Does the occurrence of resonance (by the delocalization of radical/cationic/anionic charges) induce the existence of intramolecular halogen–halogen contacts?

Abstract

Exotic intramolecular homo/hetero dihalogen bonding (C–X⋯X–C: X = Br, Cl and F) in radical, cationic and anionic five-membered ring systems was analyzed using wave functional theory (MP2/aug-cc-pVTZ) analysis. The six types (Br–Br, Cl–Cl, F–F, Br–Cl, Cl–F and Br–F) of C–X⋯X–C interactions, stabilized by resonance, were created using delocalized radical/cationic/anionic carbon atoms in corresponding five-membered ring structures. The above interactions fall under the group of ‘resonance assisted noncovalent interactions’, where the impact of resonance is to induce the existence of intramolecular dihalogen bonding, even without electrostatic interaction. Further, the paper focuses on NCI bond length and stability, as these can be tuned through substitution effects. 3D-NCI plots and the presence of Bond Critical Points (BCP) clearly confirm the existence of dihalogen bonding. Natural bond orbital (NBO) analysis reveals that the dihalogen bonding in radical/cationic/anionic systems lacks charge transfer and orbital overlapping through non-interacting lobes. Specifically, σ- and π-holes exist not only for the electron depleted regions (positive regions) but also for the electron enriched regions (negative regions). The σ- and π-holes were not utilized for the C–X⋯X–C interactions because the interactions considered were not assisted by electrostatic interactions; instead, they were only assisted by resonance. Overall, this study clearly reveals that the impact of resonance formation (by delocalization of radical, cation, and anion charges) enhances the chance of occurrence of intramolecular halogen–halogen contacts.