Novel half-integer quadrupolar solid-state NMR correlation experiments for probing atomic proximities and connectivities in disordered materials

当科学家调查问题时，就像所有优秀的侦探一样，他们需要线索来了解正在发生的事情。对于一系列关键问题，能够揭示原子周围局部环境的技术对于深入了解这个级别的结构至关重要，这通常决定了材料或分子的行为。核磁共振（NMR）光谱学在整个科学领域的重要性日益增加，因为它是一种元素特异性探针，可以区分不同位置周围环境的非常小的变化（例如，硼原子是与三个还是四个氧原子结合，从而采用三角形还是四面体排列）。核磁共振利用了所有原子中心的原子核固有的磁性：就像指南针在地球磁场中的排列一样，当放置在强磁场中时，核磁体有一个优先的方向。然而，这种偏好是微弱的，并且可以通过施加谐振无线电波，即其频率和能量与翻转核磁体所需的能量精确匹配的无线电波，使核磁体改变其方向，例如，从与磁场方向对齐到与磁场方向相反。原子核周围的电子也具有固有的磁性，并受到磁场存在的影响。重要的是，特定原子核的共振频率非常敏感地依赖于电子的这种额外响应，因此原子核充当了局部电子环境的间谍，从而产生了特定的化学键，允许它用于探测上述环境。不同核同位素的共振频率被很好地分离，因此核磁共振谱是特定于特定选择的同位素的。（一种元素可以以不同的同位素形式存在，即原子核中质子数量相同，中子数量不同。）这个项目考虑了所谓的四极核，它具有四极电子矩（即，原子核中的电荷分布不均匀）。超过三分之二的同位素都是这样的四极核，许多重要的元素，如锂、硼、氧、钠、铝只有四极核磁共振活性同位素。由于电子产生的四极矩与环境的强相互作用，导致核磁共振谱线宽，因此四极核通常是困难的。核磁共振的关键优势之一是原子核经历相互作用，传递有关其周围环境的信息。作为一个例子，偶极相互作用的产生是由于核磁铁不是孤立的，而是它们以类似的方式相互作用，当两个条形磁铁靠近时，它们要么吸引，要么排斥。一个相关的相互作用是J耦合，其中原子核之间的电子使一个原子核能够感知与它化学键合的另一个原子核。该项目将开发新的核磁共振实验，适用于固体样品，使用偶极相互作用和相关的J耦合来识别四极核之间的通过空间接近或通过键连接。检验一项好技术的标准是它是否适用于广泛的问题。在这个项目中，新的核磁共振实验将用于确定玻璃的原子尺度结构，这些玻璃可以应用于电池、牙科水泥、烤箱、望远镜镜和放射性废物的固定。材料的本体结构与其隐藏的原子尺度结构之间总是存在联系，因此更好地了解后者将有助于开发更好的材料。正是通过以问题为基础的科学家和以技术为基础的科学家之间的合作，才取得了真正的进展。

When scientists investigate problems, like all good detectives they need clues as to what is happening. For a whole range of key problems, techniques that can reveal the local environment around an atom are crucial to provide insight into the structure at this level, which often governs how a material or molecule behaves. Nuclear Magnetic Resonance (NMR) spectroscopy has increased in importance throughout the sciences as it is an element-specific probe that can distinguish very small changes in the surroundings of different sites (e.g. whether a boron atom is bonded to three or four oxygen atoms and hence adopts a trigonal or a tetrahedral arrangement). NMR exploits the inherent magnetism of atomic nuclei which are at the centre of all atoms: like the alignment of a compass needle in the Earth's magnetic field, nuclear magnets have a preferred direction when placed in a strong magnetic field. This preference, however, is weak and a nuclear magnet can be made to change its direction from, e.g., being aligned with to being aligned against the direction of the magnetic field, by applying a resonant radio wave, i.e., one whose frequency and hence energy matches precisely the energy required to flip the nuclear magnet. The electrons surrounding the atomic nucleus are also inherently magnetic and are affected by the presence of a magnetic field. Importantly, the resonant frequency of a particular nucleus depends very sensitively on this additional response of the electrons, such that the atomic nuclei act as spies of the local electron environment and hence the specific chemical bonding, allowing it to be used to probe environments as described above. The resonant frequency of different nuclear isotopes are well separated such that an NMR spectrum is specific to a particular chosen isotope. (An element can exist as different isotopes whereby there is the same number of protons but a different number of neutrons in the nucleus.) This project considers so-called quadrupolar nuclei which have a quadrupole electronic moment (i.e., there is a non-uniform distribution of electric charge in the nucleus). Over two-thirds of all isotopes are such quadrupolar nuclei, and many important elements, e.g., lithium, boron, oxygen, sodium, aluminium only have NMR-active isotopes that are quadrupolar. Quadrupolar nuclei are often difficult because the strong interaction of the quadrupole moment with the environment generated by the electrons leads to broad lines in NMR spectra. One of the key advantages for NMR is that nuclei experience interactions that convey information about their surroundings. As an example, the dipole interaction arises as the nuclear magnets are not isolated, but rather they interact in an analogous way to how two bar magnets either attract or repel when brought close together. A related interaction is the J coupling where the electrons between the nuclei enable one nucleus to sense another nucleus to which it is chemically bonded. This project will develop new NMR experiments applicable to solid samples that use dipolar interactions and related J couplings to identify through-space proximities or through-bond connectivities between quadrupolar nuclei. A test of a good technique is that it is applicable to a wide range of problems. In this project, the new NMR experiments will be used to determine the atomic-scale structure of glasses that have applications in batteries, dental cement, ovenware, telescope mirrors, and radioactive waste immobilisation. There is always a link between the bulk structure of a material and its hidden atomic-scale structure, hence a better understanding of the latter will enable better materials to be developed. It is through the partnership between problem-based and technique-based scientists that real progress is made.