Elementary reactions of the phenyl radical, CH, with CH isomers, and of benzene, CH, with atomic carbon in extraterrestrial environments

¹ Department of Chemistry, University of Hawai'i at Manoa, Honolulu, HI 95622, USA
² Department of Chemistry, University of Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium
³ Lehrstuhl für Organische Chemie 2, Ruhr-Universität Bochum, 44780 Bochum, Germany
⁴ Center for Computational Quantum Chemistry, The University of Georgia, Athens, GA, USA
⁵ Institut für Organische Chemie, Universität Erlangen-Nürnberg, 91052 Erlangen, Germany

Corresponding author: R. I. Kaiser, kaiser@gold.chem.hawaii.edu

Received: 19 February 2003
Accepted: 15 May 2003

Abstract

Binary collisions of ground state carbon atoms, C(³P_j), with benzene, C₆H₆(X¹A), and of phenyl radicals, C₆H₅(X²A₁), with methylacetylene, CH₃CCH(X¹A₁), were investigated in crossed beam experiments, ab initio calculations, and via RRKM theory to elucidate the underlying mechanisms of elementary reactions relevant to the formation of polycyclic aromatic hydrocarbons (PAHs) in extraterrestrial environments. The reactions of phenyl radicals with allene, H₂CCCH₂, and with cyclopropene, cyc-C₃H₄, as well as the reaction of benzyl radicals, C₆H₅CH₂, with acetylene, HCCH, were also investigated theoretically. The C(³P_j) atom reacts with benzene via complex formation to a cyclic, seven membered C₇H₅ doublet radical plus atomic hydrogen. Since this pathway has neither an entrance nor an exit barrier and is exoergic, the benzene molecule can be destroyed by carbon atoms even in the coldest molecular clouds. On the other hand, the reaction of phenyl radicals with methylacetylene has an entrance barrier; at high collision energies, the dynamics are at the boundary between an osculating complex and a direct pathway. Statistical calculations on the phenyl plus methylacetylene reaction demonstrate dramatic energy/temperature dependencies: at lower temperatures, the bicyclic indene isomer is the sole reaction product. But as the temperature increases to 2000 K, formation of indene diminishes in favor of substituted acetylenes and allenes, such as PhCCH, PhCCCH₃, PhCHCCH₂, and PhCH₂CCH. Also, direct H-abstraction channels become accessible, forming benzene and C₃H₃ radicals, including propargyl. Similar conclusions were reached for the reactions of phenyl radicals with the other C₃H₄ isomers, as well as for the benzyl + acetylene reaction. The strong temperature dependence emphasizes that distinct product isomers must be included in reaction networks modeling PAH formation in extraterrestrial environments.