¹⁷O NMR and Computational Study of a Tetrasiliconiobate Ion, [H_2+xSi₄Nb₁₆O₅₆]^(14−x)−

Abstract

Rates of oxygen-isotope exchange were measured in the tetrasiliconiobate ion [H_2+xSi₄Nb₁₆O₅₆]^(14−x)− to better understand how large oxide ions interact with water. The molecule has 19 nonequivalent oxygen sites and is sufficiently complex to evaluate hypotheses derived from our previous work on smaller clusters. We want to examine the extent to which individual oxygen atoms react independently with particular attention given to the order of protonation of the various oxygen sites as the pH decreases from 13 to 6. As in our previous work, we find that the set of oxygen sites reacts at rates that vary over approximately 10⁴ across the molecule at 6<pH<13 but with similar pH dependencies. There is NMR evidence of an intra- or intermolecular reaction at pH∼7, where new peaks began to slowly form without losing the ¹⁷O isotopic tag, and at pH ≤ 6 these new peaks formed rapidly. The oxygen atoms bonded to silicon atoms began to isotopically exchange at pH 9 and below. The ¹⁷O NMR peak positions also vary considerably with pH for some, but not all, nonequivalent oxygen sites. This variation could be only partly accounted by electronic calculations, which indicate that oxygen atoms should shift similarly upon protonation. Instead, we see that some sites change enormously with pH, whereas other, similarly coordinated oxygen atoms are less affected, suggesting that either some protons are exchanging so rapidly that the oxygen sites are seeing an averaged charge, or that counterions are modulating the effect of the coordinated protons.