Modeling of the nuclear parameters for H atoms in X-ray charge-density studies

Extensive and precise X‐ray diffraction data for xylitol have been used to test different approaches to estimate nuclear parameters for H atoms in charge‐density studies. The parameters from a neutron diffraction study of the same compound were taken as a reference. The resulting static charge densities obtained for the different approaches based on a multipole model were subjected to a topological analysis. The comparative analysis led to the following results. The procedure of extending the X—H bond to match bond lengths from neutron diffraction studies provides the best agreement with the neutron positional parameters. An isotropic model for the atomic displacements of H atoms is highly unsatisfactory and leads to significant deviations for the properties of the bond critical points including those that only involve non‐H atoms. Anisotropic displacement parameters for H atoms can be derived from the X‐ray data that are in agreement with the values from the neutron study, and the resulting charge‐density models are in good agreement with the reference model. The anisotropic displacement parameters for H atoms are derived from the X‐ray data as a sum of the external (rigid‐body) and internal vibrations. The external vibrations are obtained from a TLS analysis of the ADPs of the non‐H atoms and the internal vibrations from analysis of neutron diffraction studies of related compounds. The results from the analysis of positional and thermal parameters were combined to devise a `best anisotropic' model, which was employed for three other systems where X‐ray and neutron data were available. The results from the topological analysis of these systems confirm the success of the `best anisotropic' model in providing parameters for the H atoms that give charge densities in agreement with the reference models based on H‐atom parameters derived from neutron diffraction.