Halogen bonding: an electrostatically-driven highly directional noncovalent interaction

Abstract

A halogen bond is a highly directional, electrostatically-driven noncovalent interaction between a region of positive electrostatic potential on the outer side of the halogen X in a molecule R-X and a negative site B, such as a lone pair of a Lewis base or the pi-electrons of an unsaturated system. The positive region on X corresponds to the electronically-depleted outer lobe of the half-filled p-type orbital of X that is involved in forming the covalent bond to R. This depletion is labeled a sigma-hole. The resulting positive electrostatic potential is along the extension of the R-X bond, which accounts for the directionality of halogen bonding. Positive sigma-holes can also be found on covalently-bonded Group IV-VI atoms, which can similarly interact electrostatically with negative sites. Since positive sigma-holes often exist in conjunction with negative potentials on other portions of the atom's surface, such atoms can interact electrostatically with both nucleophiles and electrophiles, as has been observed in surveys of crystallographic structures. Experimental as well as computational studies indicate that halogen and other sigma-hole interactions can be competitive with hydrogen bonding, which itself can be viewed as a subset of sigma-hole bonding.

The shape of the halogen atom—anisotropy of electron distribution and its dependence on basis set and method used

“…The sigma hole can be visualized graphically by the electrostatic potential projected on the electron density isosurface [16,19]. As a result, the electrostatic nature of X-bonding was proposed [17]. However, in the literature, there are several reports based on the different interaction energy decompositions, in which it was shown that not necessarily electrostatic interaction, but HOMO-LUMO charge transfer and polarization [28], induction, and/or dispersion [27,29,30] are responsible for X-bonding.…”

“…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 sigma hole may interact with the local electron charge surplus such as lone electron pairs [17,24,25], pi-type electrons [17,24,26], or even sigma-type electrons [27]; in this way, the mechanism of formation of X-bond can be well defined. However, there are some discrepancies regarding the nature of X-bonding.…”

Communication: Competition between π⋯π interaction and halogen bond in solution: A combined 13C NMR and density functional theory study

“…In most cases, the halogen bond can be considered as a highly directional, electrostatically driven noncovalent interaction. 19 So the strong electron-withdrawing atom or atomic group geminal to the halogen atom will lead to a much stronger halogen bond. However, for the aryl and heteroaryl halides, the presence of strong electron-withdrawing atom or atomic group can also lead to a reduction of the electron density of the aromatic ring and allow for the formation of a strong π · · ·π interaction at the same time when another electron-rich aromatic ring is near the electron-deficient aromatic ring (Fig.…”

“…The hydrogen bond is probably the most important noncovalent bond. 1,2 Other important noncovalent driving forces for supramolecular assembly include π · · ·π stacking interaction, [3][4][5][6][7][8][9][10][11][12][13] halogen bond, [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] etc. Noncovalent bonds are much weaker than the covalent bonds.…”

Ab initio and DFT studies of the interaction between carbonyl and thiocarbonyl groups: the role of S···O chalcogen bonds

“…Depending on the nature of the bridging atoms [1][2][3], these interactions are commonly designed as halogen [4][5][6][7][8][9], chalcogen [10][11][12][13][14][15][16] or pnicogen [17][18][19][20] bonds. The attractive force has been attributed to an anisotropic distribution of electron density around the bridging X atom, characterized by a crown of positive electrostatic potential along the extension of the Y-X bond (σ-hole) or in areas perpendicular to it (π-hole) [6-8, 21, 22].…”