Refinement of K[HgI₃]·H₂O using non-spherical atomic form factors

Abstract

The crystal structure model for potassium triiodidomercurate(II) monohydrate, K[HgI3]·H2O, based on single-crystal data, was reported 50 years ago [Nyqvist & Johansson (1971). Acta Chem. Scand. 25, 1615–1629]. We have now redetermined this structure with X-ray diffraction data at 0.70 Å resolution collected at 153 K using Ag Kα radiation. Combined quantum mechanical methods (ORCA) and computation of non-spherical scattering form factors (NoSpherA2) allowed the refinement of the shape of the water molecule with anisotropic H atoms, despite the presence of heavy elements in the crystal. The refined shape of the water molecule via this Hirshfeld refinement is close to that determined for liquid water by neutron diffraction experiments. Moreover, the Laplacian of the electron density clearly shows how electron density accumulates along the O—H σ-valence bonds in the water molecule.

How do density functionals affect the Hirshfeld atom refinement?

“…1–6 In particular, the implementation of the Hirshfeld atom refinement (HAR) 7 in Olex2, 8 where non-spherical atomic form factors are computed from ab initio molecular calculations, 9 has potentially made this technique a routine tool that could be run in a computer desktop. 10 As this implementation has raised the possibility to read molecular orbitals calculated with different codes or quantum mechanical methods, the next question arises for a crystallographer: which is the appropriate level of theory needed for my refinement?…”

Uncovering the role of non-covalent interactions in solid-state photoswitches by non-spherical structure refinements with NoSpherA2

“…The NoSpherA2 (Non-Spherical Atoms in Olex2) approach, implemented in the crystal structure renement soware Olex2, 12 has already been applied to several interesting crystallographic problems, including those involving metal atoms. [13][14][15] The soware utilises Hirshfeld atom renement (HAR) to calculate non-spherical atomic form factors, then renes these non-spherical atom shapes against the experimental electron density obtained by SCXRD. 9 Through this combination of quantum mechanical calculations and experimental electron density renement, a greatly improved crystallographic model is obtained that can provide new insight into the intra-and intermolecular bonding.…”