ON THE FORMATION OF SILACYCLOPROPENYLIDENE (c-SiC₂H₂) AND ITS ROLE IN THE ORGANOSILICON CHEMISTRY IN THE INTERSTELLAR MEDIUM

Abstract

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 2 H 2 ) can be synthesized in the gas phase under single-collision conditions via the reaction of the silylidyne radical (SiH) with acetylene (C 2 H 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 2 ), silacyclopropenylidene might even represent the missing link between simple molecular precursors and silicon-carbide-rich interstellar grains.

Untangling the reaction dynamics of the silylidyne radical (SiH; X2Π) with acetylene (C2H2; X1Σg+)

“…Considering the collision energy of 25.3 ± 0.5 kJ mol -1 , we can therefore derive the reaction to be exoergic by -14.7 ± 8.5 kJ mol -1 . We also noticed that P(E T ) depicts a distribution maximum around 9.0 ± 3.0 kJ mol -1 indicating a loose exit transition state upon decomposition of the SiC 2 H 3 complex to the final products; [33] this is consistent with an earlier study of the D1-silylidyne -acetylene reaction [19]. Considering the CM angular distribution, T(θ), intensity is found to span over the complete angular range from 0° to 180°; this finding is indicative of indirect scattering dynamics involving the unimolecular decomposition of SiC 2 H 3 complex(es) [33].…”

“…Similarly, the isovalent cyano (CN) and silicon nitride (SiN) radicals revealed significant discrepancies in reactivity with unsaturated hydrocarbons such as acetylene (C 2 H 2 ) and ethylene (C 2 H 4 ), by bonding the carbon in the cyano (CN) and nitrogen in the silicon nitride (SiN) to the unsaturated hydrocarbons [12][13][14][15][16][17]. Considering the title reaction, the diverse chemical bonding of silicon analogous species is also evident, when comparing the silylidyne-acetylene (SiH-C 2 H 2 ) and methylidyne-acetylene (CH-C 2 H 2 ) potential energy surfaces (PESs) [18,19] as well as the cyclopropenylidene (c-CC 2 H 2 ) and silacyclopropenylidene (c-SiC 2 H 2 ) isomers in particular [20,21]. Here, cyclopropenylidene presents as a transition state, while silacyclopropenylidene has been found to be a global minimum.…”

“…Previously, Parker et al investigated the reaction of laser-ablation generated D1silylidyne radicals (SiD) with acetylene exploiting the crossed beam approach [19]. However, various aspects of this reaction have remained unanswered so far.…”

Probing the dust formation zone in IRC+10216 with the SMA

“…Presolar SiC grains found in meteorites are believed to have originated in carbon stars [3,4]. Chemical equilibrium calculations on the production of these molecules have been carried out in the last few decades [5] and also more recently [6,7].…”

Combined Experimental and Theoretical Study on the Formation of the Elusive 2-Methyl-1-silacycloprop-2-enylidene Molecule under Single Collision Conditions via Reactions of the Silylidyne Radical (SiH; X2Π) with Allene (H2CCCH2; X1A1) and D4-Allene (D2CCCD2; X1A1)

“…1 × 10 -10 ). Based on these considerations, Wakelam, 23 Ziurys, 15, 30 Millar, 2, 29 and Kaiser 37,38 et al…”