Har, Taam And Bodd Refinements Of Model Crystal Structures Using Cu K Alpha And Mo K Alpha X-Ray Distraction Data

The Independent Atom Model (IAM) of electron density is used in routine X-ray data analysis. However, this model does not give a quantitative description of the electron-density distribution. A better model that allows for modelling of aspherical charge density deformations is introduced by the Hansen-Coppens variant of the multipole model of electron density. However, the application of this model requires crystals of excellent quality and high-resolution XRD data which are quite often difficult criteria to fulfil. Therefore, Mo K alpha and Cu K alpha data of three model compounds (tricyclic imide, xylitol and methyluracil) were refined using IAM and new methods which enabled the refinement and reconstruction of charge density based on the Cu K alpha data. These methods were the Bond-Oriented Deformation Density (BODD) model, Hirshfeld Atom Refinement (HAR) and the Transferable Aspherical Atom Model (TAAM). The final results were compared to the model obtained from neutron diffraction experiments. Our results demonstrated not only that Cu K alpha data may be refined using BODD, HAR and TAAM methods, but also revealed systematic errors arising from the use of Cu K alpha data. These errors were a result of the limited information in the low-resolution data set that manifested as higher values for the anisotropic displacement parameters (ADPs) and smaller maxima and minima of the residual electron density for the Cu K alpha data compared to the Mo K alpha data. Notably, these systematic errors were much less significant than those found for the IAM. Therefore, the application of B ODD, HAR and TAAM on Cu K alpha data has a more significant influence on the final results of refinement than for the Mo K alpha data.