Challenges and capabilities of quantum crystallography for locating hydrogen atoms in transition metal hydrides

Transition metal (TM) hydrides are versatile compounds with multiple applications in catalysis, energy conversion, and the search for hydrogen storage materials or superconductors. Therefore, determining the structure of these compounds with X -ray diffraction is essential for many areas of research. However, this is hampered by the challenges of locating the position of hydrogen with X - rays, which are even more aggravated in the case of hydrogen atoms bonded to a TM atom. Furthermore, collecting high - quality, let alone high-resolution X -ray d ata, for TM hydrides, is an arduous task. It is also difficult to collect neutron data that could provide reliable information about hydrogen positions. TM hydride complexes are also computationally demanding, which makes them difficult to analyze using quantu m crystallographic methods. Hirshfeld atom refinement (HAR) is a quantum crystallographic method that has been proven to locate hydrogen atoms bonded to light elements with accuracy and precision very close to that of neutron experiments, based on standard resolution X - ray data [1]. In some cases, HAR has been reported to improve the positions of hydrogen atoms in TM - H bonds considerably [1, 2], as compared to the Independent Atom Model (IAM). The goal of this study was to evaluate the capabilities of HAR in terms of establishing the positions of hydrogen atoms, especially in TM - H bonds, and to investigate the influence of different parameters adjustable in the refinement on the final result. The following factors were considered: including interactions with the crystal environment, taking into account relativistic effects, changing the DFT functional used for wave function calculations (B3LYP, PBE, M06 - 2X), and selecting the basis set. Another aspect considered in the study was the role of treatment of hydrogen thermal