Realising the combined potential of solid-state NMR and structural databases

晶体结构数据库是固态研究人员的重要工具。最初被建立为实验确定的结构的储存库，包含在数据库中的大型数据集，例如剑桥结构数据库（CSD），通过“数据挖掘”的发展，已经成为研究的主题。然而，这些数据库的用处在很大程度上取决于其所含数据的质量。在绝大多数情况下，结构通过X射线衍射（XRD）获得。虽然XRD是建立晶体材料三维结构的卓越工具，但XRD研究仍存在一些困难，晶体学家需要一些技巧来建立正确的结构。例如，氢原子对X射线的散射非常弱，并且具有相同（OH vs F）或非常相似（Si vs Al）电子数的碎片很难区分。此外，晶体的规则有序的任何破坏都会对结构解决方案产生重大挑战;衍射不是理解这种“无序”的天然工具。过去，XRD专家使用“R因子”等指标来评估所提出的结构与实验数据的吻合程度，但理想情况下，独立的实验证据将用于验证晶体结构。近年来，我们和其他研究小组已经证明，固态核磁共振（SS-NMR）现在可以非常有效地用于区分不同的可能晶体结构。量子化学的发展（主要是通过密度泛函理论）使得核磁共振光谱能够以极高的精度进行计算。由于核磁共振谱对局部结构的微小变化非常敏感--一个皮米（10^-12 m）量级的偏差就会使谱发生可测量的变化--因此即使晶体结构溶液中很小的缺陷也能被识别出来。此外，不同类型的“无序”（例如，由于原子的运动或不规则的原子位置）对NMR谱有明显而独特的影响。该提案旨在通过固态NMR和计算化学开发验证晶体结构的系统方法。我们将建立哪些NMR实验是必需的，以区分晶体结构的解决方案，也“验证”的结构解决方案。这将涉及创建“NMR置信参数”，该参数将测量结构与可用NMR数据兼容的程度，并且这些参数的有效性将通过更传统的基于衍射的工具进行验证。通过采取系统的方法，我们将能够展示如何使用NMR来解决不同类型的结构模糊性，并展示NMR作为传统衍射研究的补充的价值。

Databases of crystal structures are essential tools for researchers working in the solid state. Initially established as repositories of experimentally determined structures, the large data sets contained within databases, such as the Cambridge Structural Database (CSD), have become the subject of research in their own right through the development of "data mining". The usefulness of such databases is, however, highly dependent on the quality of the data they contain. In the vast majority of cases the structures were obtained via X-ray diffraction (XRD). While XRD is the pre-eminent tool for establishing the three-dimensional structure of crystalline materials, there are areas where XRD studies struggle and some art is required on the part of the crystallographer to establish a correct structure. For instance, hydrogen atoms scatter X-rays very weakly, and fragments with the same (OH vs F) or very similar (Si vs Al) numbers of electrons are very hard to distinguish. In addition, any disruption of the regular ordering of a crystal creates major challenges for structure solution; diffraction is not the natural tool for understanding such "disorder". Historically XRD experts have used measures such as "R factors" to assess how well a proposed structure fits to the experimental data, but ideally independent experimental evidence would be used to verify crystal structures.We and other research groups have shown in recent years that solid-state nuclear magnetic resonance (SS-NMR) can now be used very effectively to distinguish between different possible crystalline structures. Developments in quantum chemistry (mostly notably through Density Functional Theory) allow NMR spectra to be calculated with excellent precision. Since the NMR spectrum is sensitive to very small changes in the local structure - deviations of the order of a picometre (10^-12 m) will change the spectrum measurably - even small imperfections in a crystal structure solution can be identified. Moreover different types of "disorder" e.g. due to the motion of atoms or irregular atomic positioning, have clear and distinct effects on the NMR spectrum.This proposal seeks to develop systematic approaches to the validation of crystal structures via solid-state NMR and computational chemistry. We will establish which NMR experiments are required in order to distinguish crystal structure solutions and also to "validate" a structural solution. This will involve the creation of "NMR confidence parameters" which will measure the extent to which a structure is compatible with the NMR data available, and the effectiveness of these parameters will be verified against more traditional diffraction-based tools. By taking a systematic approach, we will be able to show how NMR can be used to resolve the different types of structural ambiguity and show the value of NMR as a complement to conventional diffraction-based studies.