NER: Can Aluminum Nanocluster Complexes Yield Rates of Ligand Exchange at Aluminous Mineral Surfaces?

CASEYNER-0207709本提案是对NSF 01-157类纳米科学与工程倡议的响应，由地球观测组织理事会共同出资。地球化学家关心的反应通常是配体交换反应，即金属内部配位球中的一个原子被另一个原子取代。即使是像吸附和矿物溶解这样的复杂过程也是真正的配体交换反应，因为金属的配位数在这个过程中是不变的。对于不同的金属，这些反应的速度差别很大，但人们对矿物表面的反应速度知之甚少，因为矿物表面是影响天然水化学的关键部位。换句话说，粘土和金属氢氧化物矿物上的官能团也是配体。地球化学家越来越严重地依赖于预测反应性质的计算方法，因为许多关键反应很难通过实验进行探索。不幸的是，预测水-矿物界面上的配体交换速率的计算成本是巨大的，因为现实的模拟中必须包括如此多的原子。我们寻求另一种策略，试图在测量的铝络合物和纳米颗粒中的水交换速率系数与使用从头计算方法计算的络合物中的结构参数之间建立关联。这种关联是新颖的，因为它基于络合物的容易计算的性质，而不是基于昂贵的过渡态结构，并提供了一种简单且计算成本低廉的方法来预测最基本的一类地球化学反应的速率。根据假设，这种简单的方法可以扩展到纳米颗粒、胶体和粘土，并且只受到计算机模拟可行性的限制。这项研究是有风险的，因为不能保证重要的变量会随着分子大小的变化而变化。然而，如果成功，它有望成为一种廉价的方法来预测在水介质中最基本的反应的速率，涉及分子大小的巨大范围。这是一种新颖而影响深远的方法，因为它是基于基态结构的性质，而不是过渡态。

CaseyNER-0207709This proposal was received in response to the Nanoscale Science and Engineering initiative NSF 01-157, category NER, and is co-funded by the GEO Directorate. The reactions that concern geochemists are often ligand exchange reactions, where one atom in the inner-coordination sphere of a metal is replaced with another. Even complicated processes like adsorption and mineral dissolution are really ligand-exchange reactions because the coordination number of the metal is unchanged by the process. The rates of these reactions vary enormously for different metals yet very little is known about the reaction rates at mineral surfaces, which are the key sites that affect natural water chemistry. Stated differently, the functional groups on clays and metal-hydroxide minerals are ligands too. Geochemists are coming to rely heavily on computational methods of predicting reaction properties because so many of the key reactions are difficult to probe experimentally. Unfortunately, the computational cost of predicting rates of ligand exchanges at the aqueous-mineral interface are enormous because so many atoms must be included in a realistic simulation. We pursue an alternative strategy where we attempt to establish a correlation between measured rate coefficients of water exchange ( ) in aluminum complexes and nanoparticles and structural parameters in the complexes that were calculated using ab initio methods. This correlation is novel because it is based on the easily calculated properties of the complexes instead of on the costly transition-state structures and provides a simple and computationally inexpensive way to predict rates of one of the most fundamental classes of geochemical reactions. By hypothesis, this simple approach can be extended to nanoparticles, colloids, and clays and is limited only by the feasibility of computer simulations. This research is risky because there is no guarantee that the important variables will scale with molecular size. Nevertheless, if successful, it promises an inexpensive method to predict rates of the most fundamental of reactions in aqueous media across an enormous range of molecular sizes. It is a novel and far-reaching approach because it is based on properties of ground-state structures rather than transition states.