Formation of Benzene in the Interstellar Medium

Formation of benzene in the interstellar medium Brant M. Jonesa,b, Fangtong Zhanga, Ralf I. Kaisera,b,1, Adeel Jamalc, Alexander M. Mebelc, Martin A. Cordinerd, and Steven B. Charnleyd aDepartment of Chemistry, University of Hawaii, Honolulu, HI 96822; bNational Aeronautics and Space Administration Astrobiology Institute, University of Hawaii, Honolulu, HI 96822; cDepartment of Chemistry and Biochemistry, Florida International University, Miami, FL 33199; and dGoddard Center for Astrobiology, National Aeronautics and Space Administration Goddard Space Flight Center, Greenbelt, MD 20706 Edited* by F. Fleming Crim, University of Wisconsin, Madison, WI, and approved November 19, 2010 (received for review August 23, 2010) 1 0 Polycyclic aromatic hydrocarbons and related species have been lic 1,3-hexadien-5-yne isomer [HCCCHCHCHCH2;XA ] can be suggested to play a key role in the astrochemical evolution of formed via a barrierless, single collision event involving the reac- the interstellar medium, but the formation mechanism of even tion of two neutral molecules: 1,3-butadiene and the ethynyl radi- their simplest building block—the aromatic benzene molecule— cal. This reaction presents the simplest representative of a reaction has remained elusive for decades. Here we demonstrate in crossed class in which aromatic molecules with a benzene core can be molecular beam experiments combined with electronic structure formed from acyclic precursors via barrierless reactions of the and statistical calculations that benzene (C6H6) can be synthesized ethynyl radicals with substituted 1,3-butadiene molecules in the via the barrierless, exoergic reaction of the ethynyl radical and 1,3- ISM such as in cold molecular clouds like the Taurus Molecular þ → þ butadiene, C2H H2CCHCHCH2 C6H6 H, under single collision Cloud (TMC-1). conditions. This reaction portrays the simplest representative of a reaction class in which aromatic molecules with a benzene core Results can be formed from acyclic precursors via barrierless reactions of Electronic Structure Calculations. Our electronic structure calcula- ethynyl radicals with substituted 1,3-butadiene molecules. Unique tions indicate that the reaction proceeds without an entrance bar- gas-grain astrochemical models imply that this low-temperature rier (Fig. 1). Details of the calculations are compiled in Materials route controls the synthesis of the very first aromatic ring from and Methods. Reaction pathways to two isomers were identified: acyclic precursors in cold molecular clouds, such as in the Taurus formation of the aromatic benzene molecule and synthesis of the Molecular Cloud. Rapid, subsequent barrierless reactions of ben- thermodynamically less stable, acyclic 1,3-hexadien-5-yne isomer. zene with ethynyl radicals can lead to naphthalene-like structures thus effectively propagating the ethynyl-radical mediated forma- An initial addition of the ethynylic radical center to one of the tion of aromatic molecules in the interstellar medium. terminal carbon atoms of the 1,3-butadiene molecule leads to an acyclic reaction intermediate [i1], which is stabilized by 282 −1 astrochemistry ∣ polycyclic aromatic hyrdrocarbons ∣ reaction dynamics ∣ kJ mol with respect to the reactants. From here, this colli- bimolecular reaction sion complex can undergo unimolecular decomposition by emit- ting a hydrogen atom via a tight exit transition state forming an olycyclic aromatic hydrocarbons (PAHs) and related species acyclic C6H6 isomer: 1,3-hexadien-5-yne. The overall reaction 116 −1 Psuch as (de)hydrogenated, ionized, and substituted PAHs are was computed to be exoergic by kJ mol . Alternatively, presumed to be omnipresent in the interstellar medium (ISM) intermediate [i1] can isomerize to the cyclic structure [i4]. This (1, 2). PAH-like species are suggested to account for up to 30% molecule represents a singly hydrogenated benzene molecule and of the galactic interstellar carbon (2), have been implicated in the can be formed from [i1] via an initial ring closure to [i2] followed astrobiological evolution of the ISM (3), and provide nucleation by a hydrogen shift or through an initial hydrogen shift forming sites for the formation of carbonaceous dust particles (4). They [i3] followed by cyclization to [i4]. A comparison of the height have been also linked to the unidentified infrared (UIR) emission of transition states involved in the initial steps of the reaction se- bands observed in the range of 3–14 μm(3;300–700 cm−1) (5) and quence ½i1 → ½i2 → ½i4 versus ½i1 → ½i3 → ½i4 suggests that [i1] to the diffuse interstellar bands (DIBs) (6), discrete absorption preferentially undergoes ring closure followed by hydrogen features superimposed on the interstellar extinction curve ran- migration. Which of both pathways is the dominating route of ging from the blue part of the visible (400 nm) to the near-infra- benzene formation? Our statistical calculations reveal that, over red (1.2 μm). UIR bands have also been observed toward the a range of collision energies from 0 to 50 kJ mol−1, near 99% of Cigar Galaxy M82; DIBs are widespread in the ISM of the Local all the benzene molecules are formed through the reaction Group and into the distant universe (7). Current astrochemical sequence ½i1 → ½i2 → ½i4, whereas only 1% of the benzene mo- models propose that the synthesis of the simplest building block lecules are synthesized via the route involving ½i1 → ½i3 → ½i4. — ð 1 Þ — of PAHs the aromatic benzene molecule [C6H6 X A1g ] is Once formed, the cyclic intermediate [i4] emits a hydrogen atom driven by ion–molecule reactions (8) of methane (CH4), ethylene −1 þ þ via a tight exit transition state located 13 kJ mol above the (C2H4), and propargyl (C3H3) with C5H2 and C4H2 ions and þ separated products forming the aromatic benzene molecule; this involves C6H ions of unknown structures. However, the validity 5 barrier correlates well with an experimentally determined activa- of these processes remains conjecture because they have neither 18 0 Æ 1 1 −1 been verified computationally nor experimentally. Therefore, the tion energy of . kJ mol for the reversed reaction of an addition of a hydrogen atom to benzene as determined over a formation mechanisms of the simplest building block of PAH – species in interstellar space—the aromatic benzene molecule as temperature range of 298 400 K (10). Our calculations suggest detected toward the planetary nebula CRL 618 (9)—have remained elusive to date. Author contributions: R.I.K., A.M.M., and S.B.C. designed research; B.M.J., F.Z., A.J., and In this article, we present the results of crossed molecular beam M.A.C. performed research; B.M.J., R.I.K., A.J., A.M.M., M.A.C., and S.B.C. analyzed data; 2 þ reactions of D1-substituted ethynyl radicals [C2D; X Σ ]with1,3- and B.M.J. and R.I.K. wrote the paper. 1 butadiene [H2CCHCHCH2;X Ag] and its D2- and D4-substituted The authors declare no conflict of interest. counterparts, i.e., [H2CCDCDCH2]and[D2CCHCHCD2]. By *This Direct Submission article had a prearranged editor. combining these data with electronic structure calculations and 1To whom correspondence should be addressed. E-mail: [email protected]. astrochemical models, we provide compelling evidence that the This article contains supporting information online at www.pnas.org/lookup/suppl/ ð 1 Þ aromatic benzene molecule [C6H6 X A1g ] together with its acyc- doi:10.1073/pnas.1012468108/-/DCSupplemental. 452–457 ∣ PNAS ∣ January 11, 2011 ∣ vol. 108 ∣ no. 2 www.pnas.org/cgi/doi/10.1073/pnas.1012468108 Downloaded by guest on September 25, 2021 characterized the terminal carbon atoms of 1,3-butadiene releas- ing the hydrogen atom, we are focusing our attention now on the identification of the structural isomer(s) formed. The identification procedure of the isomers requires elucidating the chemical dynamics of the reaction by transforming the experimental data from the laboratory to the center-of-mass reference frame (11). The simulated distributions are overlaid in Fig. 2 with the corre- sponding center-of-mass functions visualized in Fig. 3. Let us turn our attention first to the derived center-of-mass translational energy distribution, PðET Þ. For those molecules formed without internal excitation, the high-energy cutoff of the PðET Þ resembles the sum of the absolute of the reaction exoergicity and the collision energy; this algebraic quantity is clearly dictated by the law of energy conservation. An adequate simulation of the laboratory data could not be achieved with only a single channel leading ex- clusively to the 1,3-hexadien-5-yne isomer or benzene. With only one channel pertaining to the acyclic isomer, the simulated TOF spectra were too slow, and the laboratory angular distribution was found to be too narrow. On the other hand, a one-channel fit accounting solely for the reaction energy to form the D1-benzene molecule yielded TOF spectra that were too fast and a laboratory Fig. 1. Potential energy surface for the reaction of ground state ethynyl angular distribution that was significantly broader than the data. ð 2ΣÞþ ð 1 Þ radicals [C2H X ] with 1,3-butadiene [CH2CHCHCH2 X Ag ]. Relative However, we could successfully replicate the experimental data energies are given in units of kilojoules per mole. Energies in parenthesis by utilizing a two-channel fit with the center-of-mass functions refer to the energetics of the reaction with the D1-ethynyl radicals. Also depicted in Fig. 3. Let us have a closer look at the PðET Þs. It indicated are electronic wave functions and point groups of the reactants, is important to note that the high-energy cutoffs of 150 Æ 20 and intermediates, and transition states. Red and blue lines indicate the reaction 380 Æ 20 −1 pathways to the acyclic and benzene isomers, respectively. Optimized Carte- kJ mol are in excellent agreement with the computed sian coordinates for all structures are given in the SI Text.

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