Hydrogen chemisorption on polycyclic aromatic hydrocarbons via tunnelling

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 H-2 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 HII regions. The activation barrier for the chemisorption of hydrogen atoms to graphite is prohibitively high (similar to 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 (similar to 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) cm(3) site(-1) s(-1) at temperatures as low as 40K, with a large kinetic isotope effect k(H)/k(D) approximate to 64, characteristic for tunnelling. At this temperature, chemisorption on the core of a PAH is orders of magnitude slower, similar to 10(-22.5) cm(3) 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.