Ab-initio Molecular Dynamics of Water Chemisorbed on Solid MgO

The Car-Parrinello method has been applied to study the adsorption of water on solid magnesium oxide with surface defects. Steps on an (011) base plane allow to model the experimentally observed microfaceting. In and on these steps dissociation of water into a hydroxyl group and an H-atom took place following a complicated pathway only accessible by the simulation of thermal motion. Under comparable conditions physisorption only was observed on a regular (001) plane. This solves an experimental controversy and it is in agreement with the observation, that disordered surfaces are more active in initiating the dissociation of the water molecules. Our work allows to identify an important active center.We can also account for the experimentally observed broadening and shifting to the red of the stretching mode of hydrogen bonded hydroxyl groups, and we provide a detailed explanation of the origin of this effect. This allows us to verify earlier theories of hydrogen bonding such as that of the adiabatic separation of the proton dynamics.