Halogen Bonds in Novel Polyhalogen Monoanions.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Inorganic Chemistry

Background:

• Polyhalogen monoanions, such as [X_2n+1 ]^- (where X=Cl, Br; n=1-5), represent a class of fascinating chemical species.
• Understanding the nature of bonding in these anions is crucial for predicting their reactivity and properties.
• Previous studies have hinted at complex interactions within these systems, necessitating detailed theoretical investigation.

Purpose of the Study:

• To systematically investigate the electronic structure and bonding characteristics of polyhalogen monoanions [X_2n+1 ]^-.
• To elucidate the role and nature of "halogen bonds" in these species using a valence bond approach.
• To analyze the trends in charge-transfer interactions and covalence across the series of studied anions.

Main Methods:

• Employed the block-localized wave function (BLW) method for theoretical calculations.
• Utilized valence bond (VB) analysis to dissect the bonding contributions.
• Calculated and analyzed the highest occupied molecular orbital (HOMO) energies to correlate with Lewis basicity.

Main Results:

• Identified the most stable isomers as a central halide anion (X^-) non-classically bonded to n dihalogen molecules (X₂) via halogen bonds.
• Confirmed that these halogen bonds are predominantly dative covalent interactions, driven by charge transfer (n→σ*) from the halide anion's lone pair to the X₂ molecule's antibonding orbital.
• Observed a decrease in charge-transfer interaction and covalence with increasing 'n', indicating an anti-cooperative effect and weakening Lewis basicity.

Conclusions:

• Polyhalogen monoanions exhibit halogen bonds that are fundamentally dative covalent in nature.
• The observed anti-cooperative effect leads to reduced charge transfer and covalence in larger polyhalogen anions.
• The X^-⋅⋅⋅X₂ halogen bond serves as a reliable gauge of covalence across the entire [X_2n+1 ]^- family.