Dynamical Refinement: Understanding Electron Scattering Factors

What is it about?

This article reports on a study of the 1-methyluracil molecule using electron diffraction (ED) data. The study used dynamical refinement to improve the fit of the model to the data and to observe fine details of the electrostatic potential. The results showed that the TAAM dynamical refinements on experimental 3D ED data still present relatively high values of fitting statistics compared with X-ray diffraction, but the quality of the 3D ED data is sufficient to detect minute variations of the electrostatic potential. The study used theoretical structure factors to validate the actual crystal atomic structure refinement and compare it with the refinement against experimental data. The study also compared various TAAM versions to show that the electrostatic potential is sensitive to intermolecular interactions and that optimizing the multipolar parameters for the specific structure using ab initio calculations is necessary for obtaining significant improvement over IAM.

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Why is it important?

This research is important because it demonstrates the potential of electron diffraction (ED) methods to provide detailed information about the redistribution of electron densities due to chemical bonds and intermolecular interactions. The study shows that ED data and dynamical refinement are now sufficient to detect minute variations of the electrostatic potential due to bonding interactions and even the small variations caused by intermolecular interactions. This is particularly important for understanding the functionality of small molecules and the crystalline materials they constitute. Key Takeaways: 1. Dynamical refinement provides a substantial improvement over the kinematic approach in all aspects, allowing for the observation of fine features of the electrostatic potential. 2. The TAAM dynamical refinements on experimental 3D ED data still present relatively high values of fitting statistics (R factors) compared with X-ray diffraction, especially for crystals with significant imperfections. 3. The study demonstrates the potential of ED methods to provide detailed information about the redistribution of electron densities due to chemical bonds and intermolecular interactions.