Modeling Carbon Dioxide Vibrational Frequencies in Ionic Liquids: III. Dynamics and Spectroscopy

In recent years, interest in carbon capture and sequestration has led to numerous investigations of the ability of ionic liquids to act as recyclable CO2-sorbent materials. Herein, we investigate the structure and dynamics of a model physisorbing ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([C4C1Im][PF6]), from the perspective of CO2 using two-dimensional (2D) IR spectroscopy and molecular dynamics simulations. A direct comparison of experimentally measured and calculated 2D IR line shapes confirms the validity of the simulations and spectroscopic calculations. Taken together, the simulations and experiments reveal new insights into the interactions of a CO2 solute with the surrounding ionic liquid and how these interactions manifest in the 2D IR spectra. In particular, higher CO2 asymmetric stretch vibrational frequencies are associated with softer, less populated solvent cages and lower frequencies are associated with tighter, more highly populated solvent cages. The CO2 interacts most...