Hydrogenated polycyclic aromatic hydrocarbons (HnPAHs) as catalysts for hydrogenation reactions in the interstellar medium: a quantum chemical model

Abstract

The sticking of H atoms onto dust grains and large hydrocarbon molecules has received considerable attention because it is thought to govern the formation of H₂ and other H-containing molecules in the interstellar medium. Using the density functional theory (DFT) approximation, we have investigated the capacity of neutral hydrogenated polycyclic aromatic hydrocarbons (H_nPAH) to catalyze simple hydrogenation reactions by acting as a source of atomic hydrogen. In particular, the interaction of OH and CO with H₁-anthracene (singly hydrogenated) and H₁₄-anthracene (fully hydrogenated) to form H₂O and HCO was modeled following the Eley–Rideal mechanism. In this process, a hydrogen atom is abstracted from the H_nPAH molecule forming the corresponding hydrogenated compound. The results were compared to the most known case of the H_nPAH-catalyzed formation of H₂. It was observed that whereas H₂ is formed by overcoming activation barriers of approximately 0.02 and 0.10 eV with H₁-anthracene and H₁₄-anthracene, respectively, H₂O is produced in a barrierless fashion with both hydrocarbon molecules. The production of HCO was found to be a highly unfavorable process (with activation barriers of 0.73 eV and 3.13 eV for H₁- and H₁₄-anthracene, respectively). Complementary calculations performed using the rest of the H_n-anthracene molecules (from 2 to 13 extra H atoms) showed that in all the cases the reaction with OH is barrierless as well. This efficient mechanism could therefore be a possible route for water formation in the cold interstellar medium.