Unimolecular dissociation of anthracene and acridine cations: The importance of isomerization barriers for the C₂H₂ loss and HCN loss channels Available to Purchase

The loss of C₂H₂ is a low activation energy dissociation channel for anthracene (C₁₄H₁₀) and acridine (C₁₃H₉N) cations. For the latter ion another prominent fragmentation pathway is the loss of HCN. We have studied these two dissociation channels by collision induced dissociation experiments of 50 keV anthracene cations and protonated acridine, both produced by electrospray ionization, in collisions with a neutral xenon target. In addition, we have carried out density functional theory calculations on possible reaction pathways for the loss of C₂H₂ and HCN. The mass spectra display features of multi-step processes, and for protonated acridine the dominant first step process is the loss of a hydrogen from the N site, which then leads to C₂H₂/HCN loss from the acridine cation. With our calculations we have identified three pathways for the loss of C₂H₂ from the anthracene cation, with three different cationic products: 2-ethynylnaphthalene, biphenylene, and acenaphthylene. The third product is the one with the overall lowest dissociation energy barrier. For the acridine cation our calculated pathway for the loss of C₂H₂ leads to the 3-ethynylquinoline cation, and the loss of HCN leads to the biphenylene cation. Isomerization plays an important role in the formation of the non-ethynyl containing products. All calculated fragmentation pathways should be accessible in the present experiment due to substantial energy deposition in the collisions.