title: Hydrogen atoms can be located accurately and precisely by x-ray crystallography.
abstract: Precise and accurate structural information on hydrogen atoms is crucial to the study of energies of interactions important for crystal engineering, materials science, medicine, and pharmacy, and to the estimation of physical and chemical properties in solids. However, hydrogen atoms only scatter x-radiation weakly, so x-rays have not been used routinely to locate them accurately. Textbooks and teaching classes still emphasize that hydrogen atoms cannot be located with x-rays close to heavy elements; instead, neutron diffraction is needed. We show that, contrary to widespread expectation, hydrogen atoms can be located very accurately using x-ray diffraction, yielding bond lengths involving hydrogen atoms (A-H) that are in agreement with results from neutron diffraction mostly within a single standard deviation. The precision of the determination is also comparable between x-ray and neutron diffraction results. This has been achieved at resolutions as low as 0.8 Å using Hirshfeld atom refinement (HAR). We have applied HAR to 81 crystal structures of organic molecules and compared the A-H bond lengths with those from neutron measurements for A-H bonds sorted into bonds of the same class. We further show in a selection of inorganic compounds that hydrogen atoms can be located in bridging positions and close to heavy transition metals accurately and precisely. We anticipate that, in the future, conventional x-radiation sources at in-house diffractometers can be used routinely for locating hydrogen atoms in small molecules accurately instead of large-scale facilities such as spallation sources or nuclear reactors.
authors.affiliation: Biological and Chemical Research Centre, Chemistry Department, University of Warsaw, Zwirki i Wigury 101, 02-089 Warszawa, Poland.
authors.forename: Magdalena
authors.initials: M
authors.lastname: Woińska
authors.affiliation: Institut für Anorganische Chemie und Kristallographie, Fachbereich 2-Biologie/Chemie, Universität Bremen, Leobener Str. NW2, 28359 Bremen, Germany.
authors.forename: Simon
authors.initials: S
authors.lastname: Grabowsky
authors.affiliation: Biological and Chemical Research Centre, Chemistry Department, University of Warsaw, Zwirki i Wigury 101, 02-089 Warszawa, Poland.
authors.forename: Paulina M
authors.initials: PM
authors.lastname: Dominiak
authors.affiliation: Biological and Chemical Research Centre, Chemistry Department, University of Warsaw, Zwirki i Wigury 101, 02-089 Warszawa, Poland.
authors.forename: Krzysztof
authors.initials: K
authors.lastname: Woźniak
authors.affiliation: School of Chemistry and Biochemistry, University of Western Australia, 35 Stirling Highway, Perth, Western Australia 6009, Australia.
authors.forename: Dylan
authors.identifier: 0000-0002-3349-5834
authors.initials: D
authors.lastname: Jayatilaka
mesh_terms: D018360:Crystallography, X-Ray; D006859:Hydrogen / Q000737:chemistry*; D006860:Hydrogen Bonding; D008958:Models, Molecular; D009502:Neutrons; D014961:X-Ray Diffraction
chemical_list: D006859:Hydrogen
keywords: Hirshfeld atom refinement; X-ray diffraction; crystallography; hydrogen atoms; neutron diffraction; quantum crystallography