Reactivity of coronene with O-atoms, a possible route to ketene in the interstellar medium

Abstract

PAHs are one of the important components of the carbonaceous matter of the Universe. They are not detected in the darkest regions of the Interstellar Medium and one possible reason could be their chemical transformation through gas phase reactions In particular, their oxidation was considered ineffective because the reaction barriers appear to be too high, based on combustion studies conducted at high temperatures. For the first time, we experimentally studied the oxidation of Coronene, a PAH archetype, at low temperature (50 K), as well as the oxidation of hydrogenated Coronenes. It appears that reactivity is higher than expected and that the fragmentation of coronene is a significant channel of the oxidation. Furthermore, hydrogenated coronenes are very reactive to oxygen. To understand the experimental data, DFT calculations were performed. They confirm a low oxidation barrier (0.11 eV) and show that oxygen is preferentially inserted at the periphery of the coronene and propose a reaction mechanism for fragmentation also involving a hydrogen atom. An estimate of the orders of magnitude shows that PAH oxidation may explain part of the decrease in their abundances in warm environments.