Top-down formation of ethylene from fragmentation of superhydrogenated polycyclic aromatic hydrocarbons

Abstract

Context. Fragmentation is an important decay mechanism for polycyclic aromatic hydrocarbons (PAHs) under harsh interstellar conditions and represents a possible formation pathway for small molecules such as H2, C2H2, and C2H4. Aims. Our aim is to investigate the dissociation mechanism of superhydrogenated PAHs that undergo energetic processing and the formation pathway of small hydrocarbons. Methods. We obtain, experimentally, the mass distribution of protonated tetrahydropyrene (C16H15+, [py + 5H]+) and protonated hex-ahydropyrene (C16H17+, [py + 7H]+) upon collision-induced dissociation (CID). The infrared (IR) spectra of their main fragments are recorded by infrared multiple-photon dissociation (IRMPD). Extended tight-binding (GFN2-xTB) based molecular dynamics (MD) simulations were performed in order to provide the missing structure information for this experiment and to identify fragmentation path ways. The pathways for fragmentation were further investigated at a hybrid density functional theory (DFT) and dispersion-corrected level. Results. A strong signal for loss of 28 mass units of [py + 7H]+ is observed both in the CID experiment and the MD simulation, while [py + 5H]+ shows a negligible signal for the product corresponding to a mass loss of 28. The 28 mass loss from [py + 7H]+ is assigned to the loss of ethylene (C2H4) and a good fit between the calculated and experimental IR spectrum of the resulting fragment species is obtained. Further DFT calculations show favorable kinetic pathways for loss of C2H4 from hydrogenated PAH configurations involving three consecutive CH2 molecular entities. Conclusions. This joint experimental and theoretical investigation proposes a chemical pathway of ethylene formation from fragmentation of superhydrogenated PAHs. This pathway is sensitive to hydrogenated edges (e.g., the degree of hydrogenation and the hydrogenated positions). The inclusion of this pathway in astrochemical models may improve the estimated abundance of ethylene.