Ultrafast vibrational spectroscopy (2D-IR) of CO2 in ionic liquids: Carbon capture from carbon dioxide's point of view

The CO2 nu(3) asymmetric stretching mode is established as a vibrational chromophore for ultrafast two-dimensional infrared (2D-IR) spectroscopic studies of local structure and dynamics in ionic liquids, which are of interest for carbon capture applications. CO2 is dissolved in a series of 1-butyl-3-methylimidazolium-based ionic liquids ([C(4)C(1)im][X], where [X](-) is the anion from the series hexafluorophosphate (PF6-), tetrafluoroborate (BF4-), bis-(trifluoromethyl) sulfonylimide (Tf2N-), triflate (TfO-), trifluoroacetate (TFA(-)), dicyanamide (DCA(-)), and thiocyanate (SCN-)). In the ionic liquids studied, the nu(3) center frequency is sensitive to the local solvation environment and reports on the timescales for local structural relaxation. Density functional theory calculations predict charge transfer from the anion to the CO2 and from CO2 to the cation. The charge transfer drives geometrical distortion of CO2, which in turn changes the nu(3) frequency. The observed structural relaxation timescales vary by up to an order of magnitude between ionic liquids. Shoulders in the 2D-IR spectra arise from anharmonic coupling of the nu(2) and nu(3) normal modes of CO2. Thermal fluctuations in the nu(2) population stochastically modulate the nu(3) frequency and generate dynamic cross-peaks. These timescales are attributed to the breakup of ion cages that create a well-defined local environment for CO2. The results suggest that the picosecond dynamics of CO2 are gated by local diffusion of anions and cations. (C) 2015 AIP Publishing LLC.