Hydrogen chemisorption on polycyclic aromatic hydrocarbons via tunnelling

Abstract

The chemisorption of hydrogen atoms on polycyclic aromatic hydrocarbons (PAHs) is studied at low temperatures via quantum mechanical tunnelling through reaction barriers. PAHs are ubiquitous in the interstellar medium and may exist in various charge states as well as hydrogenation states. PAHs have been suggested to catalyze H2 formation in photon-dominated regions via chemisorbed hydrogen atoms. Hydrogenated PAHs are also implicated by the relative strengths of the infrared bands in protoplanetary nebulae, reflection nebulae and H ii regions. The activation barrier for the chemisorption of hydrogen atoms to graphite is prohibitively high (∼5000 K) at low to moderate temperatures for this reaction to occur classically. On PAHs, however, edge sites are more flexible and can accommodate the incoming hydrogen atom more easily, resulting in a lower barrier. Combined with a further rate enhancement via tunnelling, hydrogen chemisorption on PAH edges may become feasible in various regions in the interstellar medium. We present harmonic quantum transition state theory calculations, which incorporate tunnelling, on pyrene as a model PAH system. Indeed the relatively low (∼2000 K) classical activation barriers for hydrogen atom chemisorption on edge sites combined with strong tunnelling give rise to non-negligible rates of the order of 10−16–10−18 cm3 site−1 s−1 at temperatures as low as 40 K, with a large kinetic isotope effect kH/kD≈ 64, characteristic for tunnelling. At this temperature, chemisorption on the core of a PAH is orders of magnitude slower, ∼10−22.5 cm3 site−1 s−1 even for the lightest H isotope. The addition of H atoms to PAH edge sites via tunnelling could be efficient enough to contribute H-PAH formation, although other processes may be more important.