Role of Charge Transfer in Halogen Bonding

Area of Science:

• Supramolecular chemistry
• Crystal engineering
• Drug design

Background:

• Halogen bonding is crucial in supramolecular assembly and drug design.
• The nature of halogen bonding, specifically the dominance of electrostatics versus charge transfer, remains debated.
• The sigma-hole concept supports electrostatic attraction, but evidence for charge transfer's importance exists.

Purpose of the Study:

• To quantitatively evaluate the charge transfer effect in halogen bonding using the block-localized wave function (BLW) method.
• To investigate the influence of charge transfer on the geometry, spectral properties, and energetics of halogen bonded complexes.
• To determine the critical role of charge transfer in accurately describing halogen bonds.

Main Methods:

• Utilized the block-localized wave function (BLW) method, a variant of ab initio valence bond theory.
• Applied the M06-2X-D3/6-311+G(d,p) level of theory with a def2-SVP basis set for iodine.
• Analyzed a series of halogen bonded complexes of the type Y3C-X···Z (X = Br, I; Y = F, Cl, Br; Z = F-, Cl-, Br-, I-, NMe3).

Main Results:

• Charge transfer significantly shortens the X···Z bonding distance and lengthens the C-X bonds.
• The inclusion of charge transfer is critical for accurately describing halogen bond strengths and geometries.
• Models considering only electrostatics and polarization capture only 45-60% of the binding strengths and result in elongated distances.

Conclusions:

• Charge transfer interaction plays a critical role in halogen bonding, contrary to claims that it is unimportant.
• Accurate modeling of halogen bonds requires the explicit inclusion of charge transfer effects.
• The study provides quantitative evidence for the significant contribution of charge transfer to halogen bond characteristics.