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Formation of benzene in the interstellar medium.

ABSTRACT: Polycyclic aromatic hydrocarbons and related species have been suggested to play a key role in the astrochemical evolution of the interstellar medium, but the formation mechanism of even their simplest building block--the aromatic benzene molecule--has remained elusive for decades. Here we demonstrate in crossed molecular beam experiments combined with electronic structure and statistical calculations that benzene (C(6)H(6)) can be synthesized via the barrierless, exoergic reaction of the ethynyl radical and 1,3-butadiene, C(2)H + H(2)CCHCHCH(2) ? C(6)H(6) + H, under single collision conditions. This reaction portrays the simplest representative of a reaction class in which aromatic molecules with a benzene core can be formed from acyclic precursors via barrierless reactions of ethynyl radicals with substituted 1,3-butadiene molecules. Unique gas-grain astrochemical models imply that this low-temperature route controls the synthesis of the very first aromatic ring from acyclic precursors in cold molecular clouds, such as in the Taurus Molecular Cloud. Rapid, subsequent barrierless reactions of benzene with ethynyl radicals can lead to naphthalene-like structures thus effectively propagating the ethynyl-radical mediated formation of aromatic molecules in the interstellar medium.

SUBMITTER: Jones BM

PROVIDER: S-EPMC3021054 | biostudies-other | 2011 Jan

REPOSITORIES: biostudies-other

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Formation of benzene in the interstellar medium.

Jones Brant M BM Zhang Fangtong F Kaiser Ralf I RI Jamal Adeel A Mebel Alexander M AM Cordiner Martin A MA Charnley Steven B SB

Proceedings of the National Academy of Sciences of the United States of America 20101227 2

Polycyclic aromatic hydrocarbons and related species have been suggested to play a key role in the astrochemical evolution of the interstellar medium, but the formation mechanism of even their simplest building block--the aromatic benzene molecule--has remained elusive for decades. Here we demonstrate in crossed molecular beam experiments combined with electronic structure and statistical calculations that benzene (C(6)H(6)) can be synthesized via the barrierless, exoergic reaction of the ethyny ...[more]

PMID: 21187430

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Project description:Transition-metal-catalyzed carbon-carbon (C-C) bond formation is an important reaction in pharmaceutical and organic chemistry. However, the reaction process is composed of multiple steps and is expensive owing to the presence of transition metals. This study proposes a lithium-catalyzed C-C coupling reaction of two benzene molecules (Bz) to form a biphenyl molecule, which is a transition-metal-free reaction, based on ab initio and direct ab initio molecular dynamics (AIMD) calculations. The static ab initio calculations indicate that the reaction of two Bz molecules with Li- ions (reactant state, RC) can form a stable sandwiched complex (precomplex), where the Li- ion is sandwiched by two Bz molecules. The complex formation reaction can be expressed as 2Bz + Li - → Bz(Li -)Bz, where the C-C distance between the Bz rings is 2.449 Å. This complex moves to the transition state (TS) via the structural deformation of Bz(Li-)Bz, where the C-C distance is shortened to 2.118 Å. The barrier height was calculated to be -9.9 kcal/mol (relative to RC) at the MP2/6-311++G(d,p) level. After TS, the C(sp3)-C(sp3) single bond was completely formed between the Bz rings (the C-C bond distance was 1.635 Å) (late complex). After the dissociation of H2 from the late complex, a biphenyl molecule was formed: the C(sp2)-C(sp2) bond. The calculations suggest that the C-C bond coupling of Bz occurred spontaneously from 2Bz + Li-, and biphenyl molecules were directly formed without an activation barrier. Direct AIMD calculations show that the C-C coupling reaction also takes place under electron attachment to Li(Bz)2: Li(Bz)2 + e- → [Li-(Bz)2]ver → precomplex → TS → late complex, where [Li-(Bz)2]ver is the vertical electron capture species of Li(Bz)2. Namely, the C-C coupling reaction spontaneously occurred in Li(Bz)2 owing to electron attachment. Similar C-C coupling reactions were also observed for halogen-substituted benzene molecules (Bz-X, X = F and Cl). Furthermore, this study discusses the mechanism of C-C bond formation in electron capture based on the theoretical results.

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Formation of benzene in the interstellar medium.

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Formation of benzene in the interstellar medium.

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