2021ApJ...908L..40Y

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, C_s, 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.