Refinement of anomalous dispersion correction parameters in single-crystal structure determinations.

Area of Science:

• Crystallography
• Materials Science
• Spectroscopy

Background:

• Anomalous dispersion corrections in X-ray diffraction are crucial for accurate crystal structure determination.
• Tabulated dispersion values are approximations, neglecting specific electronic and spatial environments.
• Discrepancies arise when X-ray excitation energy is near an element's absorption edge.

Purpose of the Study:

• To develop and implement a method for refining anomalous dispersion terms directly from experimental data.
• To improve the accuracy of crystal structure refinements, especially near absorption edges.
• To validate the refined parameters against independent X-ray absorption spectroscopy measurements.

Main Methods:

• Synchrotron multi-wavelength single-crystal X-ray diffraction on molybdenum hexacarbonyl (Mo(CO)₆).
• X-ray absorption spectroscopy (XAS) experiments around the molybdenum K edge.
• Full-matrix least-squares refinement incorporating dispersive (f') and absorptive (f'') terms as independent parameters.
• Implementation of the refinement procedure in the OLEX2 software suite.

Main Results:

• The dispersive and absorptive terms of anomalous dispersion were successfully refined as independent parameters.
• Refined parameters showed good agreement with independently measured X-ray absorption spectra.
• Crystallographic models derived using the refined parameters demonstrated significant improvement over those using tabulated values.

Conclusions:

• Direct refinement of anomalous dispersion terms from diffraction data provides more accurate corrections.
• The new method in OLEX2 enhances the reliability of crystal structure determination.
• This approach is particularly beneficial for compounds with elements near their absorption edges.