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

核磁共振（NMR）光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。这是一些玻璃的情况下，用来掩盖高放射性废物从核电反应堆。当废物流中存在过多的钼时，其中一些会安全地融入耐用玻璃中，但也有一些会分离成含有放射性离子的次级相，并可被地下水溶解。为了避免形成这样的相，有必要精确地了解是什么控制了Mo进入玻璃相的整合，以及为什么其中一些形成水溶性晶体。该研究计划的一部分将使用核磁共振方法，该方法可以专门检测两种类型的原子彼此的接近程度，以确定导致相分离的原子级化学相互作用。作为温度的函数，钼吸引放射性铯离子进入分离相的过程可以用光谱观察到。这一知识将使废玻璃组合物和生产工艺的优化，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降解，确保人类和环境的安全。这项研究的第二部分将解决与未成对电子自旋固体的复杂的NMR光谱的解释。这种顺磁性材料很难通过NMR观察到，甚至更难使用常规方法解释，因为顺磁性相互作用往往压倒通常用于结构解释的其他相互作用。然而，在化学、生物学、地质学和材料科学中，许多感兴趣的系统都是顺磁性的，不能用NMR进行常规研究。我们将使用计算化学来绘制电子自旋密度及其与NMR中观察到的原子核的相互作用，作为预测实验观察到的频率偏移和峰强度的手段。这种方法最初将被应用于众所周知的分子系统，这些系统在几何形状上相似，但具有不同的金属中心，因此具有不同的电子结构，从而允许直接比较未成对电子的作用。然后，这一策略将被扩展到研究局部分子结构的细微变化如何影响NMR信号，以了解无序矿物和玻璃（包括用于核废料处理的材料）中的顺磁效应。这项研究有望将顺磁相互作用从障碍变为机遇，为核磁共振光谱学的研究开辟了一个广阔的新材料范围。

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.