Flack parameter

Paramètre de Flack (Fr). Flack-Parameter (Ge). Parametro di Flack (It). フラック変数 (Ja). Parámetro de Flack (Sp).

Definition

The Flack parameter is the molar fraction [math]x[/math] in the defining equation [math]C=(1-x)X + x\bar X[/math], where [math]C[/math] represents an oriented two-domain-structure crystal, twinned by inversion, consisting of an oriented domain structure [math]X[/math] and an oriented inverted domain structure [math]\bar X[/math]. In reciprocal space, the Flack parameter [math]x[/math] is defined by the structure-amplitude equation

[math]G^2(h,k,l,x)=(1-x)|F(h,k,l)|^2 + x|F({\bar h}, {\bar k}, {\bar l})|^2[/math].

For a multidomain-structure twin of a chiral crystal structure, an equivalent Flack parameter may be calculated according to the method of Flack and Bernardinelli (1999).

See also

• Absolute structure

Related chemical terms

The IUPAC Basic Terminology of Stereochemistry contains definitions of related chemical terms of use to the crystallographer, viz: absolute configuration, chiral, chirality, chirality sense, enantiomer, enantiomerically pure, enantiomorph, racemate, racemic, racemic conglomerate and relative configuration.

References

• Flack, H. D. and Bernardinelli, G. (1999). Acta Cryst. A55, 908–915. Absolute structure and absolute configuration
• IUPAC (1996). Basic Terminology of Stereochemistry. http://www.chem.qmul.ac.uk/iupac/stereo