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)

Abstract

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, Cs, 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.

Crossed Beam Experiments and Computational Studies of Pathways to the Preparation of Singlet Ethynylsilylene (HCCSiH; X1A′): The Silacarbene Counterpart of Triplet Propargylene (HCCCH; X3B)

“…The astronomical detection of silicon carbides (SiC, c-SiC 2 , c-SiC 3 , and SiC 4 ) 33 along with silane (SiH 4 ) 34 and methylsilane (CH 3 SiH 3 ) 35 toward IRC+10216 documents a striking gas-phase chemistry, whose initial silicon−carbon bond formation mechanisms are only beginning to emerge. 36 The prospective identification of hydrogenated silicon−carbon clusters such as singlet ethynylsilylene via rotational spectroscopy exploiting the Atacama Large Millimeter/submillimeter Array would exploit circumstellar envelopes as natural laboratories for an exotic silicon− carbon chemistry under extreme conditions. This would provide an opportunity to identify fundamental elementary reactions leading to silicon−carbon bond formation in deep space by guiding astronomical observations with both laboratory experiments under single-collision conditions and theoretical investigations.…”

“…The facile preparation and identification of gas-phase singlet ethynylsilylene (HCCSiH; p3 ; 1 A′) also has significant implications for the chemistry in circumstellar environments of carbon-rich asymptotic giant branch (AGB) stars such as IRC+10216. The astronomical detection of silicon carbides (SiC, c-SiC 2 , c-SiC 3 , and SiC 4 ) along with silane (SiH 4 ) and methylsilane (CH 3 SiH 3 ) toward IRC+10216 documents a striking gas-phase chemistry, whose initial silicon–carbon bond formation mechanisms are only beginning to emerge . The prospective identification of hydrogenated silicon–carbon clusters such as singlet ethynylsilylene via rotational spectroscopy exploiting the Atacama Large Millimeter/submillimeter Array would exploit circumstellar envelopes as natural laboratories for an exotic silicon–carbon chemistry under extreme conditions.…”

The Formation of Monosubstituted Cyclopropenylidene Derivatives in the Interstellar Medium via Neutral–Neutral Reaction Pathways

“…Following the astrophysical detection of c-C 3 HC 2 H (Cernicharo et al 2021), a mechanistic study of its formation found that c-C 3 HC 2 H may form through a gas-phase reaction between c-C 3 H 2 and the ethynyl radical (Fortenberry 2021). These results open a veritable Pandora's box for further astrochemical analysis regarding the formation of c-C 3 H 2 and its family of derivatives (He et al 2022;Yang et al 2021).…”

Chemical Bonding Perspective on Low-Lying SiC4H2 Isomers: Conceptual Quantum Chemical Views

“…35,36 Owing to this reason, the chemistry of organosilicon compounds is considered a fundamental problem in astrochemistry. 34,37,38 In the present work, we have explored the chemical bonding nature of seven low-lying isomers of SiC 4 H 2 , which are 1ethynyl-3-silacycloprop-1(2)-en-3-ylidene (1), diethynylsilylidene (2), 2-methylenesilabicyclo[1.1.0]but-1(3)-en-4-ylidene (3), 1-sila-1,2,3,4-pentatetraenylidene (4), 1,3-butadiynylsilylidene (5), 4-sila-2-methylenebicyclo[1.1.0]but-1(3)-en-4-ylidene (6), and 3-ethynyl-1-silapropadienylidene (7), as depicted with Wiberg bond indices (WBI) in Figure 1 and listed in Table S1. In 1998, Maier and co-workers had trapped 1-ethynyl-3 silacycloprop-1(2)-en-3-ylidene (1) and diethynyl-silylidene (2) experimentally in an Ar matrix at 10 K by flash pyrolysis of triethynylsilane [HSi(C 2 H) 3 ] and 1,1-diethynyl 2,2,2-trimethyldisilane [Me 3 Si−SiH(CCH) 2 ].…”