H2O DISSOCIATION AT LOW-COORDINATED SITES ON (MGO)(N) CLUSTERS, N=4,8

We investigate the interactions of one and two H2O molecules at three- coordinated sites of (MgO)(4) and (MgO)(8) clusters. Energies of adsor ption are calculated and compared to similar work involving periodic a nd embedded cluster systems. The cluster model results predict that H2 O chemidissociates without a barrier directly onto adjacent three-coor dinated sites. The chemidissociation energy per water molecule is obse rved to be greater upon the dissociation of two neighboring water mole cules compared to the dissociation of a single water molecule: a posit ive cooperativity effect. Correlation energy treated at the MP2 level of theory reduces the predicted chemidissociation energy by approximat ely 10 kcal/mol, and it is found that the MP2 single-point energies ev aluated at the RHF-optimized geometries, MP2/RHF, differ from the ener gy of the MP2-optimized structures by approximately 1 kcal/mol.