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.