Gas-Phase Formation of Silicon Dicarbide (c-SiC2): 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-SiC2)─forms via reaction of the simplest silicon-bearing radical, silylidyne (SiH), with dicarbon (C2). 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-SiC2 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.