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)，以根据FeIII-OH2键长预测Fe(III)-氧化物矿物上的配体取代速率，该键长可以计算或测量。在上一个资助期，我们通过实验证明，在FeIII-OH2键长和溶剂交换速率之间可能存在这样的LFER。他们还使用了纳米尺寸的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”模拟方法与大纳米团簇中Al(III)金属水交换速率的实验数据结合起来。这项工作首次表明，表面的速度非常快，令人惊讶。现在，研究人员希望将这项工作扩展到Fe(III)(氢化物)氧化物材料。他们使用具有特别有用的FeIII-OH2键长和水稳定性的水溶液络合物来建立速率和键长的实验标度。在上一个资助期，他们掌握了一种用于Fe(III)等顺磁性金属的17O-核磁共振展宽方法。这种方法对地球化学来说是新的，但在其他方面是很好理解和可靠的。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17O-核磁共振测量，他们使用了一种新的电喷雾电离质谱仪(ESI-MS)方法，允许跟踪溶解分子中氧-同位素交换的速率。通过ESI-MS，我们跟踪了氧桥上的反应和成键的有机配体中的羧酸氧。通过这种方法，地球化学家可以研究一系列以前不可能实现的化合物，比如Mn(IV，III)-oxo原子团簇。在这个意义上，以及在其他意义上，这项研究对地球化学既是开创性的，也是变革性的。这项研究也是变革性的，因为水的交换率是描述水反应的最基本的时间尺度-这些结合水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必要的步骤。这项研究具有变革性，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种方法。他们正试图使水中的地球化学在分子尺度上实现量化。其学术价值在于对水中反应性的基础研究，他们回答了这样的问题：在足够大到与地球化学相关的系统中，什么类型的计算成本最低的方法可以产生隐藏的反应路径？这项研究还与人们对纳米团簇在天然水化学中、在运输毒物或作为毒物本身以及作为无定形材料的基本构件方面的认识增加有关。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使用，包括胶体化学、纳米科学、材料科学和医学(铁蛋白等金属蛋白和紫色酸性磷酸酶等酶与这些团簇相似)。此外，UCD的招生人数不成比例地来自美国的大量新移民，并吸引他们进入化学和地球化学专业。这些基金将非传统但才华横溢的学生带入地球科学，这些学生构成了加州大学的人口。

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.