The Reaction of Polycyclic Aromatic Hydrocarbon Cations with Hydrogen Atoms: The Astrophysical Implications

The reaction of the naphthalene cation, C 10 H + 8 , with H atoms is exothermic by about 60 kcal mol -1 and yields the 1-hydronaphthalene cation C 10 H + 9 . The addition of a second H atom is exothermic by about 45 kcal mol -1 , leading to the 1, 2-dihydronaphthalene cation C 10 H + 10 , which has less aromatic character than the parent naphthalene cation. Such products have been identified from the C—H stretching modes of the unidentified infrared bands. The abstraction of H from the 1-hydronaphthalene cation is found to have little, if any, barrier. This suggests that the polycyclic aromatic hydrocarbon (PAH) cations can catalyze the recombination of H atoms in the gas phase. Since the barrier to abstracting a deuterium is larger than hydrogen, reactions of hydrogen with PAH cations could enhance the degree of deuterium enrichment in PAHs.