Characterization of fluorine-centred `F...O' σ-hole interactions in the solid state

Area of Science:

• Solid-state chemistry and crystallography
• Computational chemistry and molecular modeling
• Supramolecular chemistry

Background:

• Intermolecular interactions, particularly halogen bonds, are vital for crystal engineering.
• The role of fluorine in non-covalent interactions is increasingly recognized but less understood than other halogens.
• Specific C-F...O contacts are rare and their influence on crystal packing requires detailed investigation.

Purpose of the Study:

• To characterize the unique intermolecular C(sp3/sp2)-F...O contacts in 1-(3-nitrophenyl)-2,2,2-trifluoroethanone (A1) and (E)-4-((4-fluorophenyl)diazenyl)phenol (A2).
• To elucidate the nature, strength, and role of these contacts in crystal packing using theoretical methods.
• To compare solid-state F...O contact features with gas-phase models.

Main Methods:

• X-ray crystallography for structural analysis.
• Theoretical charge-density calculations, including multipole modeling.
• Quantum Theory of Atoms in Molecules (QTAIM), Hirshfeld surface analysis, reduced density gradient (RDG) analysis, and PIXEL energy decomposition.

Main Results:

• Identification of rare intermolecular C(sp3)-F...O and C(sp2)-F...O contacts in the studied molecular crystals.
• Charge-density analysis confirmed electron depletion on fluorine atoms and revealed subtle electronic arrangements minimizing electrostatic repulsion.
• Intermolecular energy decomposition and RDG analysis indicated the F...O contacts are primarily dispersive in nature, influenced by mutual atomic polarization.

Conclusions:

• The study highlights the significance of rare C-F...O contacts in dictating crystal packing.
• Theoretical methods provide deep insights into the electronic and dispersive nature of these interactions.
• Atomic polarizability plays a crucial role in the observed intermolecular F...O interactions within molecular crystals.