Water formation on interstellar silicates: role of Fe2+/H2 interactions in the O + H2 → H2O reaction

Water is the most abundant molecule in solid state of the interstellar medium, and its presence is
critically important for life in space. Interstellar water is thought to be effectively synthesised by
reactions occurring on the surfaces of interstellar grains, as gas-phase reactions are not efficient
enough to justify its high abundance. In the present work, DFT simulations have been performed to
investigate the formation of interstellar water through the O + H2 → H2O reaction on olivinic silicate
surfaces that contain Fe2+ cations. The surfaces have been modeled adopting both a periodic and
cluster approach. The study focuses on: i) the stability of the surface models as a function of the
electronic states (i.e., quintuplet, triplet and singlet) arising from the presence of the Fe2+ centers, ii)
the adsorption of H2 on the silicate surfaces and its likely activation due to the Fe2+/H2 interactions,
and iii) characterising the energy profiles of the H2O formation reaction complemented with kinetics
that include tunneling effects. Results indicate that quintuplet is the most stable electronic state
in all the bare surface models. H2 adsorption, however, do not show a clear trend on the relative
stabilities of the H2/surface complexes with the electronic states, which is in general more favourable
on singlet state surfaces. Finally, reactions simulated on the periodic surfaces show elementary high
energy barriers but the reaction is kinetically feasible (considering the long lifetime of interstellar
clouds) due to the dominance of tunnelling. In contrast, on the nanocluster models, tunneling
effects cannot contribute due to the presence of endoenergetic elementary steps. It is predicted that
the reactions on the nanoclusters is only possible if the energy released during the adsorption of the
O atom is used to overcome the energy barriers.