Electronic Structures of Small (RuO₂)_n (n = 1-4) Nanoclusters and Their Anions and the Hydrolysis Reactions with Water

Group 8 (RuO₂)_n (n = 1-4) nanoclusters, their anions, and the hydrolysis reactions of the neutral clusters have been studied with the density functional theory (DFT) as well as coupled cluster CCSD(T) theory. The ground state is predicted to be a singlet and a doublet for the neutral RuO₂ clusters and anionic clusters, respectively. The CCSD(T) method is required to predict the correct ground state. The calculated singlet-triplet gaps (<15 kcal/mol) and fluoride affinities (<95 kcal/mol) are smaller than those of the group 4 (MO₂)_n and group 6 (MO₃)_n metal oxide clusters. The electron affinities range from 2.2 to 3.4 eV, showing that the RuO₂ clusters are quite reducible. Clustering energies and heats of formation are calculated. The water physisorption energies are predicted to be -10 to -20 kcal/mol with the adsorption energy for the singlet being generally more exothermic than that for the triplet. The hydrolysis reactions are exothermic for the monomer and dimer clusters and are slightly endothermic or neutral for the trimer and tetramer. H₂O is readily dissociated on the monomer and dimer but not on the trimer and tetramer. The physisorption and chemisorption energies are less exothermic, and the barriers for the hydrolysis reactions are larger for RuO₂ nanoclusters than for the corresponding group 4 ZrO₂ nanoclusters.