interScience CHRAITY-00 Review Article The Use of X-ray Crystallography to Determine Absolute Configuration ABSTRACT Estal backgroud the deterination fc lute configuratior n are defined and olute-str ant scattering dand the insights obtained fro of a Bijvoet intensity ratio op s,XRD intens sity me ment oftware of the comp on to and econfiguration determination using combined XRD and CD measur mination from light-atom structures.Chirality 20:681-690.20082007 Wiley-Liss.Ine. KEY WORDS:absolute structure;crystal structure:resonant scattering INTRODUCTION ten for the person who has sufficient knowledge of x-ray X-ray diffraction (RD)of single crystals has the mination are the following questions: the model cr he eometry,bond dis crystal stn ave be s a fine Absolute- grntiot nation,which dep nds on being able small dif raction intens represent the enceisnot guaran hae mall i ulk and山 icient measured crystal been su experimentalist to the material studied to claim that an absolute com try,atomic positions,interatomic cement parh 2007 Wiley-Liss.Ine.Review Article The Use of X-ray Crystallography to Determine Absolute Configuration H. D. FLACK* AND G. BERNARDINELLI Laboratoire de Cristallographie, University of Geneva, Switzerland ABSTRACT 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 pa￾rameter 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 phys￾ical 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 differ￾ence 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 abso￾lute-configuration determination are described. Characterization of the compounds and crystals by the physicochemical measurement of optical rotation, CD spectra, and enan￾tioselective 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 clar￾ify the technique. The review concludes with comments on absolute-configuration deter￾mination from light-atom structures. Chirality 20:681–690, 2008. VC 2007 Wiley-Liss, Inc. KEY WORDS: absolute structure; crystal structure; resonant scattering INTRODUCTION X-ray diffraction (XRD) of single crystals has the capacity to distinguish between the enantiomorphs of a chiral crystal structure and the enantiomers of a chiral molecule. The technique may be applied to compounds of a vast range of chemical composition. Essential chemical information such as the molecular geometry, bond dis￾tances and angles, and the packing of the molecules in the crystal are part and parcel of the results of the analysis. However there are limitations. Absolute-configuration determination is a fine detail of crystal-structure determi￾nation, which depends on being able to identify small dif￾fraction intensity differences between two crystal-structure models of opposite chirality. With compounds containing only light atoms a significant difference is not guaranteed. The physical reason that these differences are small is described in the section Resonant scattering and its effect on the diffraction intensities. Clearly it makes no sense to claim that an absolute con- figuration has been determined unless the gross features of the structure and its determination, such as intensity measurements, symmetry, atomic positions, interatomic distances, and atomic displacement parameters have been evaluated and shown not to be in error. This review is writ￾ten for the person who has sufficient knowledge of X-ray crystallography to accomplish this essential step. Of particular relevance in absolute-configuration deter￾mination are the following questions: • Does the model crystal structure properly represent the crystal structure inside the crystal(s) that have been measured? Is the crystal structure chiral? Is the model that of the real crystal structure and not its enantio￾morph? Is the compound enantiomerically pure? Is the assumed space-group symmetry neither too low nor too high? • Does the model crystal structure properly represent the bulk product from which the crystal was grown? • Have the bulk and the measured single crystal been suf- ficiently characterized or fingerprinted to enable another experimentalist to correctly identify the material studied? *Correspondence to: H. D. Flack, Laboratoire de Cristallographie, 24 quai Ernest-Ansermet, CH-1211 Gene`ve 4, Switzerland. E-mail: howard.flack@unige.ch Received for publication 14 May 2007; Accepted 16 July 2007 DOI: 10.1002/chir.20473 Published online 8 October 2007 in Wiley InterScience (www.interscience.wiley.com). CHIRALITY 20:681–690 (2008) VC 2007 Wiley-Liss, Inc