Halogen Bonding Interactions between Brominated Ion Pairs and CO2 Molecules: Implications for Design of New and Efficient Ionic Liquids for CO2 Absorption

In recent years, several novel halogenated liquids with characteristics of ionic liquids (ILs) were reported. To explore their performance in the absorption of CO2, in this work, quantum chemical calculations at DFT level have been carried out to investigate halogen bonding interactions between experimentally available brominated ion pairs and CO2 molecules. It is shown that, as compared to B3LYP, the functional PBE yields geometrical and energetic data more close to those of MP2 for cation-CO2 systems. The cation of brominated ILs under study can interact with CO2 molecules through Br center dot center dot center dot O interactions, possibly making an important impact on the physical solubility of CO2 in brominated ILs. The optimized geometries of the complexes of the ion pair with CO2 molecules are quite similar to those of the corresponding complexes of the cation, especially for the essentially linear C-Br center dot center dot center dot O contacts. However, much weaker halogen bonds are predicted in the former systems, as indicated by the longer intermolecular distances and the smaller interaction energies. Charges derived from NBO analysis reveal the origin of the different optimized conformations and halogen bonding interactions for the CO2 molecule. Based on the electrostatic potential results, the substitution of hydrogen atoms with fluorine atoms constituting the cation is then applied to enhance halogen bond strength. The QTAIM analysis further validates the existence of halogen bonding interaction in all complexes. The topological properties at the halogen bond critical points indicate that the Br center dot center dot center dot O interactions in the complexes are basically electrostatic in nature and belong to conventional weak halogen bonds. This study would be helpful for designing new and effective ILs for CO2 physical absorption.