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

Abstract

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...

Definition of the Halogen Bond (IUPAC Recommendations 2013): A Revisit

“…Despite the presence of many records in the CCDC highlighted in this revisit, theoretical studies centering on the halogen-bond-forming ability of halogen’s electrophilic π-/p-holes (or π-/p-belts) in molecular entities with nucleophiles are rare and surely an important area worthy of further investigation. The development of a hole/belt may be realized as a result of “bond polarization” within the domain of the molecule, a view consonant with that of others …”

“…The development of a hole/belt may be realized as a result of "bond polarization" within the domain of the molecule, a view consonant with that of others. 335 We believe that invoking the concept of the σand π-/p-holes and orbital-based charge transfer characteristics, among others, assists in the recognition and basic understanding of the halogen bond and other noncovalent interactions. The formation of halogen bonds between sites of opposite charge capacity fits within the theoretical framework of a Coulombic attraction, but it should be appreciated that the overall stability of the complex chemical system is the result of a delicate balance between different types of attraction (i.e., electrostatics, dispersion and induction/polarization) and repulsion (e.g., exchange repulsion), in which the net interaction/binding energy is difficult to dissect.…”

Enhancing Effects of the Cyano Group on the C-X∙∙∙N Hydrogen or Halogen Bond in Complexes of X-Cyanomethanes with Trimethyl Amine: CH3−n(CN)nX∙∙∙NMe3, (n = 0–3; X = H, Cl, Br, I)

“…Tuning the strength of the hydrogen or halogen bond interaction for a fixed acceptor or Lewis base can be made through the interplay of a number of factors pertaining the D-H or the D-Hal donor group. With more than one type of halogen atom that can be used in D-Hal, the halogen bond also renders greater flexibility for fine-tuning compared with the hydrogen bond [ 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 ]. One factor that can be used to fine tune the D-H or the D-Hal donor ability is the electronegativity of the atom covalently bonded to hydrogen or halogen.…”

“…In the case of the halogen bond donor ability of D-Hal, it has been found that both the size and the magnitude of the positive electrostatic potential (known as the σ-hole) of the halogen atom increases with the increasing electronegativity of the atom covalently bonded to the halogen atom in D-Hal. It is worth noting that the σ-hole on the halogen atom acts as the Lewis acid site that interacts with a suitable electron donor species acting as the Lewis base, and that the location of the σ-hole (opposite to the covalent D-Hal bond) results in the highly distinctive linearity of the halogen bond [ 9 , 32 , 44 ]. Further studies have demonstrated that the ability of a halogen atom to engage in halogen bonding correlates with the size of the σ-hole.…”

Exploring Orthogonality between Halogen and Hydrogen Bonding Involving Benzene

“…This interaction, schematized as R–X···B (X = Cl, Br, or I; B = Lewis base; R = substituent), has been explained by the existence of a region of positive electrostatic potential, named σ-hole, on the outermost surface of the covalently-bonded halogen atom and narrowly confined on the elongation of the R–X covalent bond axis [ 20 ]. Its presence has been recently demonstrated through valence bond spin-coupled calculations [ 21 ], allowing to get a rigorous ab initio validation of the qualitative models previously proposed [ 20 ]. The key role of the σ-hole in activating XB has been particularly emphasized by molecular mechanics/molecular dynamics simulations of halogen-bonded ligand–protein systems.…”