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.
Highlights
The riding hydrogen model is widely used in refining smallmolecule X-ray diffraction data
X-ray diffraction data to high resolution, collected at temperatures of 9, 30, 50, 75, 100, 150, 200 and 250 K, 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
H atoms that share the same invariom name are in the same covalent bonding environment and have the same number of next-nearest nonH neighbours, so it was used for classification throughout
Summary
The riding hydrogen model is widely used in refining smallmolecule X-ray diffraction data. Predicted H-atom positions usually lead to comparable figures of merit to a free refinement of H-atom positional parameters. This holds even for high-quality X-ray data, extending far into reciprocal space, since the scattering contribution of hydrogen is small and limited in resolution. Predicted positions, e.g. by SHELXL (Sheldrick, 2008), have been used for ‘invariom’ (Dittrich et al, 2004) aspherical-atom refinements (Schurmann et al, 2012; Propper et al, 2013) Such H-atom treatment, in combination with elongating X—H vectors to bond distances computed by quantum chemical optimizations of model compounds, Acta Cryst. A70, 309–316 provides structures of high quality from conventional diffraction data
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