Quantum crystallographic charge density of urea.

Area of Science:

• Crystallography
• Quantum Chemistry
• Materials Science

Background:

• Standard X-ray crystallography uses free-atom models, which inaccurately represent shared electron density in real molecules.
• Accurate charge density modeling is crucial for understanding molecular interactions and properties in crystalline solids.

Purpose of the Study:

• To develop and validate a quantum crystallographic charge density model for crystalline urea.
• To compare the accuracy and information content of the quantum model against traditional spherical atom and multipole models.
• To assess the impact of the quantum model on atomic displacement parameter refinement.

Main Methods:

• High-level quantum theory calculations were used to generate a periodic charge density model for crystalline urea.
• The quantum model was refined against ultra-high-resolution experimental X-ray diffraction data.
• Comparison with spherical atom and multipole refinement models was performed.

Main Results:

• The quantum crystallographic model achieved agreement with experimental data comparable to the multipole model, using a similar number of free parameters.
• The static charge density from the quantum model revealed distinct features compared to the multipole model, offering new insights.
• Hydrogen atomic displacement parameters from the quantum model closely matched those from neutron crystallography.

Conclusions:

• Quantum crystallography offers a feasible and beneficial approach for integrating quantum chemical calculations into X-ray crystallographic refinement.
• This method provides a more accurate representation of charge density and improves the determination of atomic displacement parameters.
• Quantum crystallography holds potential for advancing the understanding of molecular structures and properties in crystalline materials.