The effect of the heteroatom (X=P, As, Si and Ge) on the geometrical and electronic properties of α-Keggin polyoxometalates (M=Mo, W and Nb) – A DFT investigation

Abstract

The α-Keggin polyoxometalates (K-POMs) are an important class of materials that have been envisaged as building blocks of nanostructures and functional materials with potential application in catalysis. They are heteropolyanions with formula [XM12O40]n− consisting of a metal oxide cage with a heteroatom in the center assuring high symmetry structure. Density functional calculations have been performed to investigate how the heteroatoms (P, As, Si, Ge) affect the geometry and the electronic structure of the K-POMs. It has been shown that the K-POMs act like a rigid cage trapping the heteroatom in the center. The change of the heteroatom slightly affects the M−O distances. In fact, the change in the ionic radii of the heteroatom is damped by the change of the M−OX distances. The Bader’s Quantum Theory of Atoms in Molecules (QTAIM) analysis indicate that the interatomic bonding in the POMs are characterized by closed shell interactions typical of larger ionic character. TD-DFT simulated spectra show that the heteroatom does not affect appreciably the absorption energies, in good agreement with the experimental data. However, for the Nb-POMs, the simulated spectra indicate that they are structured in a larger range of wavelength for X = P and As, while for X = Si and Ge the Nb-POMS present a well defined peak in agreement with the available experimental data. Charge transfer is observed for the allowed electronic transitions localized in the cage region with no participation of the heteroatom.