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

TL;DR

This study uses molecular dynamics simulations to explore how coronene and similar PAHs fragment and isomerize under interstellar conditions, revealing key reaction pathways and the effects of heteroatom substitutions.

Contribution

It provides new insights into PAH fragmentation and isomerization processes in space, especially for substituted coronene molecules, using semi-empirical simulations at high temperatures.

Findings

H2, CO, HCN, and CH2 are major fragments.

Most molecules retain ring structures after dissociation.

Heteroatom substitutions facilitate structural transformations.

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.