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arXiv 1701.04640astro-ph.GA
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N-甲基甲酰胺的转动光谱学、试探性星际探测和化学模拟

Rotational spectroscopy, tentative interstellar detection, and chemical modelling of N-methylformamide

A. Belloche, A. A. Meshcheryakov, R. T. Garrod, V. V. Ilyushin, E. A. Alekseev, R. A. Motiyenko, L. Margulès, H. S. P. Müller, K. M. Menten

摘要:

N-甲基甲酰胺,CH 3 NHCHO,可能是星际前生物化学的重要分子,因为它含有一个肽键。CH_3NHCHO最稳定的反式构象的转动光谱由于甲基扭转的低能垒而被强的扭转-转动相互作用所复杂化。我们用哈尔科夫和里尔的两台吸收光谱仪测量了45- 630 GHz的转动光谱。采用Rho轴法和RAM 36程序进行了分析。我们用ALMA进行的谱线测量寻找N-甲基甲酰胺朝向热分子核Sgr B2(N2)。天文结果被放入一个更广泛的天体化学背景下的气体颗粒化学动力学模型的帮助下。CH_3 NHCHO反式构象的实验室数据集由J ≤ 62的9469个线频率组成,包括第一和第二激发扭转态的转动光谱的第一次分配。所有这些线都在实验精度范围内拟合。我们报告了对Sgr B2(N2)的CH 3 NHCHO的初步检测。我们发现CH 3 NHCHO的丰度比NH 2CHO低一个数量级以上,比CH 3 NCO低两倍,但仅比CH 3CONH 2略低。的化学模型表明,通过NH + CO对谷物的HNCO的有效形成是实现所观察到的气相丰度的CH 3 NCO的必要步骤。CH 3 NHCHO的产生可以通过直接添加官能团自由基或通过CH 3 NCO的氢化在颗粒上进行。如果确认检测到CH 3 NHCHO,则与其更稳定的结构异构体乙酰胺相比,该分子的丰度仅略微不足,并且模型丰度对化学动力学参数的敏感性表明,这两种分子的形成受动力学而不是热平衡控制。

英文摘要:

N-methylformamide, CH3NHCHO, may be an important molecule for interstellar pre-biotic chemistry because it contains a peptide bond. The rotational spectrum of the most stable trans conformer of CH3NHCHO is complicated by strong torsion-rotation interaction due to the low barrier of the methyl torsion. We use two absorption spectrometers in Kharkiv and Lille to measure the rotational spectra over 45--630 GHz. The analysis is carried out using the Rho-axis method and the RAM36 code. We search for N-methylformamide toward the hot molecular core Sgr B2(N2) using a spectral line survey carried out with ALMA. The astronomical results are put into a broader astrochemical context with the help of a gas-grain chemical kinetics model. The laboratory data set for the trans conformer of CH3NHCHO consists of 9469 line frequencies with J <= 62, including the first assignment of the rotational spectra of the first and second excited torsional states. All these lines are fitted within experimental accuracy. We report the tentative detection of CH3NHCHO towards Sgr B2(N2). We find CH3NHCHO to be more than one order of magnitude less abundant than NH2CHO, a factor of two less abundant than CH3NCO, but only slightly less abundant than CH3CONH2. The chemical models indicate that the efficient formation of HNCO via NH + CO on grains is a necessary step in the achievement of the observed gas-phase abundance of CH3NCO. Production of CH3NHCHO may plausibly occur on grains either through the direct addition of functional-group radicals or through the hydrogenation of CH3NCO. Provided the detection of CH3NHCHO is confirmed, the only slight underabundance of this molecule compared to its more stable structural isomer acetamide and the sensitivity of the model abundances to the chemical kinetics parameters suggest that the formation of these two molecules is controlled by kinetics rather than thermal equilibrium.

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