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

点击翻译按钮获取中文摘要

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.