Browsing by Author "Kleemiss, F"

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    Accurate H-atom parameters for the two polymorphs of L-histi­dine at 5, 105 and 295 K
    (International Union of Crystallography, 2021-10-01) Novelli, G; McMonagle, CJ; Kleemiss, F; Probert, MR; Puschmann, H; Grabowsky, S; Maynard-Casely, HE; McIntyre, GJ; Parsons, S
    The crystal structure of the monoclinic polymorph of the primary amino acid L-histi­dine has been determined for the first time by single-crystal neutron diffraction, while that of the orthorhombic polymorph has been reinvestigated with an untwinned crystal, improving the experimental precision and accuracy. For each polymorph, neutron diffraction data were collected at 5, 105 and 295 K. Single-crystal X-ray diffraction experiments were also performed at the same temperatures. The two polymorphs, whose crystal packing is interpreted by intermolecular interaction energies calculated using the Pixel method, show differences in the energy and geometry of the hydrogen bond formed along the c direction. Taking advantage of the X-ray diffraction data collected at 5 K, the precision and accuracy of the new Hirshfeld atom refinement method im­ple­mented in NoSpherA2 were probed choosing various settings of the functionals and basis sets, together with the use of explicit clusters of molecules and enhanced rigid-body restraints for H atoms. Equivalent atomic coordinates and aniso­tropic displacement parameters were com­pared and found to agree well with those obtained from the corresponding neutron structural models.© International Union of Crystallography
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    Fast and accurate quantum crystallography: from small to large, from light to heavy
    (American Chemical Society, 2019-10-21) Malaspina, LA; Wieduwilt, EK; Bergmann, J; Kleemiss, F; Meyer, B; Ruiz-López, MF; Pal, R; Hupf, E; Beckmann, J; Piltz, RO; Edwards, AJ; Grabowsky, S; Genoni, A
    The coupling of the crystallographic refinement technique Hirshfeld atom refinement (HAR) with the recently constructed libraries of extremely localized molecular orbitals (ELMOs) gives rise to the new quantum-crystallographic method HAR-ELMO. This method is significantly faster than HAR but as accurate and precise, especially concerning the free refinement of hydrogen atoms from X-ray diffraction data, so that the first fully quantum-crystallographic refinement of a protein is presented here. However, the promise of HAR-ELMO exceeds large molecules and protein crystallography. In fact, it also renders possible electron-density investigations of heavy elements in small molecules and facilitates the detection and isolation of systematic errors from physical effects. © 2019 American Chemical Society