Abstract The formation mechanism of linear and isopropyl cyanide (hereafter n-PrCN and i-PrCN, respectively) in the interstellar medium (ISM) has been proposed from the reaction between some previously detected small cyanides/cyanide radicals and hydrocarbons/hydrocarbon radicals. n-PrCN and i-PrCN are nitriles therefore, they can be precursors of amino acids via Strecker synthesis. The chemistry of i-PrCN is especially important since it is the first and only branched molecule in ISM, hence, it could be a precursor of branched amino acids such as leucine, isoleucine, etc. Therefore, both n-PrCN and i-PrCN have significant astrobiological importance. To study the formation of n-PrCN and i-PrCN in ISM, quantum chemical calculations have been performed using density functional theory at the MP2/6-311++G(2d,p)//M062X/6-311+G(2d,p) level. All the proposed reactions have been studied in the gas phase and the interstellar water ice. It is found that reactions of small cyanide with hydrocarbon radicals result in the formation of either large cyanide radicals or ethyl and vinyl cyanide, both of which are very important prebiotic interstellar species. They subsequently react with the radicals CH2 and CH3 to yield n-PrCN and i-PrCN. The proposed reactions are efficient in the hot cores of SgrB2 (N) (where both n-PrCN and i-PrCN were detected) due to either being barrierless or due to the presence of a permeable entrance barrier. However, the formation of n-PrCN and i-PrCN from the ethyl and vinyl cyanide always has an entrance barrier impermeable in the dark cloud; therefore, our proposed pathways are inefficient in the deep regions of molecular clouds. It is also observed that ethyl and vinyl cyanide serve as direct precursors to n-PrCN and i-PrCN and their abundance in ISM is directly related to the abundance of both isomers of propyl cyanide in ISM. In all the cases, reactions in the ice have smaller barriers compared to their gas-phase counterparts.

中文翻译：

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星际介质中异丙基氰及其线性异构体形成的量子化学研究

摘要 星际介质 (ISM) 中线性和异丙基氰化物（以下分别称为 n-PrCN 和 i-PrCN）的形成机制已经从一些先前检测到的小氰化物/氰化物自由基与烃/烃自由基之间的反应中提出。n-PrCN 和 i-PrCN 是腈类，因此它们可以通过 Strecker 合成成为氨基酸的前体。i-PrCN 的化学性质特别重要，因为它是 ISM 中第一个也是唯一的分支分子，因此，它可能是分支氨基酸的前体，如亮氨酸、异亮氨酸等。 因此，n-PrCN 和 i-PrCN具有重要的天体生物学意义。为了研究 ISM 中 n-PrCN 和 i-PrCN 的形成，使用密度泛函理论在 MP2/6-311++G(2d,p)//M062X/6-311+G( 2d,p) 水平。所有提出的反应都在气相和星际水冰中进行了研究。发现小氰化物与烃自由基的反应导致形成大氰化物自由基或乙基和乙烯基氰化物，这两者都是非常重要的益生元星际物种。它们随后与自由基 CH2 和 CH3 反应生成 n-PrCN 和 i-PrCN。由于无屏障或由于存在可渗透的入口屏障，所提出的反应在 SgrB2 (N) 的热核中是有效的（其中检测到 n-PrCN 和 i-PrCN）。然而，由乙基和乙烯基氰化物形成的 n-PrCN 和 i-PrCN 在乌云中总是具有不可渗透的入口屏障；因此，我们提出的路径在分子云的深部区域效率低下。还观察到乙基和乙烯基氰作为 n-PrCN 和 i-PrCN 的直接前体，它们在 ISM 中的丰度与 ISM 中丙氰的两种异构体的丰度直接相关。在所有情况下，与气相反应相比，冰中的反应具有较小的障碍。