Ab initio and density functional electronic structure study of molybdenum oxide clusters

First principles quantum chemical calculations at various levels of theory (HF, MP2, B3LYP), using the LANL2DZ basis set, provide a satisfactory description of structural, energetic and electronic properties of molybdenum(VI) oxide species of molecular dimensions, formulated as MoO3, MoO42−, MoO4H2, MoO4Na2, Mo2O52+ and Mo2O6. The energetics of all topomers corresponding to global or local minima and saddle points in the potential energy hypersurfaces were computed at the more sophisticated QCISD(T) level as well. The proton affinity of the MoO42− dianion was found to be equal to 1544, 1536 and 1527 kJ mol−1 at the HF, MP2 and B3LYP levels of theory. The formation processes of Mo2O52+(C1) dication from MoO3 and MoO22+(C2v) dication and the Mo2O6 dimer upon dimerization of the MoO3 species are predicted to be exothermic, the energies of formation being equal to 636.0 and 388.7 kJ mol−1 at the B3LYP level, respectively. Finally, the computed spectroscopic properties (harmonic vibrational frequencies and corresponding normal modes, electronic transitions and NMR chemical shifts) of the molecular molybdenum oxides are thoroughly discussed in relation with available experimental data.