Cooperation between bound waters and hydroxyls in controlling isotope-exchange rates
Cooperation between bound waters and hydroxyls in controlling isotope-exchange rates
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结合水和羟基之间的合作控制同位素交换率
DOI：
10.1016/j.gca.2011.10.041
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发表时间：
2012
影响因子：
5
通讯作者：
W. Casey
中科院分区：
文献类型：
--
作者：
Adele F. Panasci;J. G. McAlpin;C. Ohlin;S. Christensen;J. Fettinger;R. Britt;J. Rustad;W. Casey
矿物氧化物与水性离子的不同之处在于，结合的水分子通常附着在不同的金属中心或邻近的金属中心上，从而彼此分离。相比之下，对于大多数用于建立动力学反应趋势的单体离子，例如八面体水合离子（例如 Al(H2O)63+），结合水是紧密堆积的或孪生的。由于这种结构差异，有关单体离子中配体取代的现有文献可能无法很好地指导地球化学感兴趣的反应。为了了解反应官能团的协调如何影响简单水交换反应的速率，我们合成了两种结构相似的 Rh(III) 配合物，[Rh(phen)2(H2O)2]3+[1] 和 [Rh(phen)2(H2O)Cl]2+[2]，其中 (phen)=1,10-菲咯啉。配合物 [1] 在内配位球内具有两个相邻的孪生结合水，[2] 具有与结合氯离子相邻的单个结合水。我们使用 Rh(III) 作为三价金属，而不是像 Fe(III) 或 Al(III) 这样与地球化学更相关的金属来减慢反应速率，这使得通过简单的质谱法测量同位素取代率成为可能。我们制备了同位素纯版本的分子，将它们溶解到同位素不同的水中，并根据结合水处的 18 O 和 16 O 交换程度测量交换速率。两种复合物之间的速率对 pH 值的依赖性差异很大。完全质子化分子的水交换在 298K 时的伪一级速率系数很接近：[1] 为 k0298=5×10−8(±0.5×10−8)s−1，[2] 为 k0298=2.5×10−9(±1×10−9)。 [1] 测得的焓和熵活化参数（ΔH‡ 和 ΔS‡）分别为 119(±3)kJmol−1 和 14(±1)Jmol−1K−1。单水复合物的相应参数 [2] 为 132(±3)kJmol−1 和 41.5(±2)Jmol−1K−1。由于可转移质子非常接近，可以将结合羟基转化为结合水，因此复合物 [1] 中的一种结合水去质子化时，速率会增加多个数量级。这种相互转化允许氧以结合水的形式进行交换，而不是以结合羟基的形式进行交换，后者在接近中性的 pH 条件下速度很慢。
Mineral oxides differ from aqueous ions in that the bound water molecules are usually attached to different metal centers, or vicinal, and thus separated from one another. In contrast, for most monomeric ions used to establish kinetic reactivity trends, such as octahedral aquo ions (e.g., Al(H2O)63+), the bound waters are closely packed, or geminal. Because of this structural difference, the existing literature about ligand substitution in monomer ions may be a poor guide to the reactions of geochemical interest. To understand how coordination of the reactive functional groups might affect the rates of simple water-exchange reactions, we synthesized two structurally similar Rh(III) complexes, [Rh(phen)2(H2O)2]3+[1] and [Rh(phen)2(H2O)Cl]2+[2] where (phen)=1,10-phenanthroline. Complex [1] has two adjacent, geminal, bound waters in the inner-coordination sphere and [2] has a single bound water adjacent to a bound chloride ion. We employed Rh(III) as a trivalent metal rather than a more geochemically relevant metal like Fe(III) or Al(III) to slow the rate of reaction, which makes possible measurement of the rates of isotopic substitution by simple mass spectrometry. We prepared isotopically pure versions of the molecules, dissolved them into isotopically dissimilar water, and measured the rates of exchange from the extents of18O and16O exchange at the bound waters. The pH dependency of rates differ enormously between the two complexes. Pseudo-first-order rate coefficients at 298K for water exchanges from the fully protonated molecules are close: k0298=5×10−8(±0.5×10−8)s−1for [1] and k0298=2.5×10−9(±1×10−9) for [2]. Enthalpy and entropy activation parameters (ΔH‡and ΔS‡) were measured to be 119(±3)kJmol−1, and 14(±1)Jmol−1K−1, respectively for [1]. The corresponding parameters for the mono–aquo complex, [2], are 132(±3)kJmol−1and 41.5(±2)Jmol−1K−1. Rates increase by many orders of magnitude upon deprotonation of one of the bound waters in complex [1] because of the close proximity of a transferable proton that can convert the bound hydroxyl to a bound water. This interconversion allows the oxygen to exchange as a bound water, rather than as a bound hydroxyl, which is slow at near-neutral pH conditions.