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

在星际空间中，氢主要以氢原子的形式存在。一部分氢也以氢分子的形式存在。然而，H_2在太空中的形成机制仍是一个谜。根据流行的假设，H_2是在星际颗粒表面产生的。然而，H_2形成的动力学方面不能用这个假设来解释。采用从头算分子轨道理论（PMP2/6-31G**）和密度泛函理论（B3LYP/6- 31g **）研究了多环芳烃（PAH）自由基自由基介导H_2生成的可能性，推测H_2是通过两个连续反应生成的。首先，在多环芳烃+阳离子上加入一个氢原子。接着，第二个氢原子靠近并抽离了arenium离子中的一个亚甲基氢，形成一个氢分子。使用的多环芳烃有萘、蒽、芘和冠烯。第一次反应不需要活化能。第二种反应的活化能较低，为0-3 kcal/mol。因此，这对反应可以被认为是冷空间中H_2的形成机理之一。这些反应也可能对多环芳烃分子中的氘富集有效。事实上，由于密度泛函理论高估了松散结合配合物的相关能，估计氢转移反应的准确活化能并不容易。MP2程序高估了相关能。然后我们用PMP2/6-31G**和B3LYP/6-31G**//PMP2/6-31G**计算第二反应的活化能，取平均值得到合理的活化能

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