Anti-Electrostatic Anion–Anion Interactions in Halogen Oxyanions: Are X···O Contacts Really Halogen Bonds?

Introduction: Noncovalent interactions act as a chemical glue that binds molecular entities and governs the formation of sophisticated functional materials. Beyond classical hydrogen bonding, halogen bonding, and related directional contacts involving elements across the periodic table, anti-electrostatic interactions have recently emerged as an unusual class of intermolecular interactions. These involve contacts between like-charged species that are expected to be repulsive based on simple electrostatic considerations, yet are frequently observed in crystalline solids.

Halogen oxyanions such as XO₃⁻ provide a particularly intriguing platform for examining such behavior. Numerous crystal structures display short X···O contacts between anions, often interpreted as halogen bonding. However, whether these contacts represent genuine attractive interactions or arise from environment-induced polarization remains unclear.

Results and discussion: In this work, we examined a series of anion–anion assemblies involving halogen oxyanions using density functional theory in both gas and solution phases. The optimized structures reveal that several systems stabilized in solution exhibit short X···O contacts that resemble halogen bonds. However, SAPT energy decomposition analysis shows that these interactions are characterized by large positive electrostatic contributions, and the total interaction energies are also positive when evaluated in the gas phase.

These findings indicate that the apparent stabilization of the dimers does not originate from intrinsic attractive halogen bonding. Instead, the structures are maintained by solvent screening and polarization effects that reduce Coulomb repulsion and allow weaker induction and dispersion contributions to shape the geometry. The persistence of these motifs in crystalline materials therefore arises from collective influences, including counterions, packing constraints, and polarization, rather than from intrinsic attractive forces between the isolated anions. These aspects are discussed in detail.