Theoretical study of the geometries and dissociation energies of molecular water on neutral aluminum clusters Al(n) (n = 2-25)

Geometries and dissociation energies of water molecules on Al(n) (n = 2-25) clusters were investigated using density functional theory with all electron relativistic spin-polarized calculations under the generalized gradient approximation. An extensive structure search was performed to identify the low-energy conformations of Al(n)H(2)O complexes for each size. Optimal adsorption sites were assigned for low-energy isomers of the clusters. Size and site specific dependences were studied for the Al(n)H(2)O complexes in stabilities, geometries, adsorption energies, dissociation energies, Al-O bond lengths, and other characteristic quantities. The stabilities and geometries revealed that H atom in H(2)O is not inclined to bond with Al atoms. The most stable Al(n)H(2)O configurations for each size tend to correspond to the most stable bare Al(n) cluster except of Al(6) and Al(24) clusters. The HO bond lengths increase generally 0.01 ? with respect to the isolated H(2)O in all of the adsorption complexes. The dissociation energy of an isolated H(2)O into HO and H was 5.39 eV, which decreased about two-thirds to the energy range of 0.83-2.12 eV with the help of Al(n) clusters. In spite of the fluctuations, the dissociation energies of Al(n)H(2)O complexes rise with the size increasing as a whole. In addition, we also found that the bare Al(n) clusters with high vertical ionization potentials usually have high dissociation energies of H(2)O in the corresponding adsorption models. The energetically preferred spin-multiplicity of all the odd-n Al(n)H(2)O complexes is doublet, and it is singlet for all the even-n complexes with exception of Al(2)H(2)O which is triplet.