title: Hirshfeld atom refinement for modelling strong hydrogen bonds.
abstract: High-resolution low-temperature synchrotron X-ray diffraction data of the salt L-phenylalaninium hydrogen maleate are used to test the new automated iterative Hirshfeld atom refinement (HAR) procedure for the modelling of strong hydrogen bonds. The HAR models used present the first examples of Z' > 1 treatments in the framework of wavefunction-based refinement methods. L-Phenylalaninium hydrogen maleate exhibits several hydrogen bonds in its crystal structure, of which the shortest and the most challenging to model is the O-H...O intramolecular hydrogen bond present in the hydrogen maleate anion (O...O distance is about 2.41 Å). In particular, the reconstruction of the electron density in the hydrogen maleate moiety and the determination of hydrogen-atom properties [positions, bond distances and anisotropic displacement parameters (ADPs)] are the focus of the study. For comparison to the HAR results, different spherical (independent atom model, IAM) and aspherical (free multipole model, MM; transferable aspherical atom model, TAAM) X-ray refinement techniques as well as results from a low-temperature neutron-diffraction experiment are employed. Hydrogen-atom ADPs are furthermore compared to those derived from a TLS/rigid-body (SHADE) treatment of the X-ray structures. The reference neutron-diffraction experiment reveals a truly symmetric hydrogen bond in the hydrogen maleate anion. Only with HAR is it possible to freely refine hydrogen-atom positions and ADPs from the X-ray data, which leads to the best electron-density model and the closest agreement with the structural parameters derived from the neutron-diffraction experiment, e.g. the symmetric hydrogen position can be reproduced. The multipole-based refinement techniques (MM and TAAM) yield slightly asymmetric positions, whereas the IAM yields a significantly asymmetric position.
authors.affiliation: Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland.
authors.forename: Magdalena
authors.initials: M
authors.lastname: Woińska
authors.affiliation: School of Chemistry and Biochemistry, The University of Western Australia, 35 Stirling Highway, Crawley WA 6009, Australia.
authors.forename: Dylan
authors.initials: D
authors.lastname: Jayatilaka
authors.affiliation: School of Chemistry and Biochemistry, The University of Western Australia, 35 Stirling Highway, Crawley WA 6009, Australia.
authors.forename: Mark A
authors.initials: MA
authors.lastname: Spackman
authors.affiliation: Australian Nuclear Science and Technology Organization, New Illawarra Road, Lucas Heights NSW 2234, Australia.
authors.forename: Alison J
authors.initials: AJ
authors.lastname: Edwards
authors.affiliation: Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland.
authors.forename: Paulina M
authors.initials: PM
authors.lastname: Dominiak
authors.affiliation: Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland.
authors.forename: Krzysztof
authors.initials: K
authors.lastname: Woźniak
authors.affiliation: RIKEN RSC-Rigaku Collaboration Center, RIKEN SPring-8 Center, 1-1-1, Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan.
authors.forename: Eiji
authors.initials: E
authors.lastname: Nishibori
authors.affiliation: Japan Synchrotron Radiation Research Institute (JASRI), 1-1-1, Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan.
authors.forename: Kunihisa
authors.initials: K
authors.lastname: Sugimoto
authors.affiliation: School of Chemistry and Biochemistry, The University of Western Australia, 35 Stirling Highway, Crawley WA 6009, Australia.
authors.forename: Simon
authors.initials: S
authors.lastname: Grabowsky
keywords: Hirshfeld atom refinement; aspherical scattering factors; electron density; hydrogen bonds; hydrogen maleate; structure refinement