Comparison of HAR and TAAM in Refining Electron Density of Quinone-like Crystal Structures

What is it about?

This research compares the performance of the Hirshfeld atom refinement (HAR) and transferable aspherical atom model (TAAM) approaches in crystal structure refinement and electron-density analysis for a series of quinone-like compounds characterized by single–double bond alternation. Six crystal structures were refined using three electron-density models: independent atom model (IAM), TAAM, and HAR. The study assessed molecular geometries, X—H bond distances, and atomic displacement parameters (ADPs) derived from these models, noting that HAR and TAAM produced similar geometries and improved H-atom positions compared to IAM. However, differences in ADPs between HAR and TAAM were observed, particularly for certain atoms in the F4TCNQ polymorphs, suggesting that the choice of electron-density model can impact thermodynamic property calculations. Topological analysis based on quantum theory of atoms in molecules (QTAIM) was conducted to compare electron-density distributions from HAR and TAAM, focusing on bond-critical point properties for both covalent and noncovalent interactions. The results indicate that while HAR accurately reproduces bond alternation in electron density, TAAM is less effective in these specific cases. Overall, the findings highlight the strengths and limitations of each approach for detailed crystallographic and bonding analyses.

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Why is it important?

This research compares the effectiveness of the Hirshfeld atom refinement (HAR) and transferable aspherical atom model (TAAM) approaches in crystal structure refinement, particularly focusing on their ability to accurately model electron density and chemical bonding in quinone-like compounds. Understanding the strengths and limitations of these methods is crucial for advancing quantum crystallography, where precise electron-density models inform fundamental insights into chemical bonding and the thermodynamic properties of crystalline materials. Key Takeaways: 1. The study demonstrates that HAR is able to accurately reproduce the single–double bond alternation characteristic of quinone-like systems in the electron-density distribution, whereas TAAM does not perform as effectively in these specific cases. 2. Findings reveal that both HAR and TAAM significantly improve the accuracy of hydrogen atom positions and atomic displacement parameters (ADPs) over the independent atom model (IAM); however, noticeable differences between HAR and TAAM emerge for certain atoms, especially in challenging systems like F4TCNQ polymorphs. 3. The research highlights that inadequacies in electron-density models, as evidenced by differences in ADPs and topological electron-density properties, can impact the derivation of thermodynamic properties and the assessment of polymorph stability, underscoring the need for careful method selection in crystallographic studies.