Intermolecular Proton Transfer Enabled Reactive CO₂ Capture by the Malononitrile Anion

Area of Science:

• Materials Science
• Chemical Engineering
• Computational Chemistry

Background:

• Ionic liquids (ILs) utilizing carbanions are a novel class of materials for carbon capture.
• The malononitrile carbanion ([CH(CN)2]-) exhibits high CO2 absorption capacity.
• The proton transfer mechanism in forming carboxylic acids from CO2 capture by [CH(CN)2]- remains unclear.

Purpose of the Study:

• Investigate proton transfer mechanisms in the reaction of [CH(CN)2]- with CO2.
• Determine the feasibility of intramolecular versus intermolecular proton transfer pathways.
• Understand the factors stabilizing the final carboxylic acid product.

Main Methods:

• Density functional theory (DFT) calculations were employed.
• An implicit solvation model was utilized to simulate the reaction environment.
• Energy barriers for different proton transfer pathways were computed.

Main Results:

• The intramolecular proton transfer pathway has a high energy barrier (152 kJ/mol), making it unlikely.
• Intermolecular proton transfer between two [CH(CN)2COO]- anions is more feasible with a lower activation energy (50 kJ/mol).
• The resulting [C(CN)2COOH]- dimer is stabilized by intermolecular hydrogen bonds in a Z-configuration.

Conclusions:

• Intermolecular proton transfer is the dominant mechanism for carboxylic acid formation in this CO2 capture system.
• This finding provides crucial insights for designing advanced carbanions and ILs for efficient carbon capture and conversion.
• The stabilization of the dimer through hydrogen bonding influences the overall process efficiency.