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xiaxinle

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[交流] Flack parameter什么意思，怎么求出

Flack parameter是关于什么的参数，怎么计算?
好像与手性有关???

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1楼 2009-08-09 21:23:47

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xi2004

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xiaxinle(金币+1):谢谢参与
xiaxinle(金币+7,VIP+0):thanks 8-10 10:48
paopaomeng(金币+1,VIP+0):谢谢交流 8-26 09:34

就是所谓的绝对构型因子.
表征你的结构的绝对构型的.

http://en.wikipedia.org/wiki/Flack_parameter

Flack parameter
From Wikipedia, the free encyclopedia
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This article is an orphan, as few or no other articles link to it. Please introduce links to this page from other articles related to it. (February 2009)

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.

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 imaginary parts, the Friedel pairs would have exactly the same amplitudes (i.e., the scattering intensity | F(hkl) | 2 from crystal plane (h k l) is equal to | F( − h − k − l) | 2). However, atomic scattering factors have imaginary parts due to the anomalous dispersion effect, and Friedel's law is broken by this effect.

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.

The Flack parameter is calculated during the structural refinement using the equation given below:

where x is the Flack parameter, I is the square of the scaled observed structure factor and F is the calculated structure factor.

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.

This parameter, introduced by H. D. Flack [1] became one of a standard set of values being checked for structures with noncentrosymmetric space groups.

[edit] References
^ H. D. Flack (1983). "On Enantiomorph-Polarity Estimation". Acta Cryst A39: 876–881.