From Bigger to Smaller: Unraveling the influence of size and Pauli repulsion on halogen bond directionality. The potential energy landscape is shaped by anisotropic electron density through Pauli repulsion in larger halogens, whereas the influence of electrostatic forces on bond orientation becomes more pronounced in smaller halogens.

Abstract

Halogen bonds are typically observed to have a linear arrangement with a 180° angle between the nucleophile and the halogen bond acceptor X−R. This linearity is commonly explained using the σ-hole model, although there have been alternative explanations involving exchange repulsion forces. We employ two-dimensional Distortion/Interaction and Energy Decomposition Analysis to examine the archetypal H₃N⋯X₂ halogen bond systems. Our results indicate that although halogen bonds are predominantly electrostatic, their directionality is largely due to decreased Pauli repulsion in linear configurations as opposed to angled ones in the I₂ and Br₂ systems. As we move to the smaller halogens, Cl₂ and F₂, the influence of Pauli repulsion diminishes, and the energy surface is shaped by orbital interactions and electrostatic forces. These results support the role of exchange repulsion forces in influencing the directionality of strong halogen bonds. Additionally, we demonstrate that the 2D Energy Decomposition Analysis is a useful tool for enhancing our understanding of the nature of potential energy surfaces in noncovalent interactions.