X-ray powder diffraction as a tool to solve molecular crystal structures

The way atoms, ions, and/or molecules form, on average, a long-range ordered arrangement is defined as a crystal. In a more compact and elegant definition, a material is defined as a crystal if it essentially has a sharp diffraction pattern [1].

When studying the physicochemical properties of molecular solid, e.g.,
pharmaceuticals, the knowledge of their crystal structures impart several essential characteristics, which may impact properties like dissolution rate, and stability, among others.

One of the most powerful ways to determine crystal structures is using single-
crystal X-ray diffraction. On the other hand, its success is based on the obtention and selection of “perfect” tiny crystals. But, sometimes, obtaining single crystals is problematic – or even impossible. In such cases, X-ray powder diffraction may overcome this problem.

In this talk, I will show some results obtained in the Laboratory of Crystallography and Structural Characterization of Materials (LCCEM) using global optimization methods for crystal-structure determination via a simulated annealing approach implemented in DASH [2] and/or TOPAS-Academic [3]. Several complementary computational analyses carried out with the CSD-Materials module within Mercury [4] help find some intra- and
intermolecular interactions between molecules to understand the behavior of different materials.

Acknowledgments
The author thanks the financial support from the São Paulo Research Foundation (FAPESP grant # 2021/03640-7), the National Council for Scientific and Technological Development (CNPq grant # 305601/2019-9), and the Coordination for the Improvement of Higher Education Personnel – finance code 001.

References
[1] Online Dictionary of Crystallography, International Union of Crystallography (IUCr), https://dictionary.iucr.org/Crystal (accessed on Feb 4th, 2022).
[2] W. I. F. David, K. Shankland, J. van de Streek, E. Pidcock, W. D. S. Motherwell and J. C. Cole, “DASH: a program for crystal structure determination from powder diffraction data”, J. App. Cryst. (2006) 39, 910-915.
[3] A. A. Coelho, “TOPAS and TOPAS-Academic: an optimization program integrating computer algebra and crystallographic objects written in C++”, J. Appl. Cryst. (2018) 51, 210-218.

[4] C. F. Macrae, I. Sovago, S. J. Cottrell, P. T. A. Galek, P. McCabe, E. Pidcock, M. Platings, G. P. Shields, J. S. Stevens, M. Towler and P. A. Wood, “Mercury 4.0: from visualization to analysis, design and prediction”, J. Appl. Cryst. (2020) 53, 226-235.