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必不可少的工具。最初是作为实验确定的结构的存储库建立的，包含在数据库中的大型数据集，如剑桥结构数据库（CSD），通过“数据挖掘”的发展，已经成为他们自己的研究主题。然而，这种数据库的有用性在很大程度上取决于它们所包含的数据的质量。在绝大多数情况下，结构是通过x射线衍射（XRD）获得的。虽然XRD是建立晶体材料三维结构的卓越工具，但XRD研究仍有一些领域存在困难，并且晶体学家需要一些技巧才能建立正确的结构。例如，氢原子对x射线的散射非常弱，电子数量相同（OH vs F）或非常相似（Si vs Al）的碎片很难区分。此外，任何对晶体常规秩序的破坏都会给结构解决带来重大挑战；衍射不是理解这种“无序”的自然工具。历史上，XRD专家使用诸如“R因子”之类的方法来评估所提出的结构与实验数据的匹配程度，但理想情况下，应该使用独立的实验证据来验证晶体结构。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可能的晶体结构。量子化学的发展（主要是通过密度泛函理论）使核磁共振光谱的计算具有极高的精度。由于核磁共振光谱对局部结构的微小变化非常敏感——1皮米（10^-12米）量级的偏差会显著地改变光谱——即使是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”，如由于原子的运动或不规则的原子定位，对核磁共振光谱有明显而不同的影响。本提案旨在通过固态核磁共振和计算化学开发系统的方法来验证晶体结构。我们将确定需要哪些核磁共振实验来区分晶体结构溶液，并“验证”结构溶液。这将涉及创建“核磁共振置信参数”，该参数将测量结构与可用核磁共振数据兼容的程度，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。通过采用系统的方法，我们将能够展示核磁共振如何用于解决不同类型的结构歧义，并显示核磁共振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.