Diffraction patterns anomalous dispersion effects

Anomalous X-ray dispersion effects will cause intensity differences if the structure is noncentrosymmetric and contains an atom that scatters anomalously. Friedel s Law, which states that the X-ray diffraction pattern of a crystal is centrosymmetric even if the crystal structure is not, will not be obeyed. This means that, in the presence of anomalous dispersion effects, I hkl) If the structure is... 

The X-ray intensity diffraction data of the given crystal do not allow one to specify which of the two sets describes the actual crystal structure and thus the absolute configuration of the molecule when there is no effect of anomalous X-ray dispersion. Under such conditions Friedel s law holds, which states that the X-ray intensity diffraction pattern of a crystal is centrosymmetric whether the crystal structure is centrosymmetric or not. This does not mean that a false crystal structure containing a center of symmetry is obtained as the solution of the structural problem, but rather that the X-ray analysis cannot differentiate between the two enantiomeric structures. A simple mathematical analogy is provided by the two possible square roots of a number Vj = x. 

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