Possible molecular hydrogen formation mediated by the inner and outer carbon atoms of typical PAH cations

We have been studying the possibility of H2 formation mediated by the radical cations of gaseous polycyclic aromatic hydrocarbons (PAHs). In this study, we explored the catalytic role of both outer and inner carbon atoms of some typical PAH cations at the B3LYP/6-31G** level of theory. We presumed that H2 is formed by way of two elementary reactions: the addition of an H atom to a PAH cation and the H abstraction from the resulting arenium ion by another H atom. It was found that both reactions proceed with no or very small activation energies. Refinement of the activation energies with a spin-projected MP2 procedure confirmed a very small activation energy for the second reaction. It then follows that at least some carbon atoms of a PAH cation give sites for molecular hydrogen formation in space. Interstellar PAHs are mostly large compact ones with many inner carbon atoms. Therefore, even if such carbon atoms might be less susceptible to H addition, their catalytic role can never be overlooked in interstellar chemistry