Molecular and electronic structures, Bronsted basicities, and Lewis acidities of group VIB transition metal oxide clusters

The structures and properties of transition metal oxide (TMO) clusters of the group VIB metals, (MO3)(n) (M = Cr, Mo, W; n = 1-6), have been studied with density functional theory (DFT) methods. Geometry optimizations and frequency calculations were carried out at the local and nonlocal DFT levels with polarized valence double-zeta quality basis sets, and final energies were calculated at nonlocal DFT levels with polarized valence triple-zeta quality basis sets at the local and nonlocal DFT geometries. Effective core potentials were used to treat the transition metal atoms. Two types of clusters were investigated, the ring and the chain, with the ring being lower in energy. Large ring structures ( n > 3) were shown to be fluxional in their out of plane deformations. Long chain structures (n > 3) of (CrO3)(n) were predicted to be weakly bound complexes of the smaller clusters at the nonlocal DFT levels. For M6O18, two additional isomers were also studied, the cage and the inverted cage. The relative stability of the different conformations of M6O18 depends on the transition metal as well as the level of theory. Normalized and differential clustering energies of the ring structures were calculated and were shown to vary with respect to the cluster size. Bronsted basicities and Lewis acidities based on a fluoride affinity scale were also calculated. The Bronsted basicities as well as the Lewis acidities depend on the size of the cluster and the site to which the proton or the fluoride anion binds. These clusters are fairly weak Bronsted bases with gas phase basicities comparable to those of H2O and NH3. The clusters are, however, very strong Lewis acids and many of them are stronger than strong Lewis acids such as SbF5. Bronsted acidities of M6O19H2 and M6O18FH were calculated for M) Mo and W and these compounds were shown to be very strong acids in the gas phase. The acid/base properties of these TMO clusters are expected to play important roles in their catalytic activities.