A W-183, H-1 AND O-17 NUCLEAR-MAGNETIC-RESONANCE STUDY OF AQUEOUS ISOPOLYTUNGSTATES

Isopolytungstates have been studied in aqueous solution between pH 8 and 1.5, using W-183, O-17 and H-1 NMR spectroscopy. The first polyanions to form upon acidification are the paratungstates A and B. Their resonances are largely assigned, and paratungstate B is shown to protonate with pK(a) = 4.59. Its protonated form has at least two isomers separated by a detectably slow proton-exchange process. On further acidification, paratungstate B loses one tungsten atom to give an anion with no symmetry, which is identified as the solution form of psi-metatungstate. This in turn protonates with pK(m) = 2.65, before transmuting to the known species tungstate-Y. Six metatungstate species with Keggin structures are also observed at lower pH values. Five are metastable anions which slowly convert to the well known alpha-[H2W12O40]8- ion. Two have beta-Keggin structures. In each case direct structural information is provided from the solution state. The rotational correlation times and average interproton distances of the alpha- and beta-[H2W12O40]6- species are deduced from their H-1 NMR relaxation data, and similar data are used to assist in identifying the three internally monoprotonated alpha- and beta-Keggin anions.