Formation of the Elusive Silylenemethyl Radical (HCSiH 2 ; X 2 B 2 ) via the Unimolecular Decomposition of Triplet Silaethylene (H 2 CSiH 2 ; a 3 A″).

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 3 P electronic ground state with silane (SiH 4 ; X 1 A 1 ) 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 (HCSiH 2 ; X 2 B 2 ) via the unimolecular decomposition of triplet silaethylene (H 2 CSiH 2 ; a 3 A″). The silaethylene radical was formed via hydrogen migration of the triplet silylmethylene (HCSiH 3 ; X 3 A″) 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 (CH 3 Si; X 2 A″) and methylenesilyl (CH 2 SiH; X 2 A') 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.