W. M. Keck Research Laboratory in Astrochemistry, University of Hawaii at Manoa, Honolulu, Hawaii HI 96822, USA.
Phys Chem Chem Phys. 2014 Feb 28;16(8):3399-424. doi: 10.1039/c3cp54255f.
Ice mixtures of methane and carbon monoxide were exposed to ionizing radiation in the form of energetic electrons at 5.5 K to investigate the formation of carbonyl bearing molecules in extraterrestrial ices. The radiation induced chemical processing of the mixed ices along with their isotopically labeled counterparts was probed online and in situ via infrared spectroscopy (solid state) aided with reflectron time-of-flight mass spectrometry (ReTOFMS) coupled to single photon photoionization (PI) at 10.49 eV (gas phase). Deconvolution of the carbonyl absorption feature centered at 1727 cm(-1) in the processed ices and subsequent kinetic fitting to the temporal growth of the newly formed species suggests the formation of acetaldehyde (CH3CHO) together with four key classes of carbonyl-bearing molecules: (i) alkyl aldehydes, (ii) alkyl ketones, (iii) α,β-unsaturated ketones/aldehydes and (iv) α,β,γ,δ-unsaturated ketones/α,β-dicarbonyl compounds in keto-enol form. The mechanistical studies indicate that acetaldehyde acts as the key building block of higher aldehydes (i) and ketones (ii) with unsaturated ketones/aldehydes (iii) and/or α,β-dicarbonyl compounds (iv) formed from the latter. Upon sublimation of the newly synthesized molecules, ReTOFMS together with isotopic shifts of the mass-to-charge ratios was exploited to identify eleven product classes containing molecules with up to six carbon atoms, which can be formally derived from C1-C5 hydrocarbons incorporating up to three carbon monoxide building blocks. The classes are (i) saturated aldehydes/ketones, (ii) unsaturated aldehydes/ketones, (iii) doubly unsaturated aldehydes/ketones, (iv) saturated dicarbonyls (aldehydes/ketones), (v) unsaturated dicarbonyls (aldehydes/ketones), (vi) saturated tricarbonyls (aldehydes/ketones), molecules containing (vii) one carbonyl - one alcohol (viii), two carbonyls - one alcohol, (ix) one carbonyl - two alcohol groups along with (x) alcohols and (xi) diols. Reaction pathways to synthesize these classes were derived as well. The present experiments provide clear evidence for the formation of key organic molecules--acetaldehyde, acetone, and potentially vinylalcohol--which are among the 15 carbonyl containing organic molecules detected in the interstellar medium. Despite numerous previous experimental investigations probing the effect of ionizing radiation on simple astrophysical ice representatives, our results suggest that more complex organic molecules can be formed in extraterrestrial ices than previously suggested. An outlook on further identification of individual isomers is also presented.
在 5.5K 下，将甲烷和一氧化碳的冰混合物暴露于高能电子形式的电离辐射中，以研究星际冰中含羰基分子的形成。通过红外光谱（固态）和反射飞行时间质谱（ReTOFMS）与单光子光电离（PI）相结合，在线原位探测了混合冰及其同位素标记物的辐射诱导化学处理，PI 采用 10.49eV（气相）。对处理后的冰中羰基吸收特征在 1727cm(-1) 处进行解卷积，并对新形成物质的时间增长进行动力学拟合，表明乙缩醛（CH3CHO）与四类关键含羰基分子的形成：（i）烷基醛，（ii）烷基酮，（iii）α，β-不饱和酮/醛和（iv）α，β，γ，δ-不饱和酮/α，β-二羰基化合物，呈酮式-烯醇式。机理研究表明，乙缩醛是形成更高醛（i）和酮（ii）的关键构建块，其中不饱和酮/醛（iii）和/或α，β-二羰基化合物（iv）由后者形成。新合成分子升华后，利用 ReTOFMS 以及质荷比的同位素位移来鉴定 11 个产物类，其中包含多达 6 个碳原子的分子，这些分子可以从包含多达三个一氧化碳构建块的 C1-C5 烃正式衍生而来。这些类包括（i）饱和醛/酮，（ii）不饱和醛/酮，（iii）双不饱和醛/酮，（iv）饱和二羰基（醛/酮），（v）不饱和二羰基（醛/酮），（vi）饱和三羰基（醛/酮），（vii）一个羰基-一个醇，（viii）两个羰基-一个醇，（ix）一个羰基-两个醇基团以及（x）醇和（xi）二醇。还推导了合成这些类的反应途径。本实验为关键有机分子（乙缩醛、丙酮和潜在的乙烯醇）的形成提供了明确证据，这些分子是星际介质中检测到的 15 种含羰基有机分子之一。尽管之前有许多实验研究探测了电离辐射对简单天体冰代表物的影响，但我们的结果表明，在星际冰中可以形成比之前提出的更复杂的有机分子。还提出了进一步鉴定个别异构体的展望。
