Medvedkov Iakov A, Nikolayev Anatoliy A, Goettl Shane J, Yang Zhenghai, Mebel Alexander M, Kaiser Ralf I
Department of Chemistry, University of Hawai'i at Manoa Honolulu HI 96822 USA
Samara National Research University Samara 443086 Russia.
Chem Sci. 2025 Sep 1. doi: 10.1039/d5sc04699h.
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 CH isomers is explored through the bimolecular reaction of tricarbon (C, XΣ ) with the vinyl radical (CH, XA') at a collision energy of 44 ± 1 kJ mol 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 symmetric doublet collisional complex (CCCCHCH). Subsequent low-barrier isomerization steps culminate in the resonantly stabilized 2,4-pentadiynyl-1 radical (CHCCCCH), which decomposes atomic hydrogen loss. Statistical calculations identify linear, triplet pentadiynylidene (p2, XΣ ) as the dominant product, while singlet carbenes ethynylcyclopropenylidene (p1, XA'), pentatetraenylidene (p3, XA), and ethynylpropadienylidene (p4, XA') are formed with lower branching ratios. The least stable isomer, 2-cyclopropen-1-ylidenethenylidene ('eiffelene'; p5, XA), 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) is only traceable 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.
通过将实验室动力学与宇宙可观测物联系起来，这项工作突出了高反应性物种之间的反应在塑造星际介质分子清单方面的关键作用，并为进入天体化学复杂性中光谱难以捉摸的角落打开了新窗口。利用交叉分子束技术，并结合电子结构和赖斯-拉姆斯佩格-卡塞尔-马库斯（RRKM）计算，在44±1 kJ mol的碰撞能量下，通过三碳（C，XΣ ）与乙烯基自由基（CH，XA'）的双分子反应，探索了不同CH异构体的气相形成。这个无势垒且放能的反应遵循间接动力学，由三碳加到乙烯基自由基的自由基中心引发，形成一个对称的双重态碰撞复合物（CCCCHCH）。随后的低势垒异构化步骤最终形成共振稳定的2,4-戊二炔基-1自由基（CHCCCCH），它通过原子氢损失而分解。统计计算确定线性三重态戊二炔叉（p2，XΣ ）为主要产物，而单线态卡宾乙炔基环丙烯叉（p1，XA'）、戊四烯叉（p3，XA）和乙炔基丙二烯叉（p4，XA'）以较低的分支比形成。最不稳定的异构体2-环丙烯-1-亚乙烯基（“埃菲尔烯”；p5，XA）在热力学上仍然可行，但分支比可忽略不计。目前在TMC-1中检测到的两种异构体（p1和p3）具有显著的偶极矩，使其适合射电望远镜观测，而线性戊二炔叉（p2；D）只能通过红外光谱或通过其氰基戊二炔叉衍生物（HCCCCCCN）来追踪。这项研究突出了在涉及冷分子云中两种不稳定反应物的无势垒且放能的双分子反应的低温烃类化学中可获得的异构体多样性。
