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).

Fragmentation dynamics of the doubly charged aniline: the source of kinetically excited CnH3+ ions

“…32 Interestingly, it has been observed that C 2 H 3 and C 3 H 3 are noteworthy precursors in the synthesis of the cyclopentadienyl radical, 3 which has the potential to engage in interactions with methyl radicals, leading to the expansion of benzene and ultimately culminating in the synthesis of polycyclic aromatic hydrocarbons (PAHs) inside circumstellar environments. 3,33 The present study has shown that aniline dication fragments are C n H 3 + ( n = 1, 2, and 3) and that they can participate in the ion–molecule process in the synthesis of larger molecules due to their significant range of kinetic energies if aniline is present in the ISM.…”

Exploring the Chemical Dynamics of Phenylethynyl Radical (C6H5CC; X2A1) Reactions with Allene (H2CCCH2; X1A1) and Methylacetylene (CH3CCH; X1A1)

“…The formation mechanisms of polycyclic aromatic hydrocarbons (PAHs) along with their unsaturated precursors have received considerable attention by the astrochemistry and combustion science communities. − Here, PAHs are classified as reaction intermediates and fundamental molecular building blocks in molecular mass growth processes leading ultimately to soot particles (combustion flames) and carbonaceous nanoparticles (circumstellar and interstellar grains). , Particular interest has been devoted to the propargyl radical (H 2 CCCH, X 2 B 1 ), which represents a prototype of a resonantly stabilized free radical (RSFR) and the most thermodynamically stable C 3 H 3 isomer . Recently detected in the cold Taurus Molecular Cloud (TMC-1), bimolecular propargyl–propargyl radical reactions lead to the formation of the aromatic phenyl radical (C 6 H 5 ), while a stabilization of the reaction intermediate(s) accesses benzene (C 6 H 6 ) along with its 1,5-hexadiyne, fulvene, and 2-ethynyl-1,3-butadiene isomers. − Consequently, the propargyl radical plays a major role in astrochemical − and combustion − models as a potential precursor for bottom-up synthetic pathways to PAHs and carbonaceous nanoparticles (soot, interstellar grains). However, reactions of the propargyl radical with closed-shell hydrocarbons, e.g., acetylene (C 2 H 2 ) and benzene (C 6 H 6 ), involve entrance barriers to addition typically in the range of 50–60 kJ mol –1 . − These entrance barriers limit propargyl radical reactions with closed-shell hydrocarbons to high-temperature environments like circumstellar envelopes of carbon stars and planetary nebulae as their descendants.…”

“…However, reactions of the propargyl radical with closed-shell hydrocarbons, e.g., acetylene (C 2 H 2 ) and benzene (C 6 H 6 ), involve entrance barriers to addition typically in the range of 50–60 kJ mol –1 . − These entrance barriers limit propargyl radical reactions with closed-shell hydrocarbons to high-temperature environments like circumstellar envelopes of carbon stars and planetary nebulae as their descendants. So far, the reaction of tricarbon (C 3 )formally a carbenewith the propargyl radical represents the only barrierless and exoergic pathway of the propargyl radical with a technically closed-shell ‘organic’ reactant leading to triacetylene (HCCCCCCH) and its high-energy isomer ethynylbutatrienylidene (HCCCHCC). , …”

“…So far, the reaction of tricarbon (C 3 )�formally a carbene�with the propargyl radical represents the only barrierless and exoergic pathway of the propargyl radical with a technically closed-shell 'organic' reactant leading to triacetylene (HCCCCCCH) and its high-energy isomer ethynylbutatrienylidene (HCCCHCC). 11,22 On the other hand, the 1-propynyl radical (CH 3 CC, X 2 A 1 )�a non-resonant free-radical isomer of C 3 H 3 (Figure 1)�lies 168 kJ mol −1 higher in energy than propargyl 6 and has recently been demonstrated to add without an entrance barrier to the closed-shell hydrocarbons acetylene, 23 ethylene ( C 2 H 4 ) , 2 4 m e t h y l a c e t y l e n e ( C H 3 C C H ) , a l l e n e (H 2 CCCH 2 ), 25 diacetylene (C 4 H 2 ), 26 1,3-butadiene (C 4 H 6 ), 27 and benzene (C 6 H 6 ). 28 The barrierless nature of bimolecular encounters in these systems implies that the 1propynyl radical�in strong contrast to the propargyl radical isomer�can initiate reactions forming highly unsaturated hydrocarbons�among them polyacetylenes such as methyldiacetylene 23,29 and methyltriacetylene, 26 as well as aromatics like toluene 27 and 1-phenyl-1-propyne 28 �in low-temperature environments like cold molecular clouds.…”