What Will Photo-Processing of Large, Ionized Amino-Substituted Polycyclic Aromatic Hydrocarbons Produce in the Interstellar Medium?

Collision-energy resolved tandem mass spectrometry was used to probe the trends in unimolecular fragmentation in a series of ionized amino-substituted polycyclic aromatic hydrocarbons ranging from naphthalene to pyrene. As the ring system expands, the dominant dissociation process changes from HNC loss (aniline) to H loss for 1-aminopyrene. Imaging photoelectron photoion coincidence spectroscopy of 1-aminopyrene yielded threshold photon-energy resolved breakdown curves, the Rice–Ramsperger–Kassel–Marcus modeling of which gave a 0 K activation energy, E0, for H loss of 3.8 ± 0.4 eV. Calculations at the CCSD/6-31G­(d)//B3LYP/6-31G­(d) level of theory were used to explore the possible reaction mechanisms for H, HNC, and C,N,H2 losses, and details of the reaction pathways are presented. The H atom loss was found to be due both to direct N–H bond cleavage and isomerization to form an azepine derivative.