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 N2, 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.
History
Date Created
2018-04-05Date Modified
2018-10-30Defense Date
2018-03-28Research Director(s)
Steven A. CorcelliCommittee Members
Edward J. Maginn J. Dan GezelterDegree
- Doctor of Philosophy
Degree Level
- Doctoral Dissertation
Program Name
- Chemistry and Biochemistry