Molecular hydrogen formation in the interstellar medium: the role of polycyclic aromatic hydrocarbons analysed by the reaction force and activation strain model

The formation of H₂ onto pyrene (from hydrogen atoms) was studied in the framework of the Eley-Rideal mechanism, which was fully analysed for all the adsorption sites of pyrene. The structural and electronic contributions to the activation energies were characterized through the reaction force and activation strain model (ASM). The reaction force indicates that the activation process of the hydrogen chemisorption is dominated by the structural rearrangements of reactants rather than by electronic reordering. Furthermore, the ASM shows that the structural rearrangements are driven by the approach of hydrogen to the pyrene surface, which generates a strong repulsive interaction. However, the changes in the geometry of the surface generate a minimal increase of the total energy, being negligible at the absorption sites at the edge of the surface. For that reason, the process is most kinetically favoured at the edge sites, which have lower activation energies (∼1-3 kcal mol^-1) than the internal ones (∼6 kcal mol^-1). On the other hand, the recombination of hydrogen atoms for the subsequent H₂ formation is a barrier-less process, no matter the adsorption site of the carbonaceous surface. The ASM analysis shows that the attractive interactions take place at the beginning of the recombination process, they avoid that the energy barrier can be generated by structural distortions. Both the chemisorption and the abstraction of the adsorbed hydrogen atom by an incoming H atom, throughout the entire surface, are highly exoenergetic processes, with reaction energy values in the range of -12 to -36 kcal mol^-1 and -84 to -60 kcal mol^-1, respectively.