Hydrogen migration dynamics in hydrated Al clusters: the Al17(-)·H2O system as an example

The Alm((-))·(H2O)n systems are known to undergo water splitting processes in the gas phase giving HkAlm(OH)k((-))·(H2O)n-k systems, which can generate H2. The migration of H atoms from one Al atom to another on the cluster's surface is of critical importance to the mechanism of the complete H2 production process. We have applied a combination of Molecular Dynamics and Rice-Ramsperger-Kassel-Marcus theory including tunneling effects to study the gas-phase evolution of HAl17(OH)((-)), which can be considered a model system. First, we have performed an extensive search for local minima and the connecting saddle points using a density functional theory method. It is found that in the water-splitting process Al17((-))·(H2O) → HAl17(OH)((-)), the H atom which bonds to the Al cluster losses rather quickly its excess energy, which is easily "absorbed" by the cluster because of its flexibility. This fact ultimately determines that long-range hydrogen migration is not a very fast process and that, probably, tunneling only plays a secondary role in the migration dynamics, at least for moderate energies. Reduction of the total energy results in the process being very much slowed down. The consequences on the possible mechanisms of H2 generation from the interaction of Al clusters and water molecules are discussed.