title: On the temperature dependence of H-U(iso) in the riding hydrogen model.
abstract: The temperature dependence of H-U(iso) in N-acetyl-L-4-hydroxyproline monohydrate is investigated. Imposing a constant temperature-independent multiplier of 1.2 or 1.5 for the riding hydrogen model is found to be inaccurate, and severely underestimates H-U(iso) below 100 K. Neutron diffraction data at temperatures of 9, 150, 200 and 250 K provide benchmark results for this study. X-ray diffraction data to high resolution, collected at temperatures of 9, 30, 50, 75, 100, 150, 200 and 250 K (synchrotron and home source), reproduce neutron results only when evaluated by aspherical-atom refinement models, since these take into account bonding and lone-pair electron density; both invariom and Hirshfeld-atom refinement models enable a more precise determination of the magnitude of H-atom displacements than independent-atom model refinements. Experimental efforts are complemented by computing displacement parameters following the TLS+ONIOM approach. A satisfactory agreement between all approaches is found.
authors.affiliation: Institut für Anorganische Chemie, Georg-August-Universität, Tammannstrasse 4, D-37077 Göttingen, Germany.
authors.forename: Jens
authors.initials: J
authors.lastname: Lübben
authors.affiliation: Institut für Anorganische Chemie, Georg-August-Universität, Tammannstrasse 4, D-37077 Göttingen, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Volkmann
authors.affiliation: School of Chemistry and Biochemistry, Stirling Highway 35, WA-6009 Crawley, Australia.
authors.forename: Simon
authors.initials: S
authors.lastname: Grabowsky
authors.affiliation: Bragg Institute, Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia.
authors.forename: Alison
authors.initials: A
authors.lastname: Edwards
authors.affiliation: Institut für Geowissenschaften, Abteilung Kristallographie, Goethe-Universität, Altenhöferallee 1, 60438 Frankfurt am Main, Germany.
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Morgenroth
authors.affiliation: GZG, Abteilung Kristallographie, Georg-August Universität, Goldschmidtstrasse 1, 37077 Göttingen, Germany.
authors.forename: Francesca P A
authors.initials: FP
authors.lastname: Fabbiani
authors.affiliation: Institut für Anorganische Chemie, Georg-August-Universität, Tammannstrasse 4, D-37077 Göttingen, Germany.
authors.forename: George M
authors.initials: GM
authors.lastname: Sheldrick
authors.affiliation: Institut für Anorganische und Angewandte Chemie, Martin-Luther-King-Platz 6, 20146 Hamburg, Germany.
authors.forename: Birger
authors.initials: B
authors.lastname: Dittrich
keywords: Hirshfeld-atom refinement; QM/MM computations; invariom refinement; neutron diffraction; riding hydrogen model; synchrotron radiation