Use of intensity quotients and differences in absolute structure refinement.

Area of Science:

• Crystallography
• Materials Science
• Chemistry

Background:

• Determining the absolute structure of crystals is crucial in chemistry and materials science.
• Accurate absolute structure determination is essential for understanding molecular properties and reactions.
• Current methods face challenges in precision and applicability, especially for light-element compounds.

Purpose of the Study:

• To evaluate and compare various methods for absolute structure refinement using single-crystal X-ray diffraction data.
• To assess the precision and accuracy of different computational approaches for determining the Flack parameter.
• To establish the reliability of these methods, even for challenging samples like hydrocarbons.

Main Methods:

• Single-crystal X-ray diffraction data were collected using Cu Kα radiation for 23 crystals containing only light elements (up to oxygen).
• Methods tested included conventional refinement with an inversion twin model, and estimations using intensity quotients (SHELXL2012), Bayesian methods (PLATON), numerical intensity differences (CRYSTALS), and combined differences/quotients (TOPAS-Academic).
• Restraints were implemented in some methods, explicitly coded in terms of other structural parameters.

Main Results:

• All tested methods successfully established the absolute structure, including for a hydrocarbon sample.
• Conventional refinement provided accurate Flack parameter values but with significant standard uncertainties (0.15–0.77).
• Alternative methods yielded accurate Flack parameter values with substantially higher precision.

Conclusions:

• Multiple methods can reliably determine absolute structure from X-ray diffraction data for light-element compounds.
• Advanced computational techniques offer superior precision for the Flack parameter compared to conventional refinement.
• Explicitly coding restraints in terms of structural parameters allows for simultaneous refinement of the Flack parameter and other structural variables.