title: Accurate Bond Lengths to Hydrogen Atoms from Single-Crystal X-ray Diffraction by Including Estimated Hydrogen ADPs and Comparison to Neutron and QM/MM Benchmarks.
abstract: Amino acid structures are an ideal test set for method-development studies in crystallography. High-resolution X-ray diffraction data for eight previously studied genetically encoding amino acids are provided, complemented by a non-standard amino acid. Structures were re-investigated to study a widely applicable treatment that permits accurate X-H bond lengths to hydrogen atoms to be obtained: this treatment combines refinement of positional hydrogen-atom parameters with aspherical scattering factors with constrained "TLS+INV" estimated hydrogen anisotropic displacement parameters (H-ADPs). Tabulated invariom scattering factors allow rapid modeling without further computations, and unconstrained Hirshfeld atom refinement provides a computationally demanding alternative when database entries are missing. Both should incorporate estimated H-ADPs, as free refinement frequently leads to over-parameterization and non-positive definite H-ADPs irrespective of the aspherical scattering model used. Using estimated H-ADPs, both methods yield accurate and precise X-H distances in best quantitative agreement with neutron diffraction data (available for five of the test-set molecules). This work thus solves the last remaining problem to obtain such results more frequently. Density functional theoretical QM/MM computations are able to play the role of an alternative benchmark to neutron diffraction.
authors.affiliation: Heinrich-Heine Universität Düsseldorf, Institut für Anorganische Chemie und Strukturchemie, Material- und Strukturforschung, Gebäude: 26.42, Universitätsstraße 1, 40225, Düsseldorf, Germany.
authors.forename: Birger
authors.identifier: http://orcid.org/0000-0002-4306-5918
authors.initials: B
authors.lastname: Dittrich
authors.affiliation: Heinrich-Heine Universität Düsseldorf, Institut für Anorganische Chemie und Strukturchemie, Material- und Strukturforschung, Gebäude: 26.42, Universitätsstraße 1, 40225, Düsseldorf, Germany.
authors.forename: Jens
authors.initials: J
authors.lastname: Lübben
authors.affiliation: Institut für Chemie und Biochemie-Anorganische Chemie der, Freien Universität Berlin, 14195, Berlin, Germany.
authors.forename: Stefan
authors.initials: S
authors.lastname: Mebs
authors.affiliation: Diamond Light Source, Harwell Science and Innovation Campus, Didcot, OX11 0DE, UK.
authors.forename: Armin
authors.identifier: http://orcid.org/0000-0001-8995-7324
authors.initials: A
authors.lastname: Wagner
authors.affiliation: Institut für Chemie und Biochemie-Anorganische Chemie der, Freien Universität Berlin, 14195, Berlin, Germany.
authors.forename: Peter
authors.initials: P
authors.lastname: Luger
authors.affiliation: Diamond Light Source, Harwell Science and Innovation Campus, Didcot, OX11 0DE, UK.
authors.forename: Ralf
authors.identifier: http://orcid.org/0000-0003-3372-8578
authors.initials: R
authors.lastname: Flaig
keywords: X-ray diffraction; amino acids; density functional theory; neutron diffraction; structure elucidation