Polycyclic aromatic hydrocarbons (PAHs) are fundamental molecular building blocks of fullerenes and carbonaceous nanostructures in the interstellar medium and in combustion systems. However, an understanding of the formation of aromatic molecules carrying five-membered rings—the essential building block of nonplanar PAHs—is still in its infancy. Exploiting crossed molecular beam experiments augmented by electronic structure calculations and astrochemical modeling, we reveal an unusual pathway leading to the formation of indene (C9H8)—the prototype aromatic molecule with a five-membered ring—via a barrierless bimolecular reaction involving the simplest organic radical—methylidyne (CH)—and styrene (C6H5C2H3) through the hitherto elusive methylidyne addition–cyclization–aromatization (MACA) mechanism. Through extensive structural reorganization of the carbon backbone, the incorporation of a five-membered ring may eventually lead to three-dimensional PAHs such as corannulene (C20H10) along with fullerenes (C60, C70), thus offering a new concept on the low-temperature chemistry of carbon in our galaxy.

The gas-phase reaction of the methylidyne (CH; X2Π) radical with dimethylacetylene (CH3CCCH3; X1A1g) was studied at a collision energy of 20.6 kJ mol-1 under single collision conditions with experimental results...

Complex organosulfur molecules are ubiquitous in interstellar molecular clouds, but their fundamental formation mechanisms have remained largely elusive. These processes are of critical importance in initiating a series of elementary chemical reactions, leading eventually to organosulfur molecules—among them potential precursors to iron-sulfide grains and to astrobiologically important molecules, such as the amino acid cysteine. Here, we reveal through laboratory experiments, electronic-structure theory, quasi-classical trajectory studies, and astrochemical modeling that the organosulfur chemistry can be initiated in star-forming regions via the elementary gas-phase reaction of methylidyne radicals with hydrogen sulfide, leading to thioformaldehyde (H2CS) and its thiohydroxycarbene isomer (HCSH). The facile route to two of the simplest organosulfur molecules via a single-collision event affords persuasive evidence for a likely source of organosulfur molecules in star-forming regions. These fundamental reaction mechanisms are valuable to facilitate an understanding of the origin and evolution of the molecular universe and, in particular, of sulfur in our Galaxy.

The bimolecular gas-phase reaction of the methylidyne radical (CH; X 2 Π) with 1,2-butadiene (CH 2 CCHCH 3 ; X 1 A′) was investigated at a collision energy of 20.6 kJ mol −1 under single collision conditions. Combining our laboratory data with high-level electronic structure calculations, we reveal that this bimolecular reaction proceeds through the barrierless addition of the methylidyne radical to the carbon−carbon double bonds of 1,2-butadiene leading to doublet C 5 H 7 intermediates. These collision adducts undergo a nonstatistical unimolecular decomposition through atomic hydrogen elimination to at least the cyclic 1-vinyl-cyclopropene (p5/p26), 1-methyl-3-methylenecyclopropene (p28), and 1,2-bis(methylene)cyclopropane (p29) in overall exoergic reactions. The barrierless nature of this bimolecular reaction suggests that these cyclic C 5 H 6 isomers might be viable targets to be searched for in cold molecular clouds like TMC-1.

Fullerenes (C60, C70) detected in planetary nebulae and carbonaceous chondrites have been implicated to play a key role in the astrochemical evolution of the interstellar medium. However, the formation mechanism...

Organosilicon molecules such as silicon carbide (SiC), silicon dicarbide (c-SiC2), silicon tricarbide (c-SiC3), and silicon tetracarbide (SiC4) represent basic molecular building blocks connected to the growth of silicon-carbide dust grains in the outflow of circumstellar envelopes of carbon-rich asymptotic giant branch (AGB) stars. Yet, the fundamental mechanisms of the formation of silicon carbides and of the early processes that initiate the coupling of silicon–carbon bonds in circumstellar envelopes have remained obscure. Here, we reveal in a crossed molecular beam experiment contemplated with ab initio electronic calculations that the astronomically elusive 1-ethynyl-3-silacyclopropenylidene molecule (c-SiC4H2, Cs, X1A′) can be synthesized via a single-collision event through the barrierless reaction of the silylidyne radical (SiH) with diacetylene (C4H2). This system represents a benchmark of a previously overlooked class of reactions, in which the silicon–carbon bond coupling can be initiated by a barrierless and overall exoergic reaction between the simplest silicon-bearing radical (silylidyne) and a highly hydrogen-deficient hydrocarbon (diacetylene) in the inner circumstellar envelopes of evolved carbon-rich stars such as IRC+10216. Considering that organosilicon molecules like 1-ethynyl-3-silacyclopropenylidene might be ultimately photolyzed to bare carbon–silicon clusters like the linear silicon tetracarbide (SiC4), hydrogenated silicon–carbon clusters might represent the missing link eventually connecting simple molecular precursors such as silane (SiH4) to the population of silicon-carbide based interstellar grains ejected from carbon-rich AGB stars into the interstellar medium.

The gas-phase bimolecular reaction of the methylidyne (CH; X2Π) radical with vinylacetylene (H2CCHCCH; X1A’) was conducted at a collision energy of 20.3 kJ mol-1 under single collision conditions exploiting the...

The crossed molecular beams reactions of the 1-propynyl radical (CH3CC; X2A1) with benzene (C6H6; X1A1g) and D6-benzene (C6D6; X1A1g) were conducted to explore the formation of C9H8 isomers under single-collision conditions. The underlying reaction mechanisms were unravelled through the combination of the experimental data with electronic structure and statistical RRKM calculations. These data suggest the formation of 1-phenyl-1-propyne (C6H5CCCH3) via the barrierless addition of 1-propynyl to benzene forming a low-lying doublet C9H9 intermediate that dissociates by hydrogen atom emission via a tight transition state. In accordance with our experiments, RRKM calculations predict that the thermodynamically most stable isomer – the polycyclic aromatic hydrocarbon (PAH) indene – is not formed via this reaction. With all barriers lying below the energy of the reactants, this reaction is viable in the cold interstellar medium where several methyl-substituted molecules have been detected. Its underlying mechanism therefore advances our understanding of how methyl-substituted hydrocarbons can be formed under extreme conditions such as those found in the molecular cloud TMC-1. Implications for the chemistry of the 1-propynyl radical in astrophysical environments are also discussed.