Department of Chemistry, University of California, Davis, Davis, CA 95616, USA.
Chemistry. 2011 Aug 16;17(34):9359-67. doi: 10.1002/chem.201100004. Epub 2011 Jul 1.
Rates of oxygen-isotope exchange were measured in the tetrasiliconiobate ion H(2+x)Si(4)Nb(16)O(56) 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 10(4) 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 (17)O 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 (17)O 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.
我们测量了H(2+x)Si(4)Nb(16)O(56)四硅铌酸盐离子中的氧同位素交换速率，以更好地了解大的氧离子如何与水相互作用。该分子有 19 个不等价氧原子，结构足够复杂，可以评估我们之前关于较小团簇的工作中得出的假设。我们想研究各个氧原子独立反应的程度，特别关注随着 pH 值从 13 降低到 6，各个氧原子的质子化顺序。与我们之前的工作一样，我们发现，在 6<pH<13 的范围内，分子中一组氧原子的反应速率在大约 10(4)范围内变化，但具有相似的 pH 依赖性。在 pH∼7 处存在 NMR 证据表明存在分子内或分子间反应，新峰开始缓慢形成而不会失去(17)O 同位素标记，在 pH≤6 时这些新峰迅速形成。与硅原子键合的氧原子在 pH 9 及以下开始发生同位素交换。对于一些但不是所有的不等价氧原子，(17)O NMR 峰位置也随 pH 发生很大变化。这种变化只能部分通过电子计算来解释，电子计算表明，氧原子在质子化时应该相似地移动。相反，我们看到一些位点随 pH 发生巨大变化，而其他配位氧原子则受影响较小，这表明要么一些质子交换得如此之快，以至于氧原子看到的是平均电荷，要么抗衡离子在调节配位质子的影响。
