PAH chemistry at eV internal energies. 1. H-shifted isomers

TL;DR

This study investigates hydrogen-shifted isomers of PAHs at high internal energies using DFT calculations, revealing their energetic properties and potential relevance in astrophysical environments.

Contribution

It provides detailed computational analysis of H-shifted PAH isomers, highlighting their energetic landscape and differences between neutral and cationic states, which was previously underexplored.

Findings

H-shifted isomers are 4-5 eV above regular forms

Barriers to isomerization are about 1 eV

Cationic PAHs have lower energy H-shifted minima

Abstract

The PAH family of organic compounds (polycyclic aromatic hydrocarbons), involved in several fields of chemistry, has received particular attention in astrochemistry, where their vibrational spectroscopy, thermodynamic, dynamic, and fragmentation properties are now abundantly documented. This survey aims at drawing trends for low spin-multiplicity surfaces of PAHs bearing internal energies in the range 1-10 eV. It addresses some typical alternatives to the ground-state regular structures of PAHs, making explicit possible intramolecular rearrangements leading to high-lying minima. These isomerisations should be taken into consideration when addressing PAH processing in astrophysical conditions. The first part of this double-entry study focuses on the hydrogen-shifted forms, which bear both a carbene center and a saturated carbon. It rests upon DFT calculations mainly performed on two emblematic PAH representatives, coronene and pyrene, in their neutral and mono and multi-cationic states. Systematically searched for in neutral species, these H-shifted minima are lying 4-5 eV above the regular all-conjugated forms, and are separated by barriers of about one eV. General hydrogen-shifting is found to be easier for cationic species as the relative energies of their H-shifted minima are 1-1.5 eV lower than those for neutral species. As much as possible, classical knowledge and concepts of organic chemistry such as aromaticity and Clar's rules are invoked for result interpretation.