PAH chemistry at eV internal energies. 2. Ring alteration and dissociation

TL;DR

This study investigates high-energy PAH structures and fragmentation pathways relevant to astrochemistry, revealing alternative ring configurations and dissociation mechanisms through DFT calculations on coronene and pyrene.

Contribution

It identifies and characterizes novel PAH isomers and fragmentation pathways at eV energies, expanding understanding of PAH behavior in astrophysical environments.

Findings

Identification of ethynyl and vinylidene PAH isomers

Revealed twisted and bicyclic PAH forms at high energies

Explored fragment elimination pathways for H, H2, C2H2, and C2H4

Abstract

Recognized as important interstellar constituents, polycyclic aromatic hydrocarbons (PAHs) have been intensively studied in astrochemistry and their spectroscopy, thermodynamics, dynamics, and fragmentations are now amply documented. There exists typical alternatives to the ground-state regular planar structures of PAHs, as long as they bear internal energies in the range 1-10 eV. Resulting from intramolecular rearrangements, such high-lying minima on the potential- energy surfaces should be taken into consideration in the studies of PAH processing in astrophysical conditions. Resting upon DFT calculations mainly performed on two emblematic PAH representatives, coronene and pyrene, in their neutral and mono and multi-cationic states, this second survey addresses the following alternatives: (1) opened forms containing ethynyl or 2- butynyl groups, (2) vinylidene isomers, in which phenanthrene patterns are reorganized into dibenzofulvene ones, (3) twisted forms, where external CH=CH bonds can be partly twisted, and (4) bicyclobutane forms, in which the latter are integrated in saturated bicyclic forms. A few scenarios for elimination of fragments H, H2, C2H2 and C2H4 are explored. As far as possible, familiar concepts of organic chemistry, such as aromaticity or Clar's rules, are invoked for interpretations.