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Molecular Orbital Theoretical Exploration of Possible Formation of Interstellar Molecules by Homogeneous Catalysis

均相催化形成星际分子的分子轨道理论探索

基本信息

批准号：
16550011
负责人：
AIHARA Jun-ichi
金额：
$ 2.24万
依托单位：
Shizuoka University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
2004
资助国家：
日本
起止时间：
2004 至 2005
项目状态：
已结题

项目摘要

It has been believed that molecular hydrogen in space is formed on the surface of dust particles. Our previous molecular orbital (MO) calculations revealed that polycyclic aromatic hydrocarbon (PAH) cation radicals possibly mediate the formation of molecular hydrogen (H_2) in interstellar space. With the aid of typical PAH cations, such as molecular cations of naphthalene, anthracene, pyrene, and coronene, two H atoms combined to form H_2 with little activation energy. PAH cations are also predicted to possibly mediate the formation of many interstellar molecules even at extremely low temperatures. Among these interstellar molecules are methane, acetylene, ammonia, and hydrogen isocyanide. Reaction mechanisms derived from MO calculations are as follows. First, an H atom attaches to one of the carbon atoms in a PAH cation. Then, a precursor of an interstellar molecule approaches to the H atom to form a loosely bound complex, a kind of reaction intermediate. It crosses a low-energy trans … More ition state and finally forms the interstellar molecule. For example, the naphthalene cation attracts an H atom at the 1-position and then attracts a methyl radical to the H atom, which finally splits into a methane molecule and the starting naphthalenium ion. Our preliminary MO calculations strongly suggest that fullerenes and polyatomic interstellar molecules, such as cyanopolyynes, might also catalyze the formation of H_2 and other interstellar molecules with little activation energies. In MO calculations, the B3LYP functional was found to appreciably underestimates the activation energy of an H-transfer reaction and so the combined B3LYP-PMP2 method and the KMLYP functional were employed. In summary, we found the possibility that many interstellar molecules are formed not only by ion-molecule reactions but also by homogeneous catalysis. In order to firmly establish the occurrence of this chemical process in extremely cold interstellar space, it might be necessary to determine more accurately the activation energies in individual reactions concerned. Less

人们一直认为，太空中的分子氢是在尘埃颗粒的表面形成的。我们以前的分子轨道(MO)计算表明，多环芳烃(PAH)阳离子自由基可能参与了星际空间中分子氢(H_2)的形成。在典型的多环芳烃阳离子的辅助下，两个H原子结合形成活化能很小的H_2。据预测，即使在极低的温度下，多环芳烃离子也可能参与许多星际分子的形成。这些星际分子中有甲烷、乙炔、氨和异氰化氢。从分子轨道计算得出的反应机理如下。首先，H原子连接到PAH阳离子中的一个碳原子上。然后，星际分子的前体接近氢原子，形成一种松散结合的络合物，一种反应中间体。它跨越了一个低能量的反式…更多的激发态，最终形成星际分子。例如，萘阳离子在1-位上吸引一个H原子，然后吸引一个甲基自由基到H原子上，H原子最终分裂成甲烷分子和起始的萘离子。我们的初步分子轨道计算有力地表明，富勒烯和多原子星际分子，如氰基多炔，也可能催化H_2和其他活化能很低的星际分子的形成。在分子轨道计算中，B3LYP泛函明显低估了氢转移反应的活化能，因此采用了B3LYP-PMP2方法和KMLYP泛函相结合的方法。综上所述，我们发现了许多星际分子不仅是由离子-分子反应形成的可能性，也是由均相催化形成的可能性。为了确定在极端寒冷的星际空间中发生这种化学过程，可能有必要更准确地确定有关个别反应的活化能。较少