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.

Crossed molecular beams experiments on dicarbon molecules, C2(X1sigmag+/a3piu), with unsaturated hydrocarbons acetylene (C2H2(X1sigmag+), ethylene (C2H4(X1Ag)), methylacetylene (CH3CCH(X1A1)), and allene (H2CCCH2 (X1A1)) were carried out at 18 collision energies between 10.6 and 50.3 kJ mol(-1) utilizing a universal crossed beams machine to untangle the reaction dynamics forming hydrogen deficient hydrocarbon radicals in circumstellar envelopes of carbons stars and in cold molecular clouds. We find that all reactions proceed without the entrance barrier through indirect (complex forming) scattering dynamics. Each bimolecular collision is initiated by an addition of the dicarbon molecule to the pi bond of the unsaturated hydrocarbon molecule yielding initially acyclic (triplet) and three- or four-membered cyclic collision complexes (triplet and singlet surface). On the singlet surface, the cyclic structures isomerize to form eventually diacetylene (HCCCCH; C2/C2H2), butatriene (H2CCCCH2; C2/C2H4), methyldiacetylene (CH3CCCCH; C2/CH3CCH), and pentatetraene (H2CCCCCH2; C2/H2CCCH2) intermediates. The latter were found to decompose via atomic hydrogen loss yielding the buta-1,3-diynyl [C4H(X2sigma+) HCCCC], 1-butene-3-yne-2-yl [i-C4H3(X2A') H2CCCCH], penta-2,4-diynyl-1 [C5H3(X2B1) HCCCCCH2], and penta-1,4-diynyl-3 radical [C5H3(X2B1) HCCCHCCH] under single collision conditions. The underlying characteristics of these dicarbon versus atomic hydrogen replacement pathways (indirect scattering dynamics; no entrance barrier; isomerization barriers below the energy of the separated reactants; exoergic reactions) suggest the enormous potential of the dicarbon plus unsaturated hydrocarbon reaction class to form highly hydrogen-deficient carbonaceous molecules in cold molecular clouds and in circumstellar envelopes of carbon stars. The studies therefore present an important advancement in establishing a comprehensive database of reaction intermediates and products involved in bimolecular collisions of dicarbon molecules with unsaturated hydrocarbons which can be utilized in refined astrochemical models and also in future searches of hitherto unidentified interstellar molecules. Implications of these experiments to understand related combustion processes are also addressed.

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The aromatic indene molecule (C 9 H 8 ) together with its acyclic isomers (phenylallene, 1-phenyl-1-propyne, and 3-phenyl-1-propyne) were formed via a 'directed synthesis' in situ utilizing a high temperature chemical reactor under combustion-like conditions (300 Torr, 1,200-1,500 K) through the reactions of the phenyl radical (C 6 H 5 ) with propyne (CH 3 CCH) and allene (H 2 CCCH 2 ). The isomer distributions were probed utilizing tunable vacuum ultraviolet (VUV) radiation from the Advanced Light Source by recording the photoionization efficiency (PIE) curves at mass-to-charge of m/z = 116 (C 9 H 8 + ) of the products in a supersonic expansion for both the phenyl-allene and phenyl-propyne systems; branching ratios were derived by fitting the recorded PIE curves with a linear combination of the PIE curves of the individual C 9 H 8 isomers. Our data suggest that under our experimental conditions, the formation of the aromatic indene molecule via the reaction of the phenyl radical with allene is facile and enhanced compared to the phenyl -propyne system by a factor of about seven. Reaction mechanisms and branching ratios are explained in terms of new electronic structure calculations.Our newly developed high temperature chemical reactor presents a versatile approach to study the formation of combustion-relevant polycyclic aromatic hydrocarbons (PAHs) under welldefined and controlled conditions.

We investigated the multichannel reaction of ground-state carbon atoms with acetylene, C2H2 (X1Sigmag+), to form the linear and cyclic C3H isomers (atomic hydrogen elimination pathway) as well as tricarbon plus molecular hydrogen. The experiments were conducted under single-collision conditions at three different collision energies between 8.0 kJ mol-1 and 31.0 kJ mol-1. Our studies were complemented by crossed molecular beam experiments of carbon with three isotopomers C2D2(X1Sigmag+), C2HD (X1Sigma+), and 13C2H2 (X1Sigmag+) to clarify a potential intersystem crossing (ISC), the effect of the symmetry of the reaction intermediates on the center-of-mass angular distributions, the collision energy-dependent branching ratios of the atomic versus molecular hydrogen elimination pathways, and deuterium-enrichment processes. The results are discussed in light of recent electronic structure and dynamics calculations.

We present a joint crossed molecular beam and kinetics investigation combined with electronic structure and statistical calculations on the reaction of the ground-state cyano radical, CN(X 2 Σ +), with the 1,3-butadiene molecule, H 2 CCHCHCH 2 (X 1 A g), and its partially deuterated counterparts, H 2 CCDCDCH 2 (X 1 A g) and D 2 CCHCHCD 2 (X 1 A g). The crossed beam studies indicate that the reaction proceeds via a long-lived C 5 H 6 N complex, yielding C 5 H 5 N isomer(s) plus atomic hydrogen under single collision conditions as the nascent product(s). Experiments with the partially deuterated 1,3-butadienes indicate that the atomic hydrogen loss originates from one of the terminal carbon atoms of 1,3-butadiene. A combination of the experimental data with electronic structure calculations suggests that the thermodynamically less favorable 1-cyano-1,3-butadiene isomer represents the dominant reaction product; possible minor contributions of less than a few percent from the aromatic pyridine molecule might be feasible. Low-temperature kinetics studies demonstrate that the overall reaction is very fast from room temperature down to 23 K with rate coefficients close to the gas kinetic limit. This finding, combined with theoretical calculations, indicates that the reaction proceeds on an entrance barrier-less potential energy surface (PES). This combined experimental and theoretical approach represents an important step toward a systematic understanding of the formation of complex, nitrogen-bearing molecules-here on the C 5 H 6 N PES-in low-temperature extraterrestrial environments. These results are compared to the reaction dynamics of D1-ethynyl radicals (C 2 D; X 2 Σ +) with 1,3-butadiene accessing the isoelectronic C 6 H 7 surface as tackled earlier in our laboratories.

The crossed beams reaction of ground state ethynyl radicals, C(2)H(X(2)Sigma(+)), with allene, H(2)CCCH(2)(X(1)A(1)), was conducted under single collision conditions at a collision energy of 22.0 +/- 0.4 kJ mol(-1). The center-of-mass functions were combined with earlier ab initio calculations and revealed that the reaction was barrier-less, proceeded via indirect reaction dynamics through an addition of the ethynyl radical to the terminal carbon atom of the allene molecule, and was terminated by atomic hydrogen emission via a tight exit transition state to form the ethynylallene product. The overall reaction was found to be exoergic by 93 +/- 15 kJ mol(-1). Since the reaction is barrier-less, exoergic, and all transition states involved are located below the energy level of the separated reactants, the formation of ethynylallene is predicted to take place in low temperature atmospheres of planets and their satellites such as Titan and also in cold molecular clouds via the neutral-neutral reaction of ethynyl radicals with allene. Implications to interstellar chemistry and a comparison with the chemistry of the isoelectronic cyano radical, CN(X(2)Sigma(+)), are also presented.

Polycyclic aromatic hydrocarbons (PAHs) are regarded as key intermediates in the molecular growth process that forms soot from incomplete fossil fuel combustion. Although heavily researched, the reaction mechanisms for PAH formation have only been investigated through bulk experiments; therefore, current models remain conjectural. We report the first observation of a directed synthesis of a PAH under single-collision conditions. By using a crossed-molecular-beam apparatus, phenyl radicals react with C(3)H(4) isomers, methylacetylene and allene, to form indene at collision energies of 45 kJ mol(-1). The reaction dynamics supported by theoretical calculations show that both isomers decay through the same collision complex, are indirect, have long lifetimes, and form indene in high yields. Through the use of deuterium-substituted reactants, we were able to identify the reaction pathway to indene.