Howard Flack and the Flack Parameter

Sep 262020
 

Chemistry 2020, 2(4), 796-804; DOI: 10.3390/chemistry2040052

The Flack Parameter is now almost universally reported for all chiral materials characterized by X-ray crystallography. Its elegant simplicity was an inspired development by Howard Flack, and although the original algorithm for its computation has been strengthened by other workers, it remains an essential outcome for any crystallographic structure determination. As with any one-parameter metric, it needs to be interpreted in the context of its standard uncertainty.

Some Experimental Aspects of Absolute Configuration Determination using Single Crystal X-​Ray Diffraction

Nov 012017
 

Tetrahedron Asymmetry 2017, 28(10), 1330-1336 [doi:10.1016/j.tetasy.2017.08.016]

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 crystallizes well in the space group I41.

 

Arabinitol

HUG and SQUEEZE: using CRYSTALS to incorporate resonant-scattering in the SQUEEZE structure factor contributions to determine absolute structure

Oct 232017
 

Acta Crystallographica, 2017, C73, 845–853. [ doi:10.1107/S2053229617013304 ]

Using an approximate correction to the X-ray scattering from disordered, resonantly scattering regions of crystal structures we have developed and tested a procedure (HUG) to recover the absolute structure using conventional Flack x refi nement or other post-re finement determination methods.

Why direct and post-refinement determinations of absolute structure may give different results

Sep 192016
 

Acta. Cryst. (2016) B72 661-683 (Feature Article) [ doi:10.1107/S2052520616012890 ]

surfaceDirect determination of the Flack parameter as part of the structure refinement procedure usually gives different, though similar, values to post-refinement methods. The source of this discrepancy has been probed by analysing a range of data sets taken from the recent literature. Most significantly, it was observed that the directly refined Flack (x) parameter and its standard uncertainty are usually not much influenced by changes in the refinement weighting schemes, but if they are then there are probably problems with the data or model. Post-refinement analyses give Flack parameters strongly influenced by the choice of weights. Weights derived from those used in the main least squares lead to post-refinement estimates of the Flack parameters and their standard uncertainties very similar to those obtained by direct refinement. Weights derived from the variances of the observed structure amplitudes are more appropriate and often yield post-refinement Flack parameters similar to those from direct refinement, but always with lower standard uncertainties. Substantial disagreement between direct and post-refinement determinations are strongly indicative of problems with the data, which may be difficult to identify. Examples drawn from 28 structure determinations are provided showing a range of different underlying problems. It seems likely that post-refinement methods taking into account the slope of the normal probability plot are currently the most robust estimators of absolute structure and should be reported along with the directly refined values.

Publisher’s copy

Absolute structure determination using CRYSTALS

Mar 072016
 

Acta. Cryst. (2016) C72, 261-267 [ doi:10.1107/S2053229616003570 ]

snipA study of post-refinement absolute structure determination using previously published data was carried out using the CRYSTALS software package. We show that absolute structure determination may be carried out optimally using the analyses available in CRYSTALS, and that it is not necessary to have the separate procedures absolute structure determination and no inter­est in absolute structure as proposed by Flack [Chimia (2014), 68, 26–30].

Publisher’s copy

Applications of Leverage Analysis in Structure Refinement

Apr 302012
 

J Appl. Cryst. (2012), 45, 417-429. [ doi:10.1107/S0021889812015191 ]

Leverages measure the influence that observations (intensity data and restraints) have on the fit obtained in crystal structure refinement. Further analysis enables the influence that observations have on specific parameters to be measured. The results of leverage analyses are discussed in the context of the amino acid alanine and an incomplete high-pressure data set of the complex bis(salicylaldoximato)copper(II). Leverage analysis can reveal situations where weak data are influential and allows an assessment of the influence of restraints. Analysis of the high-pressure refinement of the copper complex shows that the influence of the highest-leverage intensity observations increases when completeness is reduced, but low leverages stay low. The influence of restraints, notably those applying the Hirshfeld rigid-bond criterion, also increases dramatically. In alanine the precision of the Flack parameter is determined by medium-resolution data with moderate intensities. The results of a leverage analysis can be incorporated into a weighting scheme designed to optimize the precision of a selected parameter. This was applied to absolute structure refinement of light-atom crystal structures. The standard uncertainty of the Flack parameter could be reduced to around 0.1 even for a hydrocarbon.

Electronic reprints

Publisher’s copy

CRYSTALS Enhancements: Absolute Structure Determination

Sep 132011
 

J. Appl. Cryst.  (2011), 44, 1017-1022.    [ doi:10.1107/S0021889811034066 ]

A summary of the features for investigating absolute structure available in the crystallographic refinement program CRYSTALS is presented, together with the results of analyses of 150 light-atom structures collected with molybdenum radiation carried out with these tools. The results confirm that the Flack and Hooft parameters are strongly indicative, even when the standard uncertainties are large compared to the thresholds recommended by Flack & Bernardinelli [J. Appl. Cryst. (2000), 33, 1143–1148].

Electronic reprints

  • Oxford University Research Archive [direct pdf]

Publisher’s copy

XXII International Congress and General Assembly of the International Union of Crystallography

Aug 312011
 

Madrid 2011 IUCrThe triennial Congress of the International Union of Crystallography was held in Madrid, Spain at the end of August 2011. Chem. Cryst. was well represented with several talks and posters including:

Richard I. Cooper
Optimising X-ray Experiment Strategy on-the-fly based on Feedback from Automated Structure Solution (Presentation)

Amber L. Thompson
Battle of the Titans:  Atlas vs. Eos and Other Stories (Invited presentation at the Agilent Technologies User Forum)

Jevgenij A. Raskatov, John M. Brown & Amber L. Thompson
Chiral Selection in the Formation of Borates from Racemic Binaphthols and related Diols (Poster)

Amber L. Thompson
Influencing Absolute Structure Determination (Poster)

Chiral Selection in the Formation of Borates from Racemic Binaphthols and Related Diols

Feb 152011
 

CrystEngComm (2011), 13(8) 2923-2929.    [ doi:10.1039/c0ce00709a ]

A series of racemic or stereochemically labile chiral borate anions based on the 2,20-biphenol motif was
investigated. All borates were homochiral in the solid state, although in some cases the heterochiral
diastereomers were computed to be thermodynamically preferred (DFT). The crystallographic
preference for the homochiral diastereomer was attributed to its lower bulk, higher molecular
symmetry, and the therewith associated better packing ability.

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