Modeling Carbon Dioxide Vibrational Frequencies in Ionic Liquids

Ionic liquids have attracted a great deal of attention in the past decade due to their unique physical and chemical properties. One property that been widely investigated is ionic liquids’ high selectivity and solubility for CO₂ over other small molecule gases present in air and flue gas, such as N_2, for the purpose of carbon capture and sequestration. As such, many studies have focused on the solvation of CO₂ in ionic liquids. However, few studies have been able to directly link experiment and theory, instead relying on indirect links between bulk experimental observables and the atom‑level resolution of theory. Because CO₂ absorbs strongly in the infrared, vibrational spectroscopy is a natural tool for studying its structure and dynamics in ionic liquids. Vibrational spectroscopy, in particular 2D‑IR spectroscopy, directly reports on the length and timescales of the vibrational reporter. Additionally, using CO₂ as the vibrational reporter permits an examination of its solvation dynamics from the solute’s point of view. These length and time scales lend themselves to direct theoretical calculation, allowing simulation to provide molecular level detail to the experiments. A wide ranging, multistage collaboration between experiment and theory has developed and applied the tools required for a detailed analysis of the CO₂‑ionic liquid system in the particular case of 1‑butyl‑3‑methylimidazolium hexafluorophosphate, a model ionic liquid. It is found that the vibrational frequency of CO₂ in ionic liquids can be calculated accurately by using a quantum mechanics/molecular mechanics implementation of density functional theory to obtain the potential energy surface required for a discrete variable representation solution to the nuclear Schrödinger equation. These vibrational frequencies correlate strongly with the geometry of the CO₂ and the forces it experiences in solution. Examining the solvation of the solute, it is found that the CO₂ is preferentially solvated by the highly charged moieties in the ionic liquid, particularly the anion and the cation ring. This is unique to CO₂ among uncharged small molecule gases, as N₂ is solvated in a highly different manner. CO₂ also has similar positive energies and enthalpies of activation for frequency decorrelation, orientational randomization, and solvent cage breakup, implying that solvent cage breakup governs the other two processes.