Does Valence Shell Electron Pair Repulsion Theory holds in core electrons?

Julen Munárriz Tabuenca
Departamento de Química Física, Facultad de Ciencias, Universidad de Zaragoza, Zaragoza, Espagne
Mercredi 17 Octobre 2018, 11h00
bibliothèque LCT, tour 12 - 13, 4^ème étage

Theoretical chemistry is devoted to understand the inherent link between microscopic characteristics and observable chemical properties. For that, chemists have historically resorted to concepts, such as the chemical bond. One of its greatest hits is the capability to predict molecular geometries through the Valence Shell Electron Pair Repulsion (VSEPR) framework. This theory has become enormously popular in Chemistry due to its simplicity and relatively well performance, being even taught in high school and university courses. However, it has barely been extended to understand special distribution of core electron pairs.
Within this context, in this work, we studied the Electron Localization Function (ELF) [1] topology in the core regions of a wide set of molecules presenting VSEPR geometries as a tool for understanding the electronic distribution atomic cores, as well as its relation with molecular geometry and properties. Analysis of the (3,-3) critical points in nuclear regions yielded polyhedral which are closely related with special distribution of ligands and lone pairs bonded to the central atom, as previously stated by Gillespie et al.[2], see Figure 1. Interestingly, in highly-symmetric molecules with atoms up to the third row in the periodic table, the electronic distribution in the core region presented the shape of the dual polyhedral of the valence shell, whereas more complexes relations were obtained for less symmetric geometries, as well as for those involving heavier atoms. This pattern corresponds to a repulsion minimization of electron repulsions in the core with valence electron pairs. This way, it is clear that VSEPR also applies to core electron pairs, and we can thus drop de "V" from VSEPR.

[Picture]

Figure 1. Spatial distribution of atoms, lone pairs (obtained from ELF topology) and (3,-3) critical points of core regions (shown in purple) for PF3, SiF4 and SF6.

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References :
[1] B. Silvi and A.Savin, Nature 1994, 371, 683.
[2] J. Gillespie, S. Noury, J. Pilmé, and B. Silvi, Inorg. Chem. 2004, 43, 3248.