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）来预测Fe(III)-氧化物矿物上的配体取代率，从feii - oh2键长度可以计算或测量。在上一个资助期，我们通过实验证明了这种LFER可能存在于feii - oh2键长和溶剂交换速率之间。他们还使用了类似的纳米尺寸Al（III）离子的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簇中Al（III）金属的水交换速率的实验数据相结合。这项工作首次表明，月球表面的速度异常惊人地快。研究人员现在想把这项工作扩展到铁（III）（水合）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配合物来建立速率和键长的实验尺度。在上一个资助期内，他们掌握了顺磁性金属如Fe（III）的17O-NMR增谱方法。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，他们就会重复他们的“罕见事件”模拟，以估计实验上无法达到的Fe(III)-（水合）氧化物表面结构的速率。为了桥接氧，并补充17O-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。用ESI-MS，我们跟踪在氧桥和羧酸氧键有机配体中的反应。通过这种方法，地球化学家可以研究大量以前不可能研究的化合物，例如Mn(IV，III)-氧簇。在这个意义上，以及在其他意义上，这项研究对地球化学来说既是开创性的，也是革命性的。这项研究也具有变革性，因为水交换速率是描述水反应的最基本的时间尺度——在大多数配体取代（“吸附”）和许多电子交换之前，这些结合水的缓慢损失。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基本步骤。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，这将被地球化学家大量使用，以应对不可能进行实验的情况。他们正试图使水地球化学在分子尺度上定量。智力优势是在水反应性的基础研究中，他们回答这样的问题：“什么是最有前途的计算廉价的方法，可以在足够大的系统中产生隐藏的反应途径，从而与地球化学相关？”这项研究也与人们对天然水化学中纳米大小的团簇的认识增加有关，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（铁蛋白等金属蛋白和紫酸性磷酸酶等酶类似于这些簇）。此外，UCD的入学人数不成比例地吸引了大量新移民到美国，这吸引了他们学习化学和地球化学。这些资金将非传统但才华横溢的学生带入地球科学，这是UCD的人口组成部分。

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.