Towards the accurate calculation of 183W NMR chemical shifts in polyoxometalates: the relevance of the structure.

DFT calculations were carried out to study (183)W NMR chemical shifts in the family of the Keggin anions with formula alpha-[XW(12)O(40)](q-) (X=B, Al, Si, P, Ga, Ge, As, Zn), in the beta- and gamma-[SiW(12)O(40)](4-) geometric isomers, in the derivative Dawson anion [P(2)W(18)O(62)](6-), and in the most symmetrical Lindqvist [W(6)O(19)](2-) anion and its derivative [W(10)O(32)](4-). In this article, we show that the geometry employed in the calculation of NMR chemical shifts in polyoxotungstates is extremely important if we want to be quantitative. Using very large basis sets of QZ4P quality and taking into account the conductor-like screening model (COSMO) to account for solvent effects (aqueous and organic solutions), good geometries were found for the polyoxoanions. From these optimal geometries the (183)W NMR chemical shifts were computed with the more standard basis sets of TZP quality and including spin-orbit corrections inside the zero-order regular approximation (ZORA) to describe the relativistic effects of the internal electrons. With this strategy the mean absolute error between experimental and theoretical values was found to be less than 10 ppm, which is similar to the experimental error. We also discuss how the geometry of the polyoxoanion influences on the shielding.

Are the solvent effects critical in the modeling of polyoxoanions?

DFT calculations were driven for a set of differently charged polyoxoanions in the gas phase and in solution. We have calculated and analyzed their geometries and orbital energies to trace simple rules of behavior regarding the modeling of anions in isolated form. We discuss the quality of the results depending on the molecular charge, q, and the size of the cluster in terms of the number of metal centers, m. When the q/m ratio reaches a value of approximately 0.8, DFT calculations for the isolated anion fail to describe the gap between the band of occupied oxo orbitals and the set of unoccupied orbitals delocalized among the metal atoms. In these cases the incorporation of the stabilizing external fields generated by the solvent through continuum models improves the geometries and orbital energies.