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