Experimental Quantification of Halogen⋅⋅⋅Arene van der Waals Contacts

Area of Science:

• Supramolecular Chemistry
• Physical Organic Chemistry

Background:

• Crystallographic and computational studies suggest favorable interactions between arenes and halogens.
• Experimental quantification in solution is challenging due to steric variations of halogens.

Purpose of the Study:

• To experimentally quantify halogen⋅⋅⋅arene interactions in various solvents.
• To elucidate the driving forces behind these interactions in solution.

Main Methods:

• Synthesis of molecular balance systems.
• Quantification using 1H NMR spectroscopy across 17 solvents and mixtures.
• Analysis using partitioned symmetry-adapted perturbation theory (SAPT0) energies.

Main Results:

• Halogen⋅⋅⋅arene interactions are unfavorable in solution (ΔG = 0 to +1.5 kJ/mol), contrary to gas-phase dispersion effects.
• Dispersion forces are attenuated by solvents, leading to a trend dominated by exchange repulsion.
• These interactions are less unfavorable in high solvophobicity, low polarizability solvents.
• Halogen⋅⋅⋅arene contacts are consistently less favorable than methyl⋅⋅⋅arene contacts (ΔG = 0 to -1.4 kJ/mol).

Conclusions:

• Solvent effects significantly alter the nature and strength of halogen⋅⋅⋅arene interactions.
• Exchange repulsion, not dispersion, dictates the trend in solution, increasing with halogen size.
• Methyl⋅⋅⋅arene interactions are inherently more favorable than halogen⋅⋅⋅arene interactions in solution.