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

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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