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The Origin of the σ‐Hole in Halogen Atoms: a Valence Bond Perspective

Abstract : Ad etailed Valence Bond-Spin Coupled analysis of as eries of halogenated molecules is here reported, allowing to get a rigorous ab initio demonstration of the qualitative models previously proposed to explain the origin of halogen bonding. The concepts of σ-hole and negative belt observed around the halogen atoms in the electrostatic potential maps are here interpreted by analysis of the relevant Spin Coupled orbitals. The role of specific intermolecular interactions in driving self-assembling of molecular and macromolecular entities to build-up materials with selected properties and functionalities is largely recognized. [1] Rationalizing the nature of non covalent bonds and the mechanisms by which they act is therefore of paramount importance in view of designing new materials with improved performance and added value. Among the interactions that have encountered wide success in materials science, halogen bond (XB) [2] has assumed adominant position thanks to its recognized high directionality and selectivity, besides the attractive feature of being easily modulated. The latter property results from the possibility to vary not only the nature of the chemical environment bonded to the halogen (as it happens for hydrogen bond) but also the halogen itself. According to the IUPAC recommendation, [3] "A halogen bond occurs when there is evidence of an et attractive interaction between an electrophilic region associated with a halogen atom in amolecular entity and anucleophilic region in another, or the same, molecular entity." Halogen bond can be schematized as DX/A, where the moiety Db onded to the halogen atom Xh as al arge variability, ranging from inorganic to organic species, and the nucleophilic site Ai su sually represented by al one pair of ah eteroatom such as oxygen, nitrogen, sulfur, or by a π-electron system such as, for example, that associated with aphenyl ring. Though halogen bonding has been largely investigated from different points of view, at both theoretical and experimental levels, [2] as urprisingly low attention has been devoted to explain its physical origin. Indeed, at af irst sight, this interaction can be considered as aq uite unexpected and counterintuitive phenomenon: why should we have an attractive interaction between at ypically electronegative atom and a nucleophilic site? From ap urely quantitative view the question can be answered by looking at the interaction energies as computed by both standard and more sophisticated ab initio methods: 'numbers' allow to get insights into the existence of the interaction and its strength. Even av ery basic computational approach, such as aH artree-Fock (HF) calculation (i. e. neglecting electron correlation) with asmall basis set, is able to provide this information if no dispersive contributions are dominant. However, calculations do not respond to the need of qualitatively rationalizing the reason why this interaction is established. Chemists' understanding of reactions and recognition processes is always based on simple models that allow to predict the behavior of molecules. This is ac rucial point for an efficient design of new molecules with desired properties and functions. One of the first models to explain halogen bonding was proposed by Politzer and coworkers, [4] who highlighted an anisotropic distribution of the electrostatic potential (ESP) on the isodensity surface around ah alogen atom Xw hen covalently bonded to another atom Y. In particular, the ESP shows ap ositive region on Xa long the extension of the YÀX bond (the so-called σ-hole) and an egative belt perpendicular to the YÀXbond. For ag iven DX molecule, the extent of such anisotropy depends on both the halogen type and the electron withdrawing capability of D. Specifically, the greater the polar-izability of Xthe larger is the ESP anisotropy and, consequently, the strength of the interaction. It is therefore expected that the XB strength increases from chlorine to iodine, while fluorine is usually unable to give halogen bonding unless it is bonded to a strongly electronegative group. On the other hand, for ag iven [a] Dr.
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Davide Franchini, Alessandra Forni, Alessandro Genoni, Stefano Pieraccini, Enrico Gandini, et al.. The Origin of the σ‐Hole in Halogen Atoms: a Valence Bond Perspective. ChemistryOpen, Wiley, 2020, 9 (4), pp.445-450. ⟨10.1002/open.202000062⟩. ⟨hal-02534934⟩



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