The Reactivity of Subcolloidal Iron Clusters

亚胶体铁簇的反应性

• 批准号: 0515600
• 负责人: William Casey
• 金额: $ 40.39万
• 依托单位: University of California-Davis
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2005
• 资助国家: 美国
• 起止时间: 2005-09-15 至 2009-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0515600&HistoricalAwards=false
• 关键词: Reactivity; Subcolloidal; Iron; Clusters

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.

合成纳米级Fe（III）氢氧化物簇的方法取得了巨大进展，可以回答困扰地球化学家数十年的Fe（III）固体表面问题。这些问题包括：（i）哪些桥连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)需要什么样的精确度，即费用？我们提出了一套涉及分子簇的项目，这些分子簇具有许多Fe（III）或Al（III）原子，它们通过羟基桥连接在一起。同系分子（Fe 17、Fe 19、Al 13和Al 15）具有相似的结构核心，但Al（III）或Fe（III）金属中心的数量不同。可以选择具有或不具有结合水分子的簇。事实上，Fe（III）和Al（III）簇可以合成结构相似的簇，这是一个巨大的实验优势。Al 13和Al 15团簇是抗磁性的，并且允许NMR实验，包括17 O，27 Al，13 C和1H。甚至Fe（III）簇也可以通过同位素交换和顺磁NMR方法进行探测。我们提出的项目是：（i）通过同位素交换实验确定簇中不同氧的反应性;（ii）评估使簇在水中稳定的“吸附”有机配体的动力学，包括结合态的存在时间;特别是：（iii）评估预测氢氧化铁固体和分子反应性所需的工具。最后一点可以重新表述：“要获得精确度优于0.05埃的结构，需要多大的量子复杂度？”比0.01 A“好？在以前的研究中，我们发现键长变化0.03 A会使相邻氢氧化物氧同位素交换率变化1/103。我们处理电子相关性的方法必须有多复杂？什么构成波函数表示的充分基础？’及：“什么是最有前途的计算成本低的方法，可以在足够大的系统中产生隐藏的反应途径，与地球化学相关？”如何构造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，Al 13，Al 15，Fe 17和Fe 19簇）的相当密集的进展，并且这一系列提供了一种方法，可以系统地将小分子上可能的精确计算外推到扩展固体表面和地球化学所需的更近似的方法。这项研究将被许多学科使用，包括胶体化学，纳米科学和医学（金属蛋白质，如铁蛋白类似于这些簇）。此外，UCD的招生人数不成比例地来自美国新移民的高人口，因此这些资金将非传统但非常有才华的学生带入地球科学。我们在定量地球化学的最新技术方面培训了一批多样化的学生，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。