Elucidating the Formation of Ethynylbutatrienylidene (HCCCHCCC; X1A′) in the Taurus Molecular Cloud (TMC-1) via the Gas-phase Reaction of Tricarbon (C3) with the Propargyl Radical (C3H3)

Abstract

The recent astronomical detection of ethynylbutatrienylidene (HCCCHCCC)—a high-energy isomer of triacetylene (HCCCCCCH) and hexapentaenylidene (H2CCCCCC)—in TMC-1 puzzled the laboratory astrophysics community since proposed reaction pathways could not synthesize the ethynylbutatrienylidene (HCCCHCCC) under cold molecular cloud conditions. Exploiting a retrosynthesis coupled with electronic structure calculations and astrochemical modeling, we reveal that observed fractional abundance of ethynylbutatrienylidene (HCCCHCCC) of 1.3 ± 0.2 × 10−11 can be quantitatively replicated though the barrierless and exoergic reaction of tricarbon (C3) with the resonantly stabilized propargyl radical (C3H3) after a few 105 yr—typical ages of cold molecular clouds. Our study provides persuasive evidence that previously assumed “dead” reactants such as tricarbon (C3) and the propargyl radical (C3H3) provide fundamental molecular building blocks in molecular mass growth processes leading to exotic, high-energy isomers of hydrocarbons: ethynylbutatrienylidene (HCCCHCCC).