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。一部分氢气也以氢分子的形式存在。然而，H_2在太空中的形成机制仍然是一个谜。根据流行的假说，H_2是在星际颗粒表面产生的。然而，H_2生成的动力学方面不能用这个假设来解释。用量子化学从头算方法（PMP 2/6- 31 G **）和密度泛函理论（B3 LYP/6 - 31 G **）研究了多环芳烃（PAH）自由基阳离子介导的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，一个氢原子被加到PAH^+阳离子上。接下来，第二个氢原子接近并脱离了芳烃离子中的一个亚甲基氢，形成一个氢分子。多环芳烃采用萘，蒽，芘，并晕苯。第一反应不需要活化能。发现第二反应需要0-3 kcal/mol的小活化能。因此，这一对反应可以被认为是冷空间中可能的H_2生成机制之一。这些反应也可能是有效的富集氘的PAH分子事实上，这是不容易估计一个准确的活化能的氢转移反应，因为密度泛函理论高估了相关能量的松散结合的复合物。MP2程序高估了相关能量。然后，我们使用PMP 2/6-31 G** 和B3 LYP/6- 31 G **//PMP 2/6- 31 G ** 计算了第二反应的活化能，并将两个值平均以获得合理的活化能

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