Gas-Phase Synthesis of the Elusive Cyclooctatetraenyl Radical (C8 H7 ) via Triplet Aromatic Cyclooctatetraene (C8 H8 ) and Non-Aromatic Cyclooctatriene (C8 H8 ) Intermediates

Formation of the Elusive Silylenemethyl Radical (HCSiH2; X2B2) via the Unimolecular Decomposition of Triplet Silaethylene (H2CSiH2; a3A″)

We investigated the formation of small organosilicon molecules─potential precursors to silicon-carbide dust grains ejected by dying carbon-rich asymptotic giant branch stars─in the gas phase via the reaction of atomic carbon (C) in its ³P electronic ground state with silane (SiH₄; X¹A₁) using the crossed molecular beams technique. The reactants collided under single collision conditions at a collision energy of 13.0 ± 0.2 kJ mol^-1, leading to the formation of the silylenemethyl radical (HCSiH₂; X²B₂) via the unimolecular decomposition of triplet silaethylene (H₂CSiH₂; a³A″). The silaethylene radical was formed via hydrogen migration of the triplet silylmethylene (HCSiH₃; X³A″) radical, which in turn was identified as the initial collision complex accessed via the barrierless insertion of atomic carbon into the silicon-hydrogen bond of silane. Our results mark the first observation of the silylenemethyl radical, where previously only its thermodynamically more stable methylsilylidyne (CH₃Si; X²A″) and methylenesilyl (CH₂SiH; X²A') isomers were observed in low-temperature matrices. Considering the abundance of silane and the availability of atomic carbon in carbon-rich circumstellar environments, our results suggest that future astrochemical models should be updated to include contributions from small saturated organosilicon molecules as potential precursors to pure gaseous silicon-carbides and ultimately to silicon-carbide dust.

Gas-Phase Formation of C5H6 Isomers via the Crossed Molecular Beam Reaction of the Methylidyne Radical (CH; X2Π) with 1,2-Butadiene (CH3CHCCH2; X1A')

The bimolecular gas-phase reaction of the methylidyne radical (CH; X²Π) with 1,2-butadiene (CH₂CCHCH₃; X¹A') 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 C₅H₇ 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 C₅H₆ isomers might be viable targets to be searched for in cold molecular clouds like TMC-1.