Refinement of organic crystal structures with multipolar electron scattering factors

Area of Science:

• Crystallography
• Electron Microscopy
• Materials Science

Background:

• Electron microscopy advancements enable near-atomic resolution imaging of molecular structures.
• Analysis of high-resolution electron diffraction data requires accurate atomic scattering factors.
• The independent atom model (IAM) is a common but simplified approach.

Purpose of the Study:

• To compare the performance of the independent atom model (IAM) and the transferable aspherical atom model (TAAM) for analyzing electron diffraction data.
• To evaluate the effectiveness of TAAM in enhancing structural refinements and atomic thermal parameter determination.
• To assess the impact of data resolution on the performance of TAAM.

Main Methods:

• Utilized experimental and theoretically modelled electron structure-factor amplitudes from carbamazepine crystals.
• Performed structure refinements using both IAM and TAAM, with TAAM parameters sourced from the University at Buffalo Databank (UBDB) based on the Hansen-Coppens multipole model.
• Investigated the influence of varying diffraction data resolution on model fitting statistics.

Main Results:

• TAAM significantly improves model fitting statistics compared to IAM in electron diffraction analysis.
• TAAM enables reliable refinement of atomic thermal parameters, offering a more nuanced structural description.
• The benefits of TAAM are more pronounced when analyzing poorer-resolution electron diffraction data.

Conclusions:

• The transferable aspherical atom model (TAAM) offers a valuable enhancement over the independent atom model (IAM) for electron diffraction data analysis.
• TAAM provides a more accurate representation of molecular structure by accounting for partial atomic charges and non-spherical atomic distributions.
• This study highlights the potential of TAAM for advancing structural determination in electron microscopy, particularly with limited resolution data.