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 Absolute configuration Absolute configurations for a chiral molecule (in pure form) are most often obtained by X-ray crystallography. All enantiomerically pure chiral molecules crystallise in one of the 65 Sohncke Groups (Chiral Space Groups). Alternative techniques are Optical rotatory dispersion, vibrational circular dichroism and the use of chiral shift reagents in proton NMR. When the absolute configuration is obtained the assignment of R or S is based on the Cahn-Ingold-Prelog priority rules. Absolute configurations are also relevant to characterization of crystals. Until 1951 it was not possible to obtain the absolute configuration of chiral compounds.[2] It was at some time decided that (+)-glyceraldehyde was the (R)-enantiomer. The configuration of other chiral compounds was then related to that of (+)-glyceraldehyde by sequences of chemical reactions. For example (+)-glyceraldehyde (1) was related to (-)-glyceric acid 2 (oxidation by mercury oxide) which in turn was related to (+)-isoserine 3 (nitric acid oxidation) and bromide 4 and (-)-lactic acid 5 (zinc reduction). Because the chemical transformations did not affect the asymmetric carbon atom, this sequence demonstrated that (-)-lactic acid was also a (R)-enantiomer. - determination of relationships in absolute configuration
In 1951 Bijvoet for the first time used in X-ray crystallography the effect of anomalous dispersion, which is now referred to as resonant scattering, to determine absolute configuration.[3] The compound investigated was (+)-sodium rubidium tartrate and from its configuration (R,R) it was deduced that the original guess for (+)-glyceraldehyde was correct. See also References fr:Configuration absolue nl:Absolute configuratie pl:Konfiguracja absolutna
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