Low temperature formation of naphthalene and its role in the synthesis of PAHs (Polycyclic Aromatic Hydrocarbons) in the interstellar medium

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 109; no. 1; pp. 53 - 58
• Main Authors: Parker, Dorian S. N., Zhang, Fangtong, Kim, Y. Seol, Kaiser, Ralf I., Landera, Alexander, Kislov, Vadim V., Mebel, Alexander M., Tielens, A. G. G. M.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 03.01.2012; National Acad Sciences
• Subjects: Atoms; Chemical synthesis; Cold Temperature; Cosmic Dust; Free Radicals; Hydrogen; Interstellar medium; Isomerism; Low temperature; Mass Spectrometry; Molecular clouds; Molecular Weight; Molecules; Naphthalenes - chemical synthesis; Naphthalenes - chemistry; Phenyls; Physical chemistry; Physical Sciences; Polycyclic aromatic hydrocarbons; Polycyclic Aromatic Hydrocarbons - chemical synthesis; Polycyclic Aromatic Hydrocarbons - chemistry; Reactants; Surface Properties; Temperature; Thermodynamics
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1113827108

Polycyclic aromatic hydrocarbons (PAHs) are regarded as key molecules in the astrochemical evolution of the interstellar medium, but the formation mechanism of even their simplest prototype— naphthalene (C10H8)—has remained an open question. Here, we show in a combined crossed beam and theoretical study that naphthalene can be formed in the gas phase via a barrierless and exoergic reaction between the phenyl radical (C6H5) and vinylacetylene (CH2 = CH-C ≡ CH) involving a van-der-Waals complex and submerged barrier in the entrance channel. Our finding challenges conventional wisdom that PAH-formation only occurs at high temperatures such as in combustion systems and implies that low temperature chemistry can initiate the synthesis of the very first PAH in the interstellar medium. In cold molecular clouds, barrierless phenyl-type radical reactions could propagate the vinylacetylene-mediated formation of PAHs leading to more complex structures like phenanthrene and anthracene at temperatures down to 10 K.