Towards solution and refinement of organic crystal structures by fitting to the atomic pair distribution function.

Area of Science:

• Crystallography
• Materials Science
• Organic Chemistry

Background:

• Solving crystal structures from powder X-ray diffraction data is challenging, especially for poorly crystalline or nanocrystalline organic compounds.
• Traditional methods often require long-range order, limiting their applicability.
• Real-space methods offer an alternative but typically work with diffraction patterns, not PDF data.

Purpose of the Study:

• To develop and validate a novel method for organic crystal structure solution and refinement using the atomic pair distribution function (PDF).
• To demonstrate the method's effectiveness for materials with limited crystallinity.
• To provide an alternative to single-crystal diffraction for challenging samples.

Main Methods:

• The method involves fitting the atomic pair distribution function (PDF), derived from X-ray powder diffraction data, to model structures.
• Input requires approximate lattice parameters and molecular geometry, with molecules treated as rigid bodies.
• Molecular positions and orientations within the unit cell are optimized from random starting values.

Main Results:

• The developed PDF-fitting method successfully solved and refined crystal structures of quinacridone, naphthalene, and allopurinol.
• The structure of allopurinol was solved and refined in space group P1, featuring four independent molecules.
• Refinement of paracetamol, a flexible molecule, was achieved using bond and angle restraints, showing excellent agreement with single-crystal data.

Conclusions:

• The atomic pair distribution function (PDF) fitting method is a powerful tool for organic crystal structure determination and refinement.
• This approach is particularly valuable for analyzing poorly crystalline, nanocrystalline, or non-long-range-ordered organic materials.
• The method provides accurate structural information comparable to single-crystal diffraction.