Nuclear Magnetic Resonance Spectroscopy of Complex Solids: Paramagnetism, Disorder and Nuclear Waste Materials

核磁共振波谱是研究固体结构的重要方法。它对于分析包含多个不同相的复杂系统特别有价值，特别是当其中一个相在结构上无序时。一些用于固定核电反应堆高放射性废物的玻璃就是这样的情况。当废料中的钼过多时，有些钼会安全地融入到耐用玻璃中，但有些钼也会分离成含有放射性离子的次生相，并可被地下水溶解。为了避免这种相的形成，有必要准确地了解是什么控制了钼进入玻璃相的结合，以及为什么其中一些钼形成了水溶性晶体。这项研究计划的一部分将使用核磁共振方法，这种方法可以具体检测两种类型的原子相互距离有多近，以确定负责相分离的原子水平的化学相互作用。作为温度的函数，可以用光谱观察到Mo吸引放射性Cs离子进入不同相的过程。这些知识将有助于优化废玻璃的成分和生产工艺，以避免钼驱动的相分离。由此产生的核废料玻璃将持续数千年而不会退化，确保了人类和环境的世代安全。*这项研究的第二部分将解决对具有未配对电子自旋的固体的复杂核磁共振谱的解释。这种顺磁材料很难用核磁共振观察，甚至更难用传统方法解释，因为顺磁相互作用往往会压倒通常用于结构解释的其他相互作用。然而，化学、生物学、地质学和材料科学中的许多感兴趣的系统都是顺磁性的，不能用核磁共振进行常规研究。我们将使用计算化学来映射在核磁共振中观察到的电子自旋密度及其与原子核的相互作用，作为预测实验观察到的频移和峰值强度的一种手段。这种方法最初将应用于众所周知的分子体系，这些体系在几何上相似，但具有不同的金属中心，因此具有不同的电子结构，从而可以直接比较未配对电子的作用。然后，这一策略将被扩展到调查局部分子结构的细微变化如何影响核磁共振信号，以此作为理解无序矿物和玻璃中顺磁效应的一种方式，包括用于核废料处理的材料。这项研究有望将顺磁相互作用从障碍转化为机遇，为核磁共振光谱研究开辟了广阔的新材料范围。

Nuclear magnetic resonance (NMR) spectroscopy is an important method for studying the structure of solids. It is especially valuable for analyzing complex systems containing more than one distinct phase, particularly when one of the phases is structurally disordered. This is the case for some glasses used to immobilize high-level radioactive waste from nuclear power reactors. When too much molybdenum is present in the waste stream, some of it becomes safely incorporated into the durable glass as intended, but some also separates into secondary phases which contain radioactive ions and can be dissolved by groundwater. To avoid the formation of such phases it is necessary to understand precisely what governs the integration of Mo into the glass phase and why some of it forms water-soluble crystals. Part of this research program will use NMR methods which can specifically detect how near two types of atoms are to each other to determine the atomic-level chemical interactions responsible for phase separation. Done as a function of temperature, the process by which Mo attracts radioactive Cs ions into separate phases can be observed spectroscopically. This knowledge will enable the optimization of waste glass compositions and production processes to avoid Mo-driven phase separation. The resulting nuclear waste glasses will last for millennia without degrading, ensuring human and environmental safety through the generations. ******A second part of this research will address the interpretation of complex NMR spectra of solids with unpaired electron spins. Such paramagnetic materials are very difficult to observe by NMR and even more difficult to interpret using conventional methods, as the paramagnetic interaction tends to overwhelm the other interactions typically used for structural interpretations. And yet, many systems of interest in chemistry, biology, geology and materials science are paramagnetic and cannot be routinely studied by NMR. We will use computational chemistry to map the electron spin density and its interaction with the nuclei observed in NMR as a means to predict the frequency shifts and peak intensities observed experimentally. This approach will initially be applied to well known molecular systems which are geometrically similar but possess different metal centres and hence different electronic structure, thereby allowing a direct comparison of the role of unpaired electrons. This strategy will then be extended to investigate how subtle changes in local molecular structure affect NMR signals as a way toward understanding paramagnetic effects in disordered minerals and glasses, including materials used for nuclear waste disposal. This research is expected to turn the paramagnetic interaction from obstacle to opportunity, opening up a vast new range of materials to investigation by NMR spectroscopy.**