Triangular halogen trimers. A DFT study of the structure, cooperativity, and vibrational properties

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Molecular Modeling

Background:

• Halogen bonding is a significant non-covalent interaction.
• Understanding the cooperative effects in halogen-bonded systems is crucial for molecular design.
• Previous studies have explored dimers, but trimers offer more complex cooperative phenomena.

Purpose of the Study:

• To investigate the structural, energetic, and electronic properties of triangular halogen trimers (RX)3.
• To analyze the nature and strength of halogen-halogen interactions within these trimers.
• To determine the cooperativity and the role of electrostatic and charge-transfer components in these interactions.

Main Methods:

• Density Functional Theory (DFT) using the Perdew-Burke-Ernzerhof (PBE) functional.
• Geometry optimization of stable trimer structures.
• Calculation of vibrational frequencies and binding energies (two- and three-body terms).

Main Results:

• All investigated trimers adopt cyclic structures with halogens in a type II approach.
• Bromine-bromine interactions are weak; iodine-iodine interactions are stronger.
• Most trimers are noncooperative, but three iodine trimers show weak cooperativity.
• Electrostatic contributions dominate halogen-halogen interactions, with charge-transfer becoming more important for iodine trimers.

Conclusions:

• Triangular halogen trimers exhibit unique cyclic geometries driven by atomic charge distribution.
• The strength and cooperativity of halogen interactions vary significantly between bromine and iodine trimers.
• Electrostatics are key, but charge-transfer effects gain importance in iodine-containing systems, influencing overall interaction dynamics.