Towards the best model for H atoms in experimental charge-density refinement

Area of Science:

• Crystallography
• Materials Science
• Quantum Chemistry

Background:

• Accurate experimental charge-density studies are crucial for understanding chemical bonding.
• Modeling hydrogen (H) atoms in X-ray diffraction studies presents significant challenges due to their low scattering power.
• Different H-atom treatment methods can impact the reliability of geometric and topological parameters derived from charge density analysis.

Purpose of the Study:

• To investigate the consequences of various H-atom treatments in experimental charge-density studies.
• To compare geometric and topological parameters obtained using different H-atom models with reference X-ray/neutron diffraction data.
• To identify the optimal H-atom modeling strategy for charge-density analysis using only X-ray data.

Main Methods:

• Performed multipolar refinement on high-resolution X-ray data using four different H-atom models.
• Compared results with a reference joint high-resolution X-ray/neutron refinement.
• Evaluated a mixed refinement approach combining high-order refinement of heavy atoms, low-angle refinement of H atoms, and elongation of X-H distances, supplemented by the SHADE program for anisotropic thermal motion estimation.

Main Results:

• A mixed refinement approach, including anisotropic thermal motion estimation for H atoms, yielded parameters closest to the reference X-ray/neutron data.
• This method proved effective even for strong hydrogen bonds.
• Standardization to average neutron X-H distances alone resulted in the poorest agreement with reference values.
• Non-hydrogen atom parameters were also systematically affected by H-atom modeling choices.

Conclusions:

• Consistent and accurate H-atom treatment is essential for reliable charge-density studies, especially when neutron data is unavailable.
• The recommended approach involves mixed refinement of heavy atoms, low-angle refinement of H atoms, and estimation of their anisotropic thermal motions.
• This strategy ensures comparability of topological and integrated properties for both H and non-H atoms in multipolar refinements.