The Reactivity of Subcolloidal Iron Clusters

合成纳米级铁（III）氢氧化物簇方法的巨大进步，可以回答困扰地球化学家几十年的关于铁（III）固体表面的问题。这些问题包括：(i)哪些桥接羟基通过什么速率定律与本体溶液交换氧同位素？（ii）电子交换在哪里进行？（iii）我们的计算机模型能预测这种化学反应吗？（iv）需要多大程度的准确性，即费用？我们提出了一系列涉及分子簇的项目，这些分子簇具有许多Fe（III）或Al（III）原子，它们与羟基桥连接在一起。同源分子家族（Fe17, Fe19， Al13和Al15）具有相似的结构核心，但Al（III）或Fe（III）金属中心的数量不同。可以选择有或没有结合水分子的簇。事实上，可以合成结构相似的Fe（III）和Al（III）簇是一个巨大的实验优势。al13和Al15簇是抗磁性的，允许进行核磁共振实验，包括17O， 27Al，13C和1H。即使是铁（III）簇也可以通过同位素交换和顺磁核磁共振的方法进行探测。我们建议的项目是：(i)通过同位素交换实验确定团簇中不同氧的反应性；（ii）评估“吸附”有机配体中使团簇在水中稳定的动力学，包括在结合状态下的停留时间；特别是：（iii）评估预测铁(iii) -氢氧化物固体和分子中的反应性所需的工具。最后一点可以预先说明：“要获得精确到优于0.05 a的结构，需要多大程度的量子复杂性？”优于0.01 A'？在之前的研究中，我们发现键长0.03 a的变化会使相邻氢氧根氧同位素的交换速率变化1 -103倍。“我们处理电子相关的方法必须有多复杂？”“什么构成波函数表示的充分基础？”以及：“在足够大到与地球化学相关的系统中，能够产生隐藏反应途径的最具前景的廉价计算方法类型是什么？”如何构建一个有用的电子结构的简化表示？分子允许从小到大（单体，二聚体，四聚体，Al13, Al15， Fe17和Fe19簇）的相当密集的进展，该系列提供了一种系统地推断精确计算的方法，这种方法可以从小分子到扩展固体表面和地球化学所需的更近似的方法。这项研究的意义远远超出了地球科学。这项研究将用于许多学科，包括胶体化学、纳米科学和医学（金属蛋白如铁蛋白类似于这些簇）。此外，UCD的入学人数不成比例地来自美国的大量新移民，因此这些资金将非传统但非常有才华的学生带入了地球科学。我们在定量地球化学的最新技术方面培养了一批不同的学生，我们模糊了地球科学和无机化学之间的区别，这对培养无畏的年轻地球化学家很重要。

The enormous advances in methods to synthesize nanometer-sized Fe(III)-hydroxide clusters can answer questions about the Fe(III)-solid surfaces that havepuzzled geochemists for decades. These questions include: (i) Which bridging hydroxylsexchange oxygen isotopes with bulk solution via what rate laws? (ii) Where do electronexchangesproceed? (iii) Can our computer models predict this chemistry? (iv) Whatlevel of accuracy, that is, expense, is needed?We propose a set of projects involving molecular clusters that have many Fe(III)or Al(III) atoms linked together with sets of hydroxyl bridges. The family ofhomologous molecules (Fe17, Fe19, Al13 and Al15) have a similar structural core butvarying numbers of Al(III) or Fe(III) metal centers. Clusters can be chosen that have, ordo not have, bound water molecules. The fact that Fe(III) and Al(III) clusters can besynthesized that are structurally similar is an enormous experimental advantage. TheAl13 and Al15 clusters are diamagnetic and allow NMR experiments, including 17O, 27Al,13C and 1H. Even the Fe(III) clusters allow probing via the methods of isotope-exchangeand paramagnetic NMR.The projects that we propose are to: (i) Determine the reactivity of differentoxygens in the clusters via isotope-exchange experiments; (ii) Evaluate the dynamics inthe 'adsorbed' organic ligand that makes the clusters stable in water, including theresidence times in the bound state; and particularly: (iii) Evaluate the tools needed topredict reactivities in Fe(III)-hydroxide solids and molecules. This last point can berestated: 'What level of quantum sophistication is required to obtain a structure accurateto better than 0.05 A? Better than 0.01 A'? In previous studies, we found that a change inbond length of 0.03 A changes the adjacent hydroxide oxygen-isotope exchange rates bya factor of i-103. 'How sophisticated must our methods of treating electron correlationbe?' 'What constitutes an adequate basis for wave-function representation?' and: 'Whatare the most promising types of computationally inexpensive methods that can yieldhidden reaction pathways in systems that are sufficiently large to be geochemicallyrelevant?' How does one construct a useful reduced representation of electronicstructure? The molecules allow a fairly dense progression from small to large(monomers, dimers, tetramers, Al13, Al15, Fe17, and Fe19 clusters) and this series providesa way to systematically extrapolate accurate calculations that are possible on smallmolecules to more approximate methods needed for extended solids surfaces andgeochemistry.The implications of this research extend well beyond Earth science. This researchwill be used by many disciplines, including colloid chemistry, nanoscience, and medicine(metalloproteins such as ferritin resemble these clusters). Also, enrollment at UCD drawsdisproportionately from the high population of new immigrants to the United States, sothese funds bring nontraditional but highly talented students into the Earth Sciences. Wetrain a diverse group of students in the latest techniques of quantitative geochemistry andwe blur the distinction between Earth science and inorganic chemistry, which isimportant for the training of fearless young geochemists.