2023ApJ...945L..40M

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).