5 years ago

Halogen Bonds in Novel Polyhalogen Monoanions

Halogen Bonds in Novel Polyhalogen Monoanions
Sason Shaik, Changwei Wang, Yirong Mo, David Danovich
Polyhalogen monoanions [X2n+1]− (X=Cl and Br; n=1, 2, 3, 4, and 5) have been systematically studied using the block-localized wave function (BLW) method, which offers a valence bond (VB) analysis. For each species, the most stable isomer can be described as a central halide anion X− non-classically bonded to a number of dihalogen molecules X2 via “halogen bonds”. VB analyses confirm the dominant role of the charge-transfer interaction between the lone pair on X− and the σ-anti-bonding orbital of X2 molecule (nσ*) in X3− and higher analogues. Thus, our study demonstrates that these halogen bonds are essentially dative covalent interactions. Importantly, the charge-transfer interaction between [X2n−1]− and X2 decreases with the increasing n, in accord with the weakening of the Lewis basicity as characterized by the corresponding HOMO energy. The reduction of the charge transfer interaction underscores the reduction of covalence in halogen bonds in [X2n+1]−. This tendency highlights the anti-cooperative effect in polyhalogen monoanions. All in all, the halogen bond between X− and nX2 molecules exhibits the same trends as in X− with a single X2 molecule. In other words, halogen bonding in the larger clusters derives from the same bonding mechanism as the [X3]− anion. As such, the X−⋅⋅⋅X2 halogen bond at different bond lengths forms a gauge of covalence for the entire [X2n+1]− family. Evolution of the halogen bond:The halogen bond in [X2n+1]− is primarily covalent due to a nσ* charge-transfer interaction. As expected, covalence is not additive, and it decreases as the cluster size grows. The halogen bond between the [X2n−1]− anion and a X2 molecule exhibits the same trends as the X−⋅⋅⋅X2 halogen bond at different distance, which can thus be taken as a gauge for all [X2n+1]− species.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/chem.201701116

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