Ionic liquids hold promise for applications in carbon capture, energy storage, and catalysis. Further optimizing their physical and chemical properties requires an improved understanding of the detailed interactions between cation and anion as well as small molecules solutes. Here we present two-dimensional infrared (2D-IR) spectroscopy of thiocyanate ions and CO2 in several ionic liquids. The thiocyanate experiments show that the dynamics are sensitive to hydrogen bonding at the 2-position of the imidazolium ring and give direct access to the activating events for the onset of diffusive motion. The 2D-IR experiments detect intertial motion (on the hundreds of femtosecond timescale) as well as rattling-in-a-cage on a few picosecond timescale and slower diffusive motions on the ~30 picosecond timescale. The results are compared to theoretical simulations which predict a tens of picosecond ion cage lifetime. Note that this timescale is faster than the reorientational relaxation (on the hundreds of picoseconds timescale) determined from ultrafast vibrational pump-probe measurements, indicating that 2D-IR spectroscopy has valuable additional information on the structure and dynamics in these liquids. The asymmetric stretch of CO2 also has a strong transition dipole moment and is also amenable to nonlinear vibrational spectroscopy, though the total evolution time which can be probed is limited by the short, picosecond vibrational relaxation time. Variation in dynamics as a function of the cation and anion identity and the implications for the intermolecular interactions in this industrially promising carbon capture material will be discussed.

Ultrafast vibrational spectroscopy (2D-IR) of ionic liquids

248th National Meeting (2014)

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