Department of Chemistry, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, USA.
J Am Chem Soc. 2012 Sep 19;134(37):15371-86. doi: 10.1021/ja304786s. Epub 2012 Sep 5.
Molecular oxygen is produced from water via the following reaction of potassium ferrate (K(2)FeO(4)) in acidic solution: 4H(3)Fe(VI)O(4) + 8H(3)O(+) → 4Fe(3+) + 3O(2) + 18H(2)O. This study focuses upon the mechanism by which the O-O bond is formed. Stopped-flow kinetics at variable acidities in H(2)O and D(2)O are used to complement the analysis of competitive oxygen-18 kinetic isotope effects ((18)O KIEs) upon consumption of natural abundance water. The derived (18)O KIEs provide insights concerning the identity of the transition state. Water attack (WA) and oxo-coupling (OC) transition states were evaluated for various reactions of monomeric and dimeric ferrates using a calibrated density functional theory protocol. Vibrational frequencies from optimized isotopic structures are used here to predict (18)O KIEs for comparison to experimental values determined using an established competitive isotope-fractionation method. The high level of agreement between experimental and theoretic isotope effects points to an intramolecular OC mechanism within a di-iron(VI) intermediate, consistent with the analysis of the reaction kinetics. Alternative mechanisms are excluded based on insurmountably high free energy barriers and disagreement with calculated (18)O KIEs.
分子氧是通过酸性溶液中高铁酸钾（K(2)FeO(4)）的下列反应从水中产生的：4H(3)Fe(VI)O(4) + 8H(3)O(+) → 4Fe(3+) + 3O(2) + 18H(2)O。本研究集中于形成 O-O 键的机制。在 H(2)O 和 D(2)O 中的可变酸度下进行的停流动力学用于补充分析消耗天然丰度水时的竞争氧-18 动力学同位素效应（(18)O KIEs）。得出的(18)O KIEs提供了有关过渡态的信息。使用经过校准的密度泛函理论方案，对单体和二聚铁酸盐的各种反应评估了水攻击（WA）和氧偶联（OC）过渡态。这里使用优化的同位素结构的振动频率来预测与使用建立的竞争同位素分馏方法确定的实验值进行比较的(18)O KIEs。实验和理论同位素效应之间的高度一致性表明，在二铁（VI）中间体内存在分子内 OC 机制，这与反应动力学的分析一致。基于不可逾越的高自由能势垒和与计算的(18)O KIEs 的不一致，排除了替代机制。
