Theoretical studies on the electronic and spectroscopic properties of Keggin-structure polyoxometalates α/β-[XM₁₂O ₄₀]^n- (X=Si, P; M=Mo, W)

The Keggin-structure compounds, α/β-[XM₁₂O ₄₀]^n- (X=Si, P; M=Mo, W), are optimized by the HF and B3LYP methods, respectively. Though the geometry parameters optimized by the HF agree better with experimental values, the B3LYP calculations can present precise energies, vibrational frequencies and suitable geometries. The IPCM model is used to account for the solvent effect of water. Based on the HF optimized geometries, the absorption spectra of title compounds in the aqueous solution are obtained by the combination of CIS and IPCM methods, in which α-[PW₁₂O₄₀]^3- as an example is discussed in detail. The calculated 252 nm X¹A₁→C ¹A₁ transition of such an anion is attributed to an O _b/c/d→W (intra-framework W₁₂O₃₆) charge transfer, corresponding to 265 nm absorption in water observed in experiment. The transition property of the absorption clearly substantiates the clathrate model hypothesis that a Keggin anion may also be viewed as a neutral M ₁₂O₃₆ cage that encapsulates the internal XO ₄^n- subunit.