Abstract
A search through Crystal Structure Database was performed and the distances in contacts of X···N,O, X···H(N,O), and X···C type were collected together with the information on spatial arrangement of the interacting fragments. A detailed statistical analysis showed that the shape of the halogen atom cannot be simply concluded on the basis of interatomic distances in crystal state although originally the concept of anisotropic charge distribution around halogen nuclei was postulated on the basis of such an analysis. It was proven that the conclusions in that case strongly depend on the type of center interacting with the halogen atom. Therefore, it was postulated that the shape of the halogen atom can be estimated for the unperturbed (due to intermolecular interactions) halogen atom. For this purpose, a method was provided to make possible a numerical quantification of the anisotropy of the halogen atom on the basis of electron density measurements performed within the framework of Atoms in Molecules Quantum Theory. The anisotropy of Cl and Br atoms in H3C–X and F3C–X (X=Cl, Br) was estimated for MP2 and DFT-B3LYP methods and several different basis sets. The influence of the method and the basis set on the degree of anisotropic distribution of electron density around halogen nuclei was discussed.
Highlights
Among different noncovalent interactions, the halogen bond (X-bond) attracts particular attention of researchers because, as the hydrogen bond (H-bond), it is responsible for physical, chemical, and biologic properties of a large group of chemical species [1–16]
A detailed statistical analysis showed that the shape of the halogen atom cannot be concluded on the basis of interatomic distances in crystal state originally the concept of anisotropic charge distribution around halogen nuclei was postulated on the basis of such an analysis
We repeat such an analysis using the N and O centers as probes (Note that the number of structures collected in CSD has increased significantly since the 1990s, which increases the significance of the results obtained by means of statistical analysis)
Summary
The halogen bond (X-bond) attracts particular attention of researchers because, as the hydrogen bond (H-bond), it is responsible for physical, chemical, and biologic properties of a large group of chemical species [1–16]. It is strong enough to bind molecules into larger complexes of stable structure but it weak enough to be broken in experimental conditions or due to the processes occurring in living organisms For this reason, X-bond is considered as an interaction which can play an important role in crystal engineering [18], drug design [16, 19], and new material engineering [20]. It has been generally accepted that due to anisotropy of halogen atoms, the partial positive charge occurs in the region of halogen valence sphere being placed opposite to the covalent bond linking the halogen atom with its adjacent atom (usually it is the carbon atom or another halogen atom, but not the hydrogen atom) This partial positive charge on the valence sphere of the halogen atom is often defined within the framework of NBO theory [21] as a sigma hole—a local deficit of an electron charge (a hole) being placed opposite the sigma bond [22, 23]. The electrostatic nature of X-bonding was proposed
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