NMR Chemical Shifts of Metal Centres in Polyoxometalates: Relativistic DFT Predictions

A DFT approach incorporating relativistic corrections and solvent effects was tested for NMR calculations on transition-metal centres in polyoxometalates. For a monoplatinum decavanadate derivative and a set of dilacunary polyoxotung-states V-51 and W-183 chemical shifts were calculated at several levels of theoretical treatment regarding solvent, counterion and exchange-correlation functional. Calculations were performed first in the gas phase to model isolated ions and next in a continuum model for water to evaluate the importance of solvation for the quality of the computed chemical shifts. We show that the use of the orbital-dependent Kohn-Sham exchange-correlation functional SAOP in ZORA spin-orbit calculations with solvent effects included via COSMO substantially improves the agreement between computed results and experimental benchmarks for V-51 chemical shifts (at least, in the case of [(H2PtV9O28)-V-IV](5-)). In the case of dilacunary polyoxotungstates our calculations confirm the necessity of modelling an ion pair in which a counterion is specifically included in the POM lacuna to attain accurate predictions of the corresponding W-183 NMR spectra. We show that if the counterion is relatively small (like Li+ and Na+), the explicit location of a water molecule in its vicinity (in addition to the overall COSMO treatment) improves further the accuracy of the correlation between computed and experimental shifts (to less than 5 ppm of the encompassed 6 range). ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)