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Latest news
Our paper on the use of the t-statistic in probability plots has appeared in Acta Crystallographica section A in July 2009. See the bibliography for details and a reprint.Absolute structure determination
Since J.M. Bijvoet discovered that X-ray crystallography could be used to determine the absolute structure of molecules, this technique has been refined many times.Our quest for absolute structures of C/H/N/O crystals
This site concentrates on the discussion about a method described by Rob Hooft, Leo Straver and Anthony Spek that has been published in the Journal of Applied Crystallography in February 2008: J. Appl. Cryst. (2008). 41, 96-103 (pdf). This method gives a more reliable calculation of the absolute structure than other existing methods. Imagine: you can finally determine the absolute structure for a structure containing no atoms heavier than oxygen. Without resorting to exceptional data collection techniques. It is easy with CuKa, but in good cases it can be done even using a MoKa data set.You do not only get a qualitative assignment of the absolute structure, but also a quantitative estimate of the reliability of that assignment. The paper uses a combination of maximum likelihood estimation and Bayesian statistics to calculate:
- A value y, comparable to the value of the Flack x parameter, with its standard deviation. This does not assume any prior knowledge about the compound.
- A pair of values P2(right) and P2(wrong) expressing the likelihood that the given absolute structure is right or wrong, assuming the prior knowledge that the compound is enantiopure.
- A triplet of values P3(right), P3(twin) and P3(wrong) expressing the likelihood that the given absolute structure is right, that the crystal is a 50%/50% inversion twin, or that the absolute structure should be inverted. This assumes the prior knowledge that the crystal can not be an inversion twin with another ratio.
Further reading
- Bibliography of literature regarding absolute configuration determinations
- Frequently asked questions on the Bayesian statistics/Maximum likelihood method of determining absolute structure
- Presentations by the authors and by users of this method