Search for an exothermic halogen bond between anions

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Supramolecular Chemistry

Background:

• Anion-dimer complexes are typically metastable in the gas phase, higher in energy than separated monomers.
• Noncovalent interactions like hydrogen and halogen bonds stabilize anion pairs in condensed phases (crystals, solvents).

Purpose of the Study:

• To computationally identify stable gas-phase halogen-bonded anion dimers.
• To investigate Lewis acid candidates with iodine and carboxylate groups, modified by cyano substituents.
• To explore Lewis base interactions with chloride and amine-carboxylate anions.

Main Methods:

• Quantum chemical calculations were employed to model interactions between designed Lewis acids and Lewis bases.
• Systematic variation of carbon chain lengths and substituent patterns in Lewis acid candidates.
• Analysis of energetics for anion association reactions to determine stability.

Main Results:

• Exothermic association reactions were achieved between chloride anions and cyano-substituted alkenes and alkanes.
• Stability was enhanced when the iodine and carboxylate groups in the Lewis acid were separated by at least four carbon atoms.
• Energetics favored longer alkane chains, with energy changes (ΔE) around -30 kcal mol⁻¹.

Conclusions:

• Stable gas-phase halogen-bonded anion dimers are computationally feasible.
• Specific molecular designs, particularly longer alkanes with appropriate functionalization, can yield significantly exothermic and stable anion associations.
• This research provides a pathway for designing stable anion complexes in the gas phase.