Reaction Mechanism of Oxygen Atoms with Cyanoacetylene in the Gas Phase and on Water Ice
Reaction Mechanism of Oxygen Atoms with Cyanoacetylene in the Gas Phase and on Water Ice
复制标题
氧原子与氰乙炔在气相和水冰上的反应机理
DOI：
10.1086/501514
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发表时间：
2006
期刊：
影响因子：
--
通讯作者：
Chia
中科院分区：
文献类型：
--
作者：
H. Xie;Yi‐hong Ding;Chia
被水冰覆盖的尘埃颗粒上的原子和分子反应被认为在许多探测到的星际分子的形成中起着重要作用。水冰在某些分子-分子反应中的催化作用已经被证明。然而，人们对水催化的原子-分子反应知之甚少。本文提出了一个明显的理论线索，即在气相中，水冰可以有效地催化一个势垒消耗很大的原子-分子反应3 O + HC 3 N，使之成为无势垒的.详细的Gaussian-3势能面研究表明，气相中的3 O + HC 3 N和1 O + HC 3 N反应都能最终生成产物3 HCCN + CO，并伴有单重态-三重态的系间跃迁.然而，气态3 O + HC 3 N反应面临着相当大的入口势垒，这表明它在低温星际空间中的可能性很低。幸运的是，水分子的加入显著地将进入势垒降低到零。这是由水分子在3 O + HC 3 N的速率决定O-加成过渡态中的双重作用引起的，即，水分子既作为具有CH键的质子受体，又作为具有3 O原子的质子供体，以协同降低反应势垒。这些结果可以很好地解释最近在HC 3 N/O3/Ar混合物的光反应性研究中检测到的氰基烯酮。因此，在水冰上，最简单的氰基多炔HC 3 N可以被氧原子有效地耗尽，并可以生成难以捉摸的氰基亚甲基（3 HCCN），其在太空中的形成机制至今仍不清楚。
Reactions of atoms and molecules on dust grains coated by water ice have been considered to play an important role in the formation of many of the detected interstellar molecules. The catalytic role of water ice in some molecule-molecule reactions has been shown. Yet, there is rather little knowledge on the water-catalyzed atom-molecule reactions. In this paper, we present an evident theoretical clue that a considerably barrier-consumed atom-molecule reaction 3O + HC3N in the gas phase can be effectively catalyzed by water ice to be barrierless. Detailed Gaussian-3 potential energy surface studies show that both the 3O + HC3N and 1O + HC3N reactions in the gas phase can eventually lead to the product 3HCCN + CO with the involvement of singlet-triplet intersystem crossings. Yet the gaseous 3O + HC3N reaction faces a considerable entrance barrier, suggestive of its low likelihood in the low-temperature interstellar space. Fortunately, inclusion of water molecules significantly reduces the entrance barrier to zero. This is caused by the dual role of water molecules in the rate-determining O-addition transition state for 3O + HC3N, i.e., the water molecules act as both a proton acceptor with the CH bond and a proton donor with the 3O atom to cooperatively decrease the reaction barrier. The results can well account for the recent cyanoketene detection in a photoreactivity study of HC3N/O3/Ar mixtures. Therefore, on water ice, the simplest cyanopolyynes, HC3N, can be effectively depleted by oxygen atoms and can generate the elusive cyanomethylene (3HCCN), whose formation mechanism in space was still unclear up to now.