Organosilicon molecules are ubiquitous in the circumstellar envelope of carbon stars such as the Asymptotic Giant Branch (AGB) star IRC+10216. The formation of such species is a critical step in silicon carbide grain formation. In spite of this astrochemical relevance, the mechanisms of formation of these organosilicon molecules have largely remained elusive until now. Here we provide an ab initio computational study to buttress experimental findings concerning the synthesis of bicyclic silicon tricarbide (c-SiC3), one such organosilicon molecule observed in the circumstellar envelope of IRC+10216. To this end, we have modeled a two-part reaction network beginning with the shock-driven reaction in the inner envelope of the star. Electronically excited silicon atoms [Si(1D)] react with methylacetylene (CH3CCH) and allene (H2CCCH2) to form SiC3H2 species via molecular hydrogen (H2) loss. Subsequently we modeled the photochemical dehydrogenation of the parent SiC3H2 species to form c-SiC3, a process that would occur in the outer circumstellar envelope. To build this reaction network we utilized density functional theory (DFT) to compute structures of the reactants, products, intermediates, and transition states and refined the energetics with coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)]. This process beginning from a neutral-neutral reaction to form a partially hydrogenated intermediate followed by photodissociation to form a dehydrogenated silicon-carbon product represents a shift in the currently accepted astrochemical views and provides a new origin for circumstellar organosilicon molecules that can be synthesized around carbon-rich stars.

Computational study of the gas phase formation of c-SiC3 molecules in the circumstellar envelope of carbon stars

2019 Spring National Meeting

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