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

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 1–10eV. 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 CHCH 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.