Accurate unrestrained DDM refinement of crystal structures from highly distorted and low-resolution powder...

Area of Science:

• Crystallography
• Materials Science
• Solid-State Chemistry

Background:

• Accurate crystal structure determination is crucial for understanding material properties.
• High-resolution synchrotron X-ray powder diffraction (XRPD) provides detailed structural information.
• Traditional refinement methods can be limited by experimental data distortions.

Purpose of the Study:

• To refine the structure of a benzene:ethane co-crystal using anisotropic displacement parameters and the derivative difference method (DDM).
• To introduce and validate a new DDM weighting scheme for handling distorted XRPD data.
• To compare DDM refinement results with existing density functional theory (DFT) calculations and previous crystallographic studies.

Main Methods:

• High-resolution synchrotron X-ray powder diffraction (XRPD) data collection at 90 K.
• Structure refinement using the derivative difference method (DDM) without geometric restraints.
• Development and application of a novel DDM weighting scheme to compensate for data distortions.
• Comparison with density functional theory (DFT) calculations and previous restrained refinements.

Main Results:

• Achieved high C-C bond precision of 0.005 Å for the benzene:ethane co-crystal.
• Successfully refined H-atom positions in ethane independently.
• The new DDM weighting scheme effectively compensated for significant experimental data distortions.
• Unrestrained DDM refinement of 2-aminopyridinium fumarate-fumaric acid yielded a two-fold narrower dispersion of interatomic distances compared to restrained methods.

Conclusions:

• The derivative difference method (DDM) is a powerful technique for precise crystal structure refinement from XRPD data.
• The newly developed DDM weighting scheme enhances accuracy and robustness, particularly with challenging datasets.
• Unrestrained DDM refinement offers superior precision in interatomic distance determination compared to traditional restrained methods.