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

¹ School of Engineering Sciences in Chemistry, Biotechnology and Health, Department of Theoretical Chemistry and Biology, Royal Institute of Technology, 10691 Stockholm, Sweden
e-mail: taochen@kth.se, luo@kth.se
² Hefei National Laboratory for Physical Science at the Microscale, Department of Chemical Physics, School of Chemistry and Materials Science, University of Science and Technology of China, Hefei, 230026 Anhui, PR China
³ Department of Physics and Astronomy, University of Missouri, Columbia, MO 65211, USA
e-mail: lia@missouri.edu

Received: 8 October 2019
Accepted: 15 December 2019

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 have been 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 were 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 completely break into carbon chains. The transformation from a hexagon to a pentagon or a heptagon may occur and the heteroatomic substitutions (e.g., N- or O-substitutions) would facilitate the transformation.