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MIT5_067F09_lec6_artefact

MIT5_067F09_lec6_artefact - Artefacts An artefact is a...

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Artefacts An artefact is “a spurious observation or result arising from preparatory or investigative procedures […] any feature that is not naturally present but is a product of an extrinsic agent or method” ( Webster’s Encyclopedic Dictionary ). In crystallography an artefact is “a method-immanent unavoidable systematic error leading to incorrect observations” (Müller in Crystal Structure Refinement ). Typical artefacts: • Libration • Short C C or C N triple bonds • Hydrogen positions • Fourier truncation ripples Libration The higher the temperature, the shorter bond lengths appear to be, even though the unit cell gets slightly larger at the same time. The reason for this is libration. Especially terminal atoms show approximately circular motion. In anisotropic refinement, this motion is fitted as an ellipsoid, the center of which lies inside the circle of motion, while the atom is located on its periphery. This effect makes the bond distance appear shorter. Depending on the movement of the atoms, the correction that needs to be applied is somewhere between 0.001 to 0.1 Å. Libration is much stronger at higher temperatures and particularly pronounced for hydrogen atoms, small A U B ions, like NO 3 , BF 4 , ClO 4 , PF 6 and for –CF 3 groups.
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