Fragmentation and isomerization of polycyclic aromatic hydrocarbons in the interstellar medium: Coronene as a case study

Abstract

Aims. Due to the limitations of current computational technology, the fragmentation and isomerization products of vibrationally-excited polycyclic aromatic hydrocarbon (PAH) molecules and their derivatives are poorly studied. In this work, we investigate the intermediate products of PAHs and their derivatives as well as the gas-phase reactions relevant to the interstellar medium, with coronene as a case study. Methods. Based on the semi-empirical method of PM3 as implemented in the CP2K program, molecular dynamics simulations are performed to model the major processes (e.g., vibrations, fragmentations, and isomerizations) of coronene and its derivatives (e.g., methylated coronene, hydrogenated coronene, dehydrogenated coronene, nitrogen-substituted coronene, and oxygen-substituted coronene) at temperatures of 3000 K and 4000 K. Results. We find that the anharmonic effects are crucial for the simulation of vibrational excitation. For the molecules studied here, H2, CO, HCN, and CH2 are the major fragments. Following the dissociation of these small units, most of the molecules could maintain their ring structures, but a few molecules would break completely into carbon chains. The transformation from hexagon to pentagon or heptagon may occur and the heteroatomic substitutions (e.g., N-or O-substitutions) facilitate the transformation.

In search of universalities in the dissociative photoionization of PANHs via isomerizations

“…35 Even in processes that appear relatively simple like neutral hydrogen losses, where at first glance, one might expect that only direct bond fission of the C-H bond to play a role, hydrogen migrations leading to various C-H shifted isomers were found to precede the hydrogen dissociation channels. [36][37][38] These isomerization mechanisms have persistently appeared in different theoretical studies too [39][40][41] and possibly hint toward a universal behavior 42 behind the dissociation of this family of PA(N)Hs. However, the dynamical details of these mechanisms are not directly decipherable from most experimental spectra.…”

Top-down formation of ethylene from fragmentation of superhydrogenated polycyclic aromatic hydrocarbons

“…The key element of MD simulations to successfully describe statistical fragmentation is the choice of a proper inter-atomic potential for determining energies and forces of a given system. The semiempirical method PM3 has been utilized to study the fragmentation of pristine (Chen & Luo 2019) and functionalized (Chen et al 2020) PAHs, while the density functional-based tight binding (DFTB) has been used to study the competition between hy-drogenation and fragmentation by comparing experimental and theoretical mass spectra (Rapacioli et al 2018). It is important to consider both accuracy and computational costs when choosing the potential for running MD simulations.…”

“…Another popular method to study fragmentation is Born-Oppenheimer molecular dynamics (MD) which is capable of directly probing all the relevant dynamical processes (Simon et al 2017;Chen & Luo 2019;Chen et al 2020) and providing the mass distribution of the fragments (Simon et al 2018;Rapacioli et al 2018). MD covers all possible fragmentation pathways in an unbiased way as long as a sufficient number of runs is conducted.…”

Photoprocessing of cationic triazacoronene: dissociation characteristics of polycyclic aromatic nitrogen heterocycles in interstellar environments

“…Coronene is a highly symmetric pericondensed PAH, which has been studied in the past. 33,34 TAC contains about twice the number of heavy atoms (C, N) compared to the previously largest investigated PANHs and is thus of larger interstellar significance. Moreover, its dissociation may result in the incorporation of one or more nitrogen atoms in the fragment ions.…”