Crystal Polarity and Enantiomeric Structure
alias 
Absolute Structure and Absolute Configuration

H. D. Flack

Laboratoire de Cristallographie, University of Geneva, Switzerland

Keywords: Absolute Structure, Absolute Configuration, Polarity, Enantiomer

Data collection techniques, modelling, refinement procedures and information technology have improved to the point where it is now very common for absolute structure and absolute configuration to be determined and reported as part of structure determination on non-centrosymmetric structures.  One is almost tempted to state that Friedel's Law is no longer ever valid as measurable differences in intensity between anti-reflections are so regularly exploited. However what exactly are absolute structure and absolute configuration? Under what conditions may one or the other be determined as part of a crystal structure analysis? When is it justifiable to say that they have in fact been determined with reasonable certainty? How far can and should one trust the data reduction and refinement software being used in the analysis? To what extent may the measurement of one single crystal be characteristic of a molecular species in solution?  Is it correct to describe space group P2₁ as chiral? Are reports of crystal-structure determinations providing sufficient information to allow a reader to judge the veracity of an author's declarations? The above questions and many others have been subject to detailed examination. Some of the conclusions will be given in the presentation and later a full text will be submitted for publication in Acta Crystallographica.

As an aid to the understanding of current-day (mis)practice in the determination and reporting of absolute structure and absolute configuration,  the complete contents of Acta Crystallographica C52 (1996) have been surveyed by reading and computer searches on the corresponding CIFs.

The following actions result:

• Compendium of definitions for the terms: absolute configuration, absolute structure, chiral, chirality, conglomerate, enantiomer, enantiomorphous/enantiomorph, inversion twinning, racemate, racemic compound, polar directions
• Definition of new CIF terms: _chemical_absolute_configuration, _chemical_optical_rotation and _reflns_Friedel_coverage
• Identification of most of the common mistakes and misconceptions in this field appearing in publications
• Checking list in algorithmic form for testing CIFs
• Detailed analysis of reported values of the Flack (1983) parameter ^[1]
• A new proposal for the definition of polar and chiral directions

Reference: [1] - Flack, H. D. (1983). Acta Cryst. A39, 879-881.

Keywords: Absolute Structure, Absolute Configuration, Polarity, Enantiomer
Postal Address: H. D. Flack, Laboratoire de Cristallographie, 24 quai Ernest-Ansermet, CH-1211 Genève 4, Switzerland.
Voice: 41 (22) 702 62 49  Fax: 41 (22) 781 21 92  E-mail: Howard.Flack@cryst.unige.ch