra of protonated acenaphthene ([ACN+H](+)) and 9,10-dihydrophenanthrene ([DHP+H](+)) have been recorded using an infrared free electron laser after the compounds were protonated by electrospray ionization and trapped in a Fourier transform ion cyclotron mass spectrometer. In both compounds, the loss of two mass units is predominant. Density functional calculations (B3LYP/6-311++G(d,p)) of the infrared spectra of all possible protonated isomers of each species showed that the observed IRMPD spectra are best fit to the isomer with the largest proton affinity and lowest relative electronic energy. Potential energy surfaces of the most stable isomers of [ACN+H](+) and [DHP+H](+) have been calculated for H and H-2 loss. The lowest energy barriers are for loss of H-2, with predicted energies 4.28 and 4.15 eV, respectively. After H-2 ejection, the adjacent aliphatic hydrogens migrate to the bare ejection site and stabilize the remaining fragment. Single H loss may occur from [ACN+H](+) but the energy required is higher. No single H loss is predicted from [DHP+H](+), only H migration around the carbon skeleton. The vibrational bands in the parent closed-shell protonated polycyclic aromatic hydrocarbons are compared to bands observed from the interstellar medium.