A detailed Valence Bond-Spin Coupled analysis of a series 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 selfassembling of molecular and macromolecular entities to buildup 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 a dominant 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 a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity." Halogen bond can be schematized as DX/A, where the moiety D bonded to the halogen atom X has a large variability, ranging from inorganic to organic species, and the nucleophilic site A is usually represented by a lone pair of a heteroatom such as oxygen, nitrogen, sulfur, or by a π-electron system such as, for example, that associated with a phenyl ring.Though halogen bonding has been largely investigated from different points of view, at both theoretical and experimental levels, [2] a surprisingly low attention has been devoted to explain its physical origin. Indeed, at a first sight, this interaction can be considered as a quite unexpected and counterintuitive phenomenon: why should we have an attractive interaction between a typically electronegative atom and a nucleophilic site?From a purely 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 a very basic computational approach, such as a Hartree-Fock (HF) calculation (i. e. neglecting electron correlation) with a small 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 p...
