scholarly journals A Bayesian approach to NMR crystal structure determination

2019 ◽  
Vol 21 (42) ◽  
pp. 23385-23400 ◽  
Author(s):  
Edgar A. Engel ◽  
Andrea Anelli ◽  
Albert Hofstetter ◽  
Federico Paruzzo ◽  
Lyndon Emsley ◽  
...  
Keyword(s):  
Crystal Structure ◽  
Chemical Shift ◽  
Bayesian Approach ◽  
Structure Determination ◽  
Bayesian Framework ◽  
Crystal Structure Determination ◽  
Structure Determinations

We introduce a Bayesian framework for quantifying the reliability of structure determinations for powdered samples on the basis of NMR experiments and chemical shift predictions (with uncertainties) for a pool of candidate structures.

scholarly journals The crystal structure of alstonite, BaCa(CO3)2: an extraordinary example of ‘hidden’ complex twinning in large single crystals

2020 ◽  
Vol 84 (5) ◽  
pp. 699-704
Author(s):  
Luca Bindi ◽  
Andrew C. Roberts ◽  
Cristian Biagioni
Keyword(s):  
Crystal Structure ◽  
Single Crystals ◽  
Structure Determination ◽  
Unit Cell ◽  
Crystal Structure Determination ◽  
Unit Cell Parameters ◽  
Space Group ◽  
Cell Parameters ◽  
Structure Determinations

AbstractAlstonite, BaCa(CO3)2, is a mineral described almost two centuries ago. It is widespread in Nature and forms magnificent cm-sized crystals. Notwithstanding, its crystal structure was still unknown. Here, we report the crystal-structure determination of the mineral and discuss it in relationship to other polymorphs of BaCa(CO3)2. Alstonite is trigonal, space group P31m, with unit-cell parameters a = 17.4360(6), c = 6.1295(2) Å, V = 1613.80(9) Å3 and Z = 12. The crystal structure was solved and refined to R1 = 0.0727 on the basis of 4515 reflections with Fo > 4σ(Fo) and 195 refined parameters. Alstonite is formed by the alternation, along c, of Ba-dominant and Ca-dominant layers, separated by CO3 groups parallel to {0001}. The main take-home message is to show that not all structure determinations of minerals/compounds can be solved routinely. Some crystals, even large ones displaying excellent diffraction quality, can be twinned in complex ways, thus making their study a crystallographic challenge.

scholarly journals Some reflections on symmetry: pitfalls of automation and some illustrative examples

2019 ◽  
Vol 75 (12) ◽  
pp. 1812-1819 ◽  
Author(s):  
William Clegg
Keyword(s):  
Crystal Structure ◽  
Structure Determination ◽  
Crystal Structure Determination ◽  
Structure Validation ◽  
X Ray Diffraction ◽  
X Ray ◽  
Structure Determinations ◽  
Basic Concepts ◽  
Software Automation

In the context of increasing hardware and software automation in the process of crystal structure determination by X-ray diffraction, and based on conference sessions presenting some of the experience of senior crystallographers for the benefit of younger colleagues, an outline is given here of some basic concepts and applications of symmetry in crystallography. Three specific examples of structure determinations are discussed, for which an understanding of these aspects of symmetry avoids mistakes that can readily be made by reliance on automatic procedures. Topics addressed include pseudo-symmetry, twinning, real and apparent disorder, chirality, and structure validation.

Crystal Structure Determination of Glycerol-Containing Lipids from Electron Diffraction Data. Use of Analog Compounds Based upon Configurational Isomers of Cyclopentane-1, 2, 3-TRIOL

1977 ◽  
Vol 35 ◽  
pp. 24-25
Author(s):  
Douglas L. Dorset ◽  
Anthony J. Hancock
Keyword(s):  
Crystal Structure ◽  
Electron Diffraction ◽  
Diffraction Data ◽  
Structure Determination ◽  
Crystal Surface ◽  
Coherent Scattering ◽  
Crystal Structure Determination ◽  
Electron Diffraction Data ◽  
Polar Group ◽  
Axis Orientation

Lipids containing long polymethylene chains were among the first compounds subjected to electron diffraction structure analysis. It was only recently realized, however, that various distortions of thin lipid microcrystal plates, e.g. bends, polar group and methyl end plane disorders, etc. (1-3), restrict coherent scattering to the methylene subcell alone, particularly if undistorted molecular layers have well-defined end planes. Thus, ab initio crystal structure determination on a given single uncharacterized natural lipid using electron diffraction data can only hope to identify the subcell packing and the chain axis orientation with respect to the crystal surface. In lipids based on glycerol, for example, conformations of long chains and polar groups about the C-C bonds of this moiety still would remain unknown.One possible means of surmounting this difficulty is to investigate structural analogs of the material of interest in conjunction with the natural compound itself. Suitable analogs to the glycerol lipids are compounds based on the three configurational isomers of cyclopentane-1,2,3-triol shown in Fig. 1, in which three rotameric forms of the natural glycerol derivatives are fixed by the ring structure (4-7).

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