On the directionality of halogen bonding

Area of Science:

• Chemical Physics
• Quantum Chemistry
• Molecular Interactions

Background:

• Halogen bonding is a significant non-covalent interaction.
• Understanding its directionality is crucial for molecular design and supramolecular chemistry.
• Previous studies have explored various aspects, but the precise origin of its high directionality requires further quantum chemical investigation.

Purpose of the Study:

• To investigate the quantum chemical origin of halogen bonding's high directionality.
• To compare the energetic contributions in linear (most stable) versus perpendicular (less stable) adduct orientations.
• To elucidate the interplay of different interaction types driving this phenomenon.

Main Methods:

• Quantum chemical calculations were performed on typical halogen bonding adducts.
• Adducts were analyzed in both linear and perpendicular orientations.
• Energy decomposition analyses (EDA) were employed to dissect interaction components.

Main Results:

• Energy decomposition revealed that the synergy between charge-transfer (CT) interactions and Pauli repulsion dictates the high directionality.
• Electrostatic contributions were found to be more favorable in the less-stable perpendicular orientation.
• The combination of attractive CT and repulsive Pauli forces in the linear orientation drives the observed stability and directionality.

Conclusions:

• The high directionality of halogen bonding is primarily governed by the interplay of charge-transfer and Pauli repulsion.
• Electrostatic forces alone do not explain the preferred linear geometry.
• This finding provides a deeper quantum chemical understanding of halogen bonding, essential for predicting and controlling molecular assembly.