Establishing the Kinetics of Aqueous Reactions at Fe(III) Molecules and Minerals

研究人员建议建立一个线性自由能关系（LFER），从FeIII-OH 2键长预测Fe（III）-氧化物矿物上的配体取代率，可以计算或测量。在上一个资助期间，我们通过实验表明，这种LFER可能存在于FeIII-OH 2键长和溶剂交换率之间。 他们还使用了纳米尺寸Al（III）离子的类似数据来建立铝表面结构的LFER。他们将“罕见事件”模拟方法与大型纳米尺寸团簇中Al（III）金属水交换速率的实验数据相结合。这项工作首次表明，在表面的速率是非常惊人的快。研究人员现在希望将这项工作扩展到Fe（III）（氢）氧化物材料。他们使用具有特别有用的FeIII-OH 2键长和水稳定性的水性络合物来建立速率和键长的实验规模。在上一个资助期内，他们掌握了一种用于Fe（III）等顺磁性金属的17 O-NMR谱线展宽方法。这种方法对地球化学来说是新的，但在其他方面很好理解和可靠。一旦建立了实验规模，他们就重复他们的“稀有事件”模拟，以估计实验上无法获得的Fe（III）-（hydr）氧化物表面结构的速率。为了桥接氧，并补充17 O-NMR测量，他们采用了一种新的电喷雾电离质谱（ESI-MS）方法，可以跟踪溶解分子中氧同位素交换的速率。通过ESI-MS，我们跟踪了键合有机配体中氧桥和羧酸氧的反应。通过这种方法，地球化学家可以研究以前不可能的大量化合物，例如Mn（IV，III）-氧代簇。在这个意义上，以及在其他意义上，这项研究是开创性的和变革性的地球化学。这项研究也是变革性的，因为水交换速率是描述水反应的最基本的时间尺度-这些结合沃茨的缓慢损失先于大多数配体取代（“吸附”）和许多电子交换。在土壤中，这些反应通常是复杂网络的一部分，但这些基本反应控制着关键步骤。这项研究是变革性的，因为反应也处于适当的规模，以改进模拟方法，地球化学家将在不可能进行实验的情况下大量使用这些方法。他们正试图使水地球化学定量在分子尺度上。智力的优点是在水反应性的基础研究，他们回答的问题，如：“什么是最有前途的类型的计算成本低的方法，可以产生隐藏的反应途径，系统是足够大的地球化学相关？“这项研究还与人们对纳米尺度的团簇在自然水化学中的认识提高有关，在运输有毒物质或作为有毒物质本身，以及作为无定形材料的基本组成部分。更广泛的影响远远超出了地球科学，因为这项研究被许多学科所使用，包括胶体化学，纳米科学，材料科学和医学（金属蛋白如铁蛋白和酶如紫色酸性磷酸酶类似于这些簇）。此外，UCD的入学率不成比例地从新移民的高人口中吸引到美国，并吸引他们进入化学和地球化学。这些资金使非传统的，但非常有才华的学生进入地球科学，其中包括UCD的人口。

Investigators propose to establish a linear-free-energy relation (LFER) to predict rates of ligand substitution on Fe(III)-oxide minerals from the FeIII-OH2 bond lengths, which can either be calculated or measured. In the last funding period we showed experimentally that such a LFER probably exists between FeIII-OH2 bond lengths and rates of solvent exchanges. They also used similar data on nanometer-size Al(III) ions to establish a LFER for aluminous surface structures. They coupled 'rare event' simulation methods to experimental data on rates of water-exchanges from Al(III) metals in large nanometersize clusters. This work showed, for the first time, that the rates at the surfaces are extraordinarily and surprisingly rapid.Investigators now want to extend the work to Fe(III) (hydr)oxide materials. They use aqueous complexes with particularly useful FeIII-OH2 bond lengths and aqueous stabilities to establish an experimental scale of rates and bond lengths. In the last funding period, they mastered a 17O-NMR line-broadening method for paramagnetic metals like Fe(III). This method is new to geochemistry, but otherwise well understood and dependable. Once the experimental scale is established, they repeat their 'rare-event' simulations to estimate rates for Fe(III)-(hydr)oxide surface structures that are experimentally inaccessible.For bridging oxygens, and to complement the 17O-NMR measurements, they employ a new ElectroSpray-Ionization Mass-Spectrometry (ESI-MS) method that allows to follow the rates of oxygen-isotope exchanges in the dissolved molecules. With ESI-MS, we follow reactions at oxo bridges and the carboxylate oxygens in a bonded organic ligand. With this method, geochemists can study an enormous range of compounds that were previously impossible, such as the Mn(IV,III)-oxo clusters. In this sense, and in others, this research is both pioneering and transformative to geochemistry.This research is also Transformative because water-exchange rates are the most fundamental timescale for describing aqueous reactions---slow loss of these bound waters precedes most ligand substitutions ('adsorptions') and many electron exchanges. In soils, these reactions are usually part of a complex network, but these elementary reactions control the essential step. This research is Transformative because reactions are also at the appropriate scale to improve methods of simulation, which will be heavily employed by geochemists for cases where experiments are impossible. They are trying to make aqueous geochemistry quantitative at the molecule scale.The Intellectual Merit is fundamental research in aqueous reactivity where they answer questions such as: 'What are the most promising types of computationally inexpensive methods that can yield hidden reaction pathways in systems that are sufficiently large to be geochemically relevant?' The research also ties to the increased awareness about nanometer-size clusters in natural water chemistry, in transporting toxicants or as toxicants themselves, and as the fundamental building block for amorphous materials.The Broader Impacts extend well beyond Earth science because this research is used by many disciplines, including colloid chemistry, nanoscience, materials science and medicine (metalloproteins such as ferritin and enzymes such as the purple-acid phosphatases resemble these clusters). Also, enrollment at UCD draws disproportionately from the high population of new immigrants to the United States and lures them into Chemistry and Geochemistry. These funds bring nontraditional but highly talented students into the Earth Sciences, which comprise the population of UCD.