Radical Isomerization upon Dissociative Electron Ionization of Anthracene and Phenanthrene

Polycyclic aromatic hydrocarbons (PAHs) are abundantly present in space. The grandPAH hypothesis states that small PAHs are photodissociated, while large symmetric PAHs survive the harsh environments in space. Moreover, it has been hypothesized that large aromatic molecules (C_<i>n</i>H_<i>m</i> with <i>n</i> ≥ 60) can convert to buckminsterfullerene (C₆₀). In this work, we test these hypotheses by studying the products formed upon dissociative electron ionization of two isomeric C₁₄H₁₀ PAHs, anthracene and phenanthrene. The fragment ions that are formed following H loss and H₂ loss are isolated in a cryogenically-cooled 22-pole ion trap and tagged with neon. Infrared predissociation spectra are recorded of the thus formed van der Waals bound complexes and the PAH dissociation fragments are identified based on a comparison with density functional theory (DFT) calculated spectra. The ionized PAHs undergo radical isomerization prior to the loss of H or H₂, resulting in a highly symmetric daughter ion that is identical for the two distinctly different parent PAHs. Moreover, the product ions are found to obey the isolated pentagon rule, which also curves fullerenes and contributes to their structural stability. We propose a mechanism for the radical isomerization based on existing molecular dynamics simulations from the literature augmented by DFT calculations. This study lends credit to the grandPAH hypothesis by showing that PAH species isomerize drastically to form a new molecule that is highly symmetric. Moreover, the formation of a daughter species that obeys the isolated-pentagon rule suggests that there is a strong chemical link between interstellar PAHs and fullerenes.