Binding energies of CO$_{2}$ with some ionic liquids*

Room temperature ionic liquids (RTILs), a novel class of materials with negligible vapor pressures and potentiality as benign solvents, may be an ideal chemical for carbon dioxide (CO$_{2})$ gas sequestration. \textit{Ab initio} computational modeling was used to investigate the molecular interactions of simple RTIL anions hexafluorophosphate (PF$_{6}^{-})$ and tetrafluoroborate (BF$_{4}^{-})$ with CO$_{2}$. Electronic potential energy surface (PES) scans of a comprehensive sampling of 1:1 anion-CO$_{2}$ orientations were computed using Spartan '02 with Dunning's correlation consistent basis sets. Qualitatively, the PES scans yielded deeper, more numerous and radially closer active sites surrounding BF$_{4}^{-}$ anion as compared with the PF$_{6}^{-}$ anion. Quantitatively, the binding energies of 17.87 kJ/mol and 25.24 kJ/mol were extracted from the identified global energy minima for the PF$_{6}^{-}$ and BF$_{4}^{-}$ systems, respectively. The smaller BF$_{4}^{-}$ anion was concluded to bind more strongly to the CO$_{2}$. However, literature-reported experimental Henry's law constants for CO$_{2}$ dissolved in imidizolium based RTILs show greater gas solvation in the PF$_{6}^{-}$ system. The discrepancy between the energetics calculation results and the experimental solvation data will be discussed.