Analysis of 18; W and 17O NMR chemical shifts in polyoxometalates by extended Hückel mo calculations

Abstract

The extended Hückel molecular orbital (EHMO) calculations have been carried out using cluster approach to polyoxo anions, i.e. calculations have been done for a single octahedron MO 6 of different symmetry and results have been used to analyze 183 W and 17 O NMR spectra. Using five d→d∗ energy differences for the individual WO 6 the sum Σ1/ ( E d→ E d∗) have been calculated and plotted against the 183 W chemical shift (from +258 to −670 ppm) for corresponding type of tungsten atom and practically a linear correlation between two parameters have been observed. This points out the electronic nature of the 183 W chemical shift. Similar correlation have been found for the 17O NMR chemical shifts (−90 to +800 ppm) when the plotted against product of the R −3 Σ1/ ( E p→ E p∗) where the R −3 bond length of the corresponding W–O bond. Increasing the 183 W nuclear magnetic shielding with the calculated electron population on tungsten atom for closely related anions has been observed, but no general tendency between δ and the calculated electronic charge if the symmetry of polyhedron is changed is expected.