A 183W, 1H and 17O nuclear magnetic resonance study of aqueous isopolytungstates

Abstract

Isopolytungstates have been studied in aqueous solution between pH 8 and 1.5, using ¹⁸³W, ¹⁷O and ¹H 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 ψ-metatungstate. This in turn protonates with pK_a= 2.65, before transmuting to the known species tungstate-Y. Six metatungstate species with Keggin structures are aslo observed at lower pH values. Five are metastable anions which slowly convert to the well known α-[H₂W₁₂O₄₀]^6– ion. Two have β-Keggin structures. In each case direct structural information is provided from the solution state. The rotational correlation times and average interproton distances of the α- and β-[H₂W₁₂O₄₀]^6– species are deduced from their ¹H NMR relaxation data, and similar data are used to assist in identifying the three internally monoprotonated α- and β-Keggin anions.