From the laboratory to space: unveiling isomeric diversity of C5H2 in the reaction of tricarbon (C3, X1Σg+) with the vinyl radical (C2H3, X2A′)

Abstract

By connecting laboratory dynamics with cosmic observables, this work highlights the critical role of reactions between highly reactive species in shaping the molecular inventory of the interstellar medium and opens new windows into the spectroscopically elusive corners of astrochemical complexity. The gas phase formation of distinct C₅H₂ isomers is explored through the bimolecular reaction of tricarbon (C₃, X¹Σ⁺_g) with the vinyl radical (C₂H₃, X²A′) at a collision energy of 44 ± 1 kJ mol⁻¹ employing the crossed molecular beam technique augmented by electronic structure and Rice–Ramsperger–Kassel–Marcus (RRKM) calculations. This barrierless and exoergic reaction follows indirect dynamics and is initiated by the addition of tricarbon to the radical center of the vinyl radical forming a C_s symmetric doublet collisional complex (CCCCHCH₂). Subsequent low-barrier isomerization steps culminate in the resonantly stabilized 2,4-pentadiynyl-1 radical (CHCCCCH₂), which decomposes via atomic hydrogen loss. Statistical calculations identify linear, triplet pentadiynylidene (p2, X³Σ⁻_g) as the dominant product, while singlet carbenes ethynylcyclopropenylidene (p1, X¹A′), pentatetraenylidene (p3, X¹A₁), and ethynylpropadienylidene (p4, X¹A′) are formed with lower branching ratios. The least stable isomer, 2-cyclopropen-1-ylidenethenylidene (‘eiffelene’; p5, X¹A₁), remains thermodynamically feasible, but exhibits negligible branching ratios. Two isomers detected in TMC-1 to date (p1 and p3) possess significant dipole moments making them amenable to radio telescopic observations, whereas linear pentadiynylidene (p2; D_∞h) is only traceable via infrared spectroscopy or through its cyanopentadiynylidene derivative (HCCCCCCN). This study highlights the isomer diversity accessed in the low temperature hydrocarbon chemistry of barrierless and exoergic bimolecular reactions involving two unstable, reactants in cold molecular clouds.