Halogen bonds in crystal TTF derivatives: an ab initio quantum mechanical study

Area of Science:

• Solid-state chemistry
• Computational chemistry
• Crystallography

Background:

• Halogen bonding is a significant non-covalent interaction in crystal engineering.
• Understanding halogen bond strength and nature is crucial for designing new materials.

Purpose of the Study:

• To calculate and compare stabilization energies of various ionic and neutral organic crystal structures featuring halogen bonds.
• To investigate the role of sigma-holes and electrostatic potential in determining halogen bond strength.
• To analyze the contribution of dispersion and electrostatic energies to the overall stabilization.

Main Methods:

• Density Functional Theory (DFT) with the D3 dispersion correction (B97D/def2-QZVP) was employed.
• Calculations were performed on crystal geometries of organic structures, ionic complexes (I3(-)···I2), and neutral complexes (I2···I2).
• Electrostatic potential was evaluated for all subsystems to deduce the nature of the bonds.

Main Results:

• Neutral X···I2 complexes, particularly I2···1,3-dithiole-2-thione-4-carboxylic acid, showed the highest stabilization energies among studied dimers.
• Ionic I3(-)···I2 and neutral I2···1,3-dithiole-2-thione-4-carboxylic acid complexes exhibited very high stabilization energies due to favorable geometry.
• Dispersion energy was consistently important, comparable to electrostatic energy, except in strong halogen bonds where electrostatics dominated.

Conclusions:

• Neutral iodine complexes can be highly stable, challenging previous assumptions.
• Favorable geometric arrangements significantly enhance halogen bond strength.
• Both dispersion and electrostatic forces are critical in stabilizing halogen-bonded systems.