Experimental observation of charge-shift bond in fluorite CaF₂

Abstract

On the basis of a multipole refinement of single-crystal X-ray diffraction data collected using an Ag source at 90 K to a resolution of 1.63 Å, a quantitative experimental charge density distribution has been obtained for fluorite (CaF). The atoms-in-molecules integrated experimental charges for Ca and F ions are +1.40 e and -0.70 e, respectively. The derived electron-density distribution, maximum electron-density paths, interaction lines and bond critical points along Ca...F and F...F contacts revealed the character of these interactions. The Ca...F interaction is clearly a closed shell and ionic in character. However, the F...F interaction has properties associated with the recently recognized type of interaction referred to as `charge-shift' bonding. This conclusion is supported by the topology of the electron localization function and analysis of the quantum theory of atoms in molecules and crystals topological parameters. The Ca...F bonded radii - measured as distances from the centre of the ion to the critical point - are 1.21 Å for the Ca cation and 1.15 Å for the F anion. These values are in a good agreement with the corresponding Shannon ionic radii. The F...F bond path and bond critical point is also found in the CaF crystal structure. According to the quantum theory of atoms in molecules and crystals, this interaction is attractive in character. This is additionally supported by the topology of non-covalent interactions based on the reduced density gradient.

Accurate crystal structures and chemical properties from NoSpherA2

“…The textbook notion that species such as CaF 2 consist of spherical ions has rarely been questioned, although there is evidence that either a multipole or a neutral-atom model may describe such compounds better than the conventional ionic model. 49,50 Electron and γ-ray diffraction have shown that there is non-sphericity of the valence electron density in KCl, LiF, and MgO. 51,52 Specifically for Ca 2+ , the formally empty d-orbitals can be partially populated to add substantial covalent bonding character to the cation–anion interactions, as, for example, in the putative Zintl-phase of CaSi 53 or the calcocenes.…”

“… 54 Here, utilizing HAR, we show that there is significant bond-directed non-spherical valence density present in the crystal structure of CaF 2 . 49 …”

Absent Diamond-to-β-SnPhase Transition for Carbon: Quantum Chemical Topology Approach

“…used the term “protocovalent bond” for the situations in which the ELF bond basin is split at the equilibrium interatomic distance . It was observed for the F−F bond in the F 2 molecule,,, and recently also in the CaF 2 crystal . The bond nature in the F 2 molecule and the reasons for its low dissociation energy are still under debate .…”

“…[38] It was observed for the FÀF bond in the F 2 molecule, [28,38,39] and recently also in the CaF 2 crystal. [40] The bond nature in the F 2 molecule and the reasons for its low dissociation energy are still under debate. [41][42][43][44][45] The analysis with the domain-averaged Fermi-hole orbitals [46] suggested that at the equilibrium distance the FÀF bond in F 2 is already partially dissociated, [43] which supports our assumption about the connection between the split of the ELIÀD attractor and the instability of the corresponding bond.…”

Novograblenovite from Radlin, Upper Silesia, Poland and its relation to ‘redikortsevite’

“…This is a much less time-consuming alternative to Bader's quantum theory of atoms in molecules space partitioning method (Bader, 1994) that we utilised for fluorite (Stachowicz et al , 2017) with an extension of the procrystal electron density for aspherical atomic contributions (Coppens, 1997). The latter approach, although computationally demanding, allows for a more detailed analysis of electron density distribution and corresponding physico-chemical properties.…”