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4. Absolute Configuration

When a crystal contains a molecule that is not superimposable upon its mirror image (its `enantiomorph'), the absolute configuration or chirality of this molecule may be determined by X-ray crystallography. This can be done if the crystal contains an atom which absorbs X-rays to an appreciable extent, so that a phase change occurs for the X-rays scattered by that atom (relative to the phase of X-rays scattered by the other atoms in the structure). This is referred to as `anomalous scattering' and causes intensities of reflections with indices hkl and -h-k-l to differ. It is possible to calculate the intensity difference for a given model, i.e. whether I(hkl) is greater than I(-h, -k, -l) or not. A comparison with experiment tells us if our model has the correct hand or not. If not, then x, y and z in the model under consideration should be reversed in sign.

It is important that the students understand the ways that absolute configuration can be described. These are listed below:

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