Hirama Mutsumi, Ishida Toshimasa, Aihara Jun-Ichi
Department of Chemistry, Faculty of Science, Shizuoka University Oya, Shizuoka 422-8529, Japan.
J Comput Chem. 2003 Sep;24(12):1378-82. doi: 10.1002/jcc.10290.
Hydrogen molecules cannot be formed readily by the association of gaseous hydrogen atoms. Possible H(2) formation mediated by the radical cations of typical polycyclic aromatic hydrocarbons (PAHs), anthracene and pyrene, was studied at the B3LYP/6-31G** level of theory. We presumed that H(2) is formed by way of two elementary reactions: the addition of an H atom to a PAH molecular cation, and the H abstraction from the resulting monohydro-PAH cation (i.e., arenium ion) by a second H atom to yield H(2). The first reaction takes place without any activation energy. The second reaction is also predicted to proceed along almost barrierless pathways, although it is far from being a typical ion-molecule reaction. There is a possibility that these reactions might constitute one of the mechanisms for H(2) formation in extremely cold interstellar space. Deuterium enrichment in PAH cations is possibly accompanied by such H(2) formation because deuteration lowers the energies of polyatomic PAH cations appreciably.
气态氢原子的结合并不能轻易形成氢分子。在B3LYP/6-31G**理论水平下，研究了典型多环芳烃（PAHs）蒽和芘的自由基阳离子介导的可能的H₂形成过程。我们推测H₂是通过两个基本反应形成的：一个H原子加成到PAH分子阳离子上，以及第二个H原子从生成的单氢-PAH阳离子（即芳鎓离子）上夺取H以生成H₂。第一个反应在没有任何活化能的情况下发生。第二个反应预计也沿着几乎无势垒的途径进行，尽管它远非典型的离子-分子反应。这些反应有可能构成极冷星际空间中H₂形成的机制之一。PAH阳离子中的氘富集可能伴随着这种H₂形成，因为氘化会显著降低多原子PAH阳离子的能量。
