ON THE FORMATION OF SILACYCLOPROPENYLIDENE (c-SiC sub(2)H sub(2)) AND ITS ROLE IN THE ORGANOSILICON CHEMISTRY IN THE INTERSTELLAR MEDIUM

• Published in: The Astrophysical journal 2013-06, Vol.770 (1), p.1-14
• Main Authors: Parker, Dorian S N, Wilson, Anthony V, Kaiser, Ralf I, Mayhall, Nicholas J, Head-Gordon, Martin, TIELENS, ALEXANDER G G M
• Format: Article
• Language: English
• Subjects: Asymptotic properties; Chemical evolution; Formations; Grains; Interstellar matter; Radicals; Silicon; Stellar envelopes
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1088/0004-637X/770/1/33

Organosilicon species such as silicon carbide and silicon dicarbide are considered as key molecular building blocks in the chemical evolution of the interstellar medium and are associated with the formation of silicon-carbide dust grains in the outflow of circumstellar envelopes of carbon-rich asymptotic giant branch (AGB) stars. However, the formation mechanisms of even the simplest silicon-bearing organic molecules have remained elusive for decades. Here, we demonstrate in crossed molecular beam experiments combined with ab initio calculations that the silacyclopropenylidene molecule (c-SiC sub(2)H sub(2)) can be synthesized in the gas phase under single-collision conditions via the reaction of the silylidyne radical (SiH) with acetylene (C sub(2)H sub(2)). This system denotes the simplest representative of a previously overlooked reaction class, in which the formation of an organosilicon molecule can be initiated via barrierless and exoergic reactions of silylidyne radicals with hydrocarbon molecules in circumstellar envelopes of evolved carbon stars such as IRC+10216. Since organosilicon molecules like silacyclopropenylidene can be eventually photolyzed to carbon-silicon clusters such as silicon dicarbide (c-SiC sub(2)), silacyclopropenylidene might even represent the missing link between simple molecular precursors and silicon-carbide-rich interstellar grains.