Negative σ-Holes on Fluorine in Molecules Revisited: Halogen Bonding or Counterintuitive, Anti-Electrostatic...

Area of Science:

• Chemistry
• Crystallography
• Computational Chemistry

Background:

• Halogen bonding involves attractive interactions between electrophilic halogen atoms and nucleophilic sites.
• Fluorine-fluorine (F···F) close contacts in crystals exhibit directional features suggesting σ-hole interactions.
• Classical halogen bonds typically require a positive site (σ-hole) on the halogen, which may be absent in F···F contacts.

Purpose of the Study:

• To investigate the nature of directional F···F interactions in various fluorine-containing molecules.
• To determine if these F···F interactions can be classified as conventional halogen bonds.
• To elucidate the driving forces and appropriate classification of these interactions.

Main Methods:

• Analysis of crystal structures from the Cambridge Structural Database.
• Computational investigations of F···F interactions involving HF, fluoromethanes, and fluorobenzenes.
• Application of molecular electrostatic surface potential (MESP) and symmetry-adapted perturbation theory (SAPT).

Main Results:

• F···F close contacts in crystals show geometries similar to σ-hole interactions.
• Computational studies reveal complex F···F interactions involving molecules like HF, CH4-nFn, and C6H6-nFn.
• Interactions between like-polarity sites are observed, challenging the classical halogen bond definition.
• These interactions are better characterized as σ-hole-centered anti-electrostatic motifs, influenced by dispersion forces.

Conclusions:

• Directional F···F interactions, while geometrically similar to halogen bonds, often involve like-polarity sites.
• These interactions are more accurately described as anti-electrostatic motifs rather than conventional halogen bonds.
• Dispersion forces play a significant role in stabilizing these F···F interactions.