Determination of partial atomic charges from single-crystal X-ray diffraction by refinement of occupancy of spherical atoms

Source abstract

We introduce refinement of occupancy of spherical atoms (ROSA) as a simple and user-friendly experimental approach for the determination of partial atomic charges from single-crystal X-ray diffraction data without the need for specialized software or computational chemistry. The approach uses the SHELX crystal structure refinement package to refine the occupancies and displacement parameters of individual atoms, resulting in a good fit to the electron density map that models bond polarity, and provides experimental atomic charges that reproduce those measured with more sophisticated charge density analysis (CDA). The approach is accessible to any moderately experienced crystallographer familiar with the SHELX suite. Seven datasets of five individual crystals of three molecular compounds were collected at 100 K and are considered as test cases; the three compounds examined are 4-(dimethylamino)pyridine, tetraphenyltin(IV) and p -dichloro- p -benzoquinone. We perform analyses of the effects of atomic positions of hydrogen atoms, vibrational parameters and structure resolution on the quality and reproducibility of the method. ROSA charges show significant reproducibility between different crystals of the same compound and phase. ROSA gives calculated atomic charges using standard-resolution Mo K α data at 0.76 Å, without correlation between occupancy and displacement parameters. These charges are similar to those obtained via high-resolution (0.55 Å) CDA, suggesting the ROSA approach is amenable for use with standard (albeit high-quality) single-crystal datasets without the need for re-collection at higher resolution. Specific recommendations are provided on handling of hydrogen atom locations and vibrational parameters for best results.