Tungsten-183 nuclear magnetic resonance spectroscopy in the study of polyoxometalates

This review focuses on the rungsten-183 NMR spectroscopy of polyoxotungstates. The W-183 nuclide is the only NMR-active natural tungsten isotope with a spin of 1/2 and natural abundance of 14.3%. It gives rise to very narrow resonance lines. The NMR spectral envelope depends on the structure and symmetry of the anion. The W-183 chemical shifts are sensitive to their surrounding environment, including central atom. electric charge and size of adjacent elements, counterions in solution and solvent. The replacement of ligand atoms in an anion by the quadrupolar vanadium nucleus or by paramagnetic ions either in a ligand site or in the central site cause the resonance signals of the tungsten atoms adjacent to the substituted atoms to shift dramatically and broaden or even disappear. The spectral envelope, relative intensity of the lines and homonuclear and heteronuclear coupling constants provide the basis to identify anionic structure. Tungsten-183 NMR spectroscopy including 2-D COSY and 2-D INADEQUATE spectroscopy are used to distinguish new synthetic species, to examine product purity, to differentiate isomers and to establish new structures in solution as well as to monitor the progress of reaction. Tungsten-183 NMR spectroscopy has already become a powerful routine tool of structural characterization for polyoxotungstates in solution. (C) 2003 Elsevier B.V. All rights reserved.