Quasi-atomic orbital analysis of halogen bonding interactions.

This article focuses on the quasi-atomic orbital (QUAO) analysis of halogen-bonded complexes formed between ammonia (NH₃) and diatomic halogen fluoride molecules (XF, where X = F, Cl, Br, I) to investigate the electronic nature of σ-hole interactions. The study finds that while the NH₃⋯F₂ complex exhibits no significant electron sharing, heavier halogens (Cl, Br, I) show increasing covalent character in the N–X bond due to electron sharing between the nitrogen lone pair and the halogen atom, accompanied by a weakening of the X–F bond. Comparisons with the corresponding NH₃X⁺ cations reveal that as the halogen size increases, the halogen-bonded complexes' electronic structure approaches that of an inner complex [NH₃X]⁺⋯F⁻, indicating a growing covalent contribution. These results suggest that halogen bonding and related σ-hole interactions involve not only noncovalent electrostatic forces but also significant covalent bonding components.