Gas-Phase Formation of Silicon Dicarbide (c-SiC ₂ ): A Key Cyclic Precursor to Silicon Carbide Grains in Space

Complex organosilicon molecules are pervasive in interstellar environments, yet the mechanisms governing their formation─particularly the initial silicon–carbon bond coupling─remain largely unexplored. Such processes initiate reaction networks that generate precursors to silicon carbide (SiC) dust grains, which enable galactic-cosmic-ray–driven synthesis of complex organic molecules, including amino acids and sugars, in deep space. Here, we combine crossed molecular beam experiments with high-level computations to show that a postulated precursor to SiC grains─cyclic silicon dicarbide (c-SiC 2 )─forms via reaction of the simplest silicon-bearing radical, silylidyne (SiH), with dicarbon (C 2 ). This barrierless, exoergic reaction links simple silicon- and carbon-bearing molecular reservoirs, initiating molecular mass growth even under the low-temperature conditions of dense molecular clouds such as G+0.693–0.027, where c-SiC 2 has recently been detected. Organosilicon species such as silicon dicarbide thus drive exoergic reaction networks culminating in SiC grain formation, bridging the gap between simple molecules and interstellar SiC dust.