Least-squares absolute-structure refinement. A case study of the effect of absorption correction, data region, stability constant and neglect of light atoms

Bernardinelli, G.; Flack, H. D.

doi:10.1107/s0108767387099859

Cited by 23 publications

(3 citation statements)

References 2 publications

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“…Early results 16,17 and subsequent experience from many crystal-structure determinations have shown that the Flack parameter is robust and converges in only a few cycles to its final value during least-squares refinement. However, as the Flack parameter is one of many parameters of the physical model of a crystal structure, the values of which are to be found by optimization based on some general criterion, it is essential in the final cycles of optimization that all parameters be varied jointly and simultaneously.…”

Section: Least-squares Refinementmentioning

confidence: 98%

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The use of X‐ray crystallography to determine absolute configuration

2007

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Essential background on the determination of absolute configuration by way of single-crystal X-ray diffraction (XRD) is presented. The use and limitations of an internal chiral reference are described. The physical model underlying the Flack parameter is explained. Absolute structure and absolute configuration are defined and their similarities and differences are highlighted. The necessary conditions on the Flack parameter for satisfactory absolute-structure determination are detailed. The symmetry and purity conditions for absolute-configuration determination are discussed. The physical basis of resonant scattering is briefly presented and the insights obtained from a complete derivation of a Bijvoet intensity ratio by way of the mean-square Friedel difference are exposed. The requirements on least-squares refinement are emphasized. The topics of right-handed axes, XRD intensity measurement, software, crystal-structure evaluation, errors in crystal structures, and compatibility of data in their relation to absolute-configuration determination are described. Characterization of the compounds and crystals by the physicochemical measurement of optical rotation, CD spectra, and enantioselective chromatography are presented. Some simple and some complex examples of absolute-configuration determination using combined XRD and CD measurements, using XRD and enantioselective chromatography, and in multiply-twinned crystals clarify the technique. The review concludes with comments on absolute-configuration determination from light-atom structures.

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Section: Least-squares Refinementmentioning

confidence: 98%

“…The twin symmetry operations are of determinant 11 for matrices 1 and 2, and 21 for matrices 3 and 4. In the nomenclature of the current analysis, one has x (17). x 2 is equivalent to the Flack x parameter for this multiply-twinned crystal.…”

Section: Determination Of Absolute Configuration Frommentioning

confidence: 99%

The use of X‐ray crystallography to determine absolute configuration

2007

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“…This is unusual because although having a complete set of Freidel pairs is undoubtedly useful, § they are not actually required when refining the Flack "x" parameter. 18 In the case of 1-deoxy-l-arabinitol, it is possible that that problem lies in the pseudo-centrosymmetric nature of the structure. The structure crystallises in the space group I4 1 , but the molecule has a pseudo inversion centre which lies between C2 and C3 (Fig.…”

Section: The Flack "X" Parameter For 1-deoxy-l-arabinitolmentioning

confidence: 99%

Some experimental aspects of absolute configuration determination using single crystal X-ray diffraction

Thompson

Jenkinson

Fleet

2017

Tetrahedron: Asymmetry

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Students of single crystal X-ray diffraction are often give advice as to how best to collect their data when attempting absolute configuration determination. These "rules" often have more grounding in gut-feeling than evidence. Thus, in an effort to provide advice and evidence that today's crystallographers can pass onto to tomorrow's young scientists, we present a systematic study of 1-deoxy-l-arabinitol, a straight chain sugar which crystalises well in the space group I4 1 .

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