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- W2949205524 abstract "The determination of the arrangements of atoms in a crystal performed by NMR defines the concept of NMR crystallography. The scope of crystallography ranges from macroscopic morphological studies of crystals to the localization of atoms in a unit cell. To perform a structure determination one can collect by NMR many pieces of information, and, by using the relation between the geometry and the actual measurement, establish the structure. This relation between NMR and geometry, nowadays, is mainly established by modeling the crystal by quantum computations, and iterating the geometrical parameters until modeling results and experimental observations are in a fair agreement. The main items of information that can be extracted quite straightforwardly by NMR on a polycrystalline powder are the distances between atoms (using the dipolar interaction); the number of inequivalent crystalline sites in the asymmetric unit; and (by using the crystallographic multiplicity of each site) the compatible space groups.The main difference between NMR and diffraction crystallographies lies in the measurement strategy. Usually, a single recording of data is necessary for diffraction, while NMR requires multiple experiments. Therefore, the structure determination strategies are usually quite different. In NMR, one first builds up a topological model of connectivities and proximities, and finishes by the determination of the metric (unit cell and atomic positions), by using distances and angles. The periodic part of the crystal is therefore built. Additionally, one can focus on nonperiodic aspects of the crystal and investigate those parts of crystals that are not considered when dealing with only the periodic lattices. Furthermore, the elements of symmetry of motion, very often associated with fractional occupancy factors or the reciprocal order of the element of symmetry (1/2, 1/3 or 1/4) can be probed, leading to an extended description of crystals.NMR can be also coupled to diffraction to perform much more accurate structure description. NMR physical observables like the chemical shift tensors and the dipolar or quadrupolar interaction may also be modeled to refine the positions of atoms. These contributions to crystallography (diffraction, modeling, and NMR) are three important components of a general crystallography that will include the description of regular structures, among which the periodic arrangements of atoms is just a subclass.Keywords:NMR crystallography;general crystallography;crystal structure;structure determination;structure refinement;polycrystalline powders;crystallography principles" @default.
- W2949205524 created "2019-06-27" @default.
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- W2949205524 date "2010-10-28" @default.
- W2949205524 modified "2023-10-17" @default.
- W2949205524 title "ChemInform Abstract: Fundamental Principles of NMR Crystallography" @default.
- W2949205524 doi "https://doi.org/10.1002/chin.201047263" @default.
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