Exploration into the Possibility of the Molecular Hydrogen Formation Mediated by Polyatomic Molecules in Interstellar Space

探索星际空间中多原子分子介导的分子氢形成的可能性

基本信息

批准号：
11640502
负责人：
AIHARA Jun-ichi
金额：
$ 2.37万
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：
1999
资助国家：
日本
起止时间：
1999 至 2001
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-11640502/
关键词：
interstellar molecule polycyclic aromatic hydrocarbon PAH PAH cation catalyst molecular hydrogen formation reaction mechanism density functional theory ab initio分子軌道計算アントラセンピレン芳香族性フラーレン

项目摘要

In interstellar space hydrogen exists mainly as hydrogen atoms. A protion of hydroegn also exists as hydrogen molecules. However, the H_2 formation mechanism in space remains quite elusive. According to the popular hypothesis, H_2 is generated on the surface of interstellar grains. However, kinetic aspects of the H_2 formation cannot be explained by this hypothesis. We investigated the possibility of the H_2 formation mediated by polycyclic aromatic hydrocarbon (PAH) radical canons using ab initio molecular orbital (PMP2/6-31G**) and density functional (B3LYP/6-31 G**) theoriesWe presumed that H_2 is formed via two consecutive reactions. First, a hydrogen atom is added to the PAH^+ cation. Next, a second hydrogen atom approaches and abstarcts one of the methylene hydrogens in the arenium ion to form a hydrogen molecule. PAHs employed were naphthalene, anthracene, pyrene, and coronene. No activation energy is needed for the first reaction. The second reaction was found to require a small activation energy of 0-3 kcal/mol. Therefore, such a pair of reactions can be considered one of the plausible H_2 formation mechanisms in cold space. These reactions might also be effective for enriching deuterium in the PAH moleculesIn fact, it is not easy to estimate an accurate activation energy for a hydrogen-transfer reaction because density functional theory overestimates correlation energy for a loosely bound complex. The MP2 procedure overestimates the correlation energy. We then evaluated the activation energy for the second reaction using PMP2/6-31 G** and B3LYP/6-31G**//PMP2/6-31G** and averaged the two values to obtain the reasonable activation energy

在星际空间中，氢主要存在于氢原子。水型的含有氢分子也存在。但是，空间中的H_2形成机制仍然难以捉摸。根据流行的假设，H_2是在星际晶粒的表面上产生的。但是，H_2形成的动力学方面不能用该假设来解释。我们研究了由多环芳族烃（PAH）自由基典型介导的H_2形成的可能性。首先，将氢原子添加到PAH+阳离子中。接下来，第二个氢原子接近并弃钉芳烃离子中的一种甲基水氢，以形成氢分子。使用的PAH是萘，蒽，吡咯和冠状的。第一个反应不需要激活能。发现第二个反应需要0-3 kcal/mol的较小活化能。因此，这样的反应可以被认为是冷空间中合理的H_2形成机制之一。这些反应也可能有效地富含pah分子素的事实中的氘，对于氢转移反应的准确活化能估计并不容易，因为密度功能理论高估了与松散结合的复合物的相关能量。 MP2程序高估了相关能量。然后，我们使用PMP2/6-31 G **和B3LYP/6-31G ** // PMP2/6-31G **评估了第二个反应的激活能，并平均两个值以获得合理的激活能