Abstract：
The fundamental reaction pathways to the simplest dialkylsubstituted aromatics-xylenes (C6 H4 (CH3 )2 )-in high-temperature combustion flames and in low-temperature extraterrestrial environments are still unknown, but critical to understand the chemistry and molecular mass growth processes in these extreme environments. Exploiting crossed molecular beam experiments augmented by state-of-the-art electronic structure and statistical calculations, this study uncovers a previously elusive, facile gas-phase synthesis of xylenes through an isomer-selective reaction of 1-propynyl (methylethynyl, CH3 CC) with 2-methyl-1,3-butadiene (isoprene, C5 H8 ). The reaction dynamics are driven by a barrierless addition of the radical to the diene moiety of 2-methyl-1,3-butadiene followed by extensive isomerization (hydrogen shifts, cyclization) prior to unimolecular decomposition accompanied by aromatization via atomic hydrogen loss. This overall exoergic reaction affords a preparation of xylenes not only in high-temperature environments such as in combustion flames and around circumstellar envelopes of carbon-rich Asymptotic Giant Branch (AGB) stars, but also in low-temperature cold molecular clouds (10 K) and in hydrocarbon-rich atmospheres of planets and their moons such as Triton and Titan. Our study established a hitherto unknown gas-phase route to xylenes and potentially more complex, disubstituted benzenes via a single collision event highlighting the significance of an alkyl-substituted ethynyl-mediated preparation of aromatic molecules in our Universe.
摘要：
在高温燃烧火焰和低温外星环境中，最简单的二烷基取代的芳烃-二甲苯（C6H4（CH3）2）-的基本反应途径仍然未知，但对于了解这些极端环境中的化学和分子质量增长过程至关重要。利用交叉分子束实验，通过最先进的电子结构和统计计算得到增强，这项研究揭示了以前难以捉摸的，通过1-丙炔基（甲基乙炔基，CH3CC）与2-甲基-1,3-丁二烯（异戊二烯，C5H8)。反应动力学是由自由基在2-甲基-1,3-丁二烯的二烯部分的无障碍添加，然后进行大量异构化（氢转移，环化），然后进行单分子分解，并通过原子氢损失进行芳构化。这种整体的反应不仅在高温环境中，例如在燃烧火焰中，而且在富含碳的渐近巨分支（AGB）恒星的星际包膜周围，都提供了二甲苯的制备。而且在低温冷分子云（10K）以及富含碳氢化合物的行星及其卫星（如Triton和Titan）的大气中也是如此。我们的研究建立了迄今为止未知的二甲苯气相路线，可能更复杂，通过单个碰撞事件的二取代苯突出了烷基取代的乙炔基介导的芳香族分子制备在我们的宇宙中的重要性。
