A Mechanistic Study on the Formation of Acetic Acid (CH3COOH) in Polar Interstellar Analog Ices Exploiting Photoionization Reflectron Time-of-flight Mass Spectrometry
A Mechanistic Study on the Formation of Acetic Acid (CH3COOH) in Polar Interstellar Analog Ices Exploiting Photoionization Reflectron Time-of-flight Mass Spectrometry
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DOI：
10.3847/1538-4357/abafa4
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发表时间：
2020-09-01
影响因子：
4.9
通讯作者：
Kaiser, Ralf, I
中科院分区：
文献类型：
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作者：
Kleimeier, N. Fabian;Eckhardt, Andre K.;Kaiser, Ralf, I
乙酸（CH 3COOH）被认为是形成最简单的氨基酸甘氨酸和肽的关键分子。它在星际介质中无处不在，并已被罗塞塔使命在彗星的热核、彗发和彗星67 P/Churyumov-Gerasimenko的表面探测到。在这里，我们提出的异构体选择性形成的乙酸在极地冰混合物的水/乙醛后，暴露于电离辐射的形式的高能电子作为代理产生的二次电子一旦银河宇宙射线通过星际冰。乙酸甚至在仅0.13 eV分子（-1）（氧化氘）和0.29 eV分子（-1）（乙醛）的低辐照剂量下也会形成，这表示分子云寿命为1 x 10（6）年。同位素取代表明，主要的形成途径是乙酰基（CH 3CO）与羟基-d（1）自由基（OD）的无障碍自由基-自由基重组，而氧插入不会产生任何可检测到的量的乙酸。这种迄今未知的反应途径将影响化学模型中不同C（2）H（4）O（2）异构体的相对丰度，旨在通过比较这些丰度来限制反应条件。与其在非极性模型冰中的形成相反，在这种极性二元冰中的形成是异构体选择性的，并且仅产生乙酸。
Acetic acid (CH3COOH) is considered a key molecule in the formation of the simplest amino acid, glycine, and consequently peptides. It is ubiquitous in the interstellar medium and has been detected toward hot cores, in the coma of comets, and on the surface of the comet 67P/Churyumov-Gerasimenko by the Rosetta mission. Here we present the isomer-selective formation of acetic acid in polar ice mixtures of water/acetaldehyde upon exposure to ionizing radiation in the form of energetic electrons as a proxy for secondary electrons generated once Galactic cosmic rays pass through interstellar ices. Acetic acid is formed even at low irradiation doses of only 0.13 eV molecule(-1)(deuterium oxide) and 0.29 eV molecule(-1)(acetaldehyde), representing molecular cloud lifetimes of 1 x 10(6)yr. Isotopic substitutions reveal that the dominant formation pathway is the barrierless radical-radical recombination of acetyl (CH3CO) with hydroxyl-d(1)radicals (OD), whereas oxygen insertion does not yield any detectable amounts of acetic acid. This hitherto unknown reaction pathway will influence the relative abundances of distinct C(2)H(4)O(2)isomers in chemical models aiming to constrain the reaction conditions by comparing these abundances. In contrast to its formation in nonpolar model ices, the formation in this polar binary ice is isomer-selective and produces acetic acid only.