Investigating the importance of edge-structure in the loss of H/H2 of PAH cations: the case of dibenzopyrene isomers

TL;DR

This study investigates how edge-structure influences dehydrogenation pathways of dibenzopyrene cations, revealing that non-planar isomers with bay regions have lower dissociation thresholds and may play a role in space H2 formation.

Contribution

It provides detailed experimental and theoretical analysis of dehydrogenation in dibenzopyrene isomers, highlighting the impact of edge-structure on dissociation energies and pathways.

Findings

Non-planar AL+ has lower dissociation thresholds than planar AE+.

H loss is the dominant dissociation channel at low energies.

Bay regions facilitate H2 formation and influence dehydrogenation pathways.

Abstract

We present a detailed study of the main dehydrogenation processes of two dibenzopyrene cation (C24H14+) isomers, namely dibenzo(a,e)pyrene (AE+) and dibenzo(a,l)pyrene (AL+). First, action spectroscopy under VUV photons was performed using synchrotron radiation in the 8-20 eV range. We observed lower dissociation thresholds for the non-planar molecule (AL+) than for the planar one (AE+) for the main dissociation pathways: H and 2H/H2 loss. In order to rationalize the experimental results, dissociation paths were investigated by means of density functional theory calculations. In the case of H loss, which is the dominant channel at the lowest energies, the observed difference between the two isomers can be explained by the presence in AL+ of two C-H bonds with considerably lower adiabatic dissociation energies. In both isomers the 2H/H2 loss channels are observed only at about 1 eV higher than H loss. We suggest that this is due to the propensity of bay H atoms to easily form H2. In addition, in the case of AL+, we cannot exclude a competition between 2H and H2 channels. In particular, the formation of a stable dissociation product with a five-membered ring could account for the low energy sequential loss of 2 hydrogens. This work shows the potential role of non-compact PAHs containing bay regions in the production of H2 in space.