On the Synthesis of the Astronomically Elusive 1-Ethynyl-3-Silacyclopropenylidene (c-SiC₄H₂) Molecule in Circumstellar Envelopes of Carbon-rich Asymptotic Giant Branch Stars and Its Potential Role in the Formation of the Silicon Tetracarbide Chain (SiC₄)

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.