Properties and Stabilities of Cyclic and Open Chains of Halogen Bonds

Abstract

Open and cyclic chains from two to eight units of ICl and IF are constructed and examined by density functional theory (DFT) calculations. These chains contain either I•••I or I•••X halogen bonds (XBs) where X refers to Cl or F. The closed rings are more stable than the open chains due to the presence of an additional XB and enhanced cooperativity. This pattern is true even for most trimers where there is sizable geometric distortion in the rings. I•••F rings are generally more stable than the corresponding I•••I cycles as the I•••F bond is stronger than I•••I even in the simple dimer. However, I•••I rings are comparable in energy to I•••Cl. It is possible to construct I•••I rings of at least as large as eight units, which are held together exclusively by XBs. On the other hand, the maximum possible size of I•••X rings is 6. Red shifts are observed in the I−X stretching frequency bands, which magnify as the chain, both cyclic and open, grows longer. The NMR chemical shielding of the I atoms increases for I•••I chains but diminishes when I•••Cl bonds are present.

Adjusting the balance between hydrogen and chalcogen bonds

“…Numerous past calculations of related systems have confirmed the dependability and accuracy of this combined approach. 15,58,67,71,[76][77][78][79][80][81][82] The aug-cc-pVDZ-PP pseudopotential 83 was applied to fourth-row Te as it takes into account certain relativistic effects. The larger triple-z aug-cc-pVTZ set was used to compute specialized electronic parameters, such as charge transfers, NBO, and AIM properties, along with the all-electron cc-pVTZ-DK3 basis for Te.…”

“…These noncovalent bonds are commonly categorized by the column of the periodic table from which this substitute bridging atom is derived. The halogen bond, for example, occurs when it is Cl, Br, or I in this position, [8][9][10][11][12][13][14][15][16] with analogous designations for pnicogen and tetrel bonds as other classifications. [17][18][19][20][21][22][23][24][25] (First-row atoms like F and N rarely participate in these bonds.…”

Competition Between the Two σ‐Holes in the Formation of a Chalcogen Bond

“…Noncovalent bonding has gained a great deal of attention of late as research has shifted to a family of interactions that share many facets with the well studied H‐bond (HB) [1–6] . The halogen bond (XB) is a prime example where the bridging H of the HB is replaced by any of the halogen group of the periodic table [7–16] . Like the HB, there is an attractive charge transfer component that shifts density from a lone pair of the nucleophile to the σ*(RX) antibonding orbital, where X refers to a halogen rather than a H atom.…”

Does a halogen bond require positive potential on the acid and negative potential on the base?

“…As study of the XB has ramped up over recent years it has come to be understood that very similar factors contribute to its stability. [8][9][10][11][12][13][14][15][16][17] The primary difference is that the overall positive charge of the bridging H is replaced by a narrower positive region that lies along the extension of the RX covalent bond, known generally as a s-hole. This polar hole is surrounded by a negative equator that leaves the X atom with an overall negative charge, which tends to make the XB somewhat more directional than a standard HB.…”