Dehydrogenation effects on the stability of aromatic units in polycyclic aromatic hydrocarbons in the interstellar medium: A computational study at finite temperature

Isomerization, ionization and fragmentation of molecular compounds in the interstellar medium can be triggered by stellar radiation and cosmic rays. In the present contribution, we examine the propensity for isomerization and the relative stability of aromatic rings in the pyrene and coronene molecules at various degrees of dehydrogenation by means of molecular modeling. Using the AIREBO reactive force field and advanced Monte Carlo techniques such as the Wang-Landau method based on suitable order parameters, entire free-energy profiles describing the isomerization pathways and equilibrium properties were calculated as a function of temperature or total energy. We generally find that hydrogenation significantly stabilizes the fully polycyclic aromatic hydrocarbon (PAH) structure, even though local dehydrogenation next to an aromatic ring favors ring opening. The formation of pentagonal rings, a typical defect motif in the polycyclic carbon skeleton, is predicted to be actually competitive with the loss of a hydrogen atom. Our investigation emphasizes the likely presence of defects in astrophysical PAHs, whose spectral features remain to be better characterized and understood.