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| In [[X-ray crystallography]], the '''Flack parameter''' is a factor used to estimate the [[absolute configuration]] of a structural model determined by single-crystal structure analysis.
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| In this approach, one determines the absolute structure of a [[noncentrosymmetric]] crystal. The processes used to decide the absolute structure use the [[anomalous dispersion effect]]. If atomic [[scattering]] factors did not have [[Complex number|imaginary parts]], the [[Friedel pair]]s would have exactly the same [[amplitude]]s (i.e., the scattering intensity <math>|F(h k l)|^{2}</math> from crystal plane (h k l) is equal to <math>|F(-h -k -l)|^{2}</math>). However, atomic scattering factors have [[imaginary part]]s due to the [[anomalous dispersion effect]], and Friedel's law is broken by this effect.
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| There are several ways to determine the absolute structure by X-ray crystallography. For example, a comparison of the intensities of [[Bijvoet pairs]] or of the [[R factors]] for the two possible structures can suggest the correct absolute structure. One of the more powerful and simple approaches is using the Flack parameter, because this single parameter clearly indicates the absolute structure.
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| The Flack parameter is calculated during the structural refinement using the equation given below:
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| :<math>\ I(hkl) = (1-x)|F(h k l)|^{2} + x|F(-h -k -l)|^{2}</math> <!--explain this formula! --> | |
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| where ''x'' is the Flack parameter, ''I'' is the square of the scaled observed structure factor and ''F'' is the calculated structure factor.
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| By determining ''x'' for all data, ''x'' is usually found to be between 0 and 1. If the value is near 0, with a small [[standard uncertainty]], the absolute structure given by the structure refinement is likely correct, and if the value is near 1, then the inverted structure is likely correct. If the value is near 0.5, the crystal may be [[racemic]] or twinned. The technique is most effective when the crystal contains both lighter and heavier atoms. Light atoms usually show only a small anomalous dispersion effect.
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| This parameter, introduced by H. D. Flack <ref>{{cite journal
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| | author = H. D. Flack
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| | title = On Enantiomorph-Polarity Estimation
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| | journal = [[Acta Cryst]]
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| | year = '''1983'''
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| | volume = A39
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| | pages = 876–881}}</ref>
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| became one of a standard set of values being checked for structures with noncentrosymmetric space groups.
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| ==References==
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| {{Reflist}}
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| [[Category:Diffraction]]
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