The ultrafast vibrational dynamics of carbon dioxide (CO₂) were studied in a series of physisorbing ionic liquids using two-dimensional infrared (2D-IR) spectroscopy. The microscopic dynamics reported by CO ₂ were found to depend strongly on the choice of anion. The timescales of spectral diffusion are attributed to the breakup of a local solvent shell (ion cage) around CO₂, and to correlate with the bulk viscosity in 1-alkyl-3-methylimidazolium ionic liquids. This correlation breaks for novel ionic liquid mixture with short alkyl sidechains and non-imidazolium head groups; however, the timescales of spectral diffusion are consistent with trends in ion transport (conductivity).

A semi-empirical spectroscopic map, developed from ab initio calculations, allowed comparison of experimental observables with molecular dynamics simulations. The observed and calculated frequency and reorientational dynamics of CO ₂ were compared, finding good correspondence. Both frequency and reorientational dynamics show multi-exponential behavior, indicating complex dynamics. Decomposition of the calculated frequency fluctuation correlation function (FFCF) into contributions from structural components of the ions confirms that the longest timescale is dominated by interactions of the anion and CO₂; however, there are substantial contributions from inertial motions involving the cation's charged head group.

Temperature-dependent 2D-IR of thiocyanate ([SCN]^–) and CO₂ in 1-alkyl-3-imidazolium bistri imide ([Im _n,1][Tf₂N], n = 2; 4; 6) ionic liquids interrogated the energetic barriers to motions around the probe molecules and the effect of increasing ionic liquid heterogeneity on the observed dynamics. Both [SCN]^– and CO₂ show a strong correlation of microscopic dynamics with viscosity in each ionic liquid studied; however, the spectral diffusion of [SCN]^– in [Im_2,1][Tf ₂N] is offset from those of [SCN]^– in the longer chain ionic liquids, potentially because of rotational motions. Additionally, both [SCN]^– and CO₂ show activated behavior in their spectral diffusion, with the activation barrier being dominated by the slowest resolved relaxation processes. The calculated barriers for both CO₂ and [SCN]^– correspond broadly to those for ion self-diffusion from MD and NMR studies. [SCN]^– shows a decrease in E_a for n = 6, possibly because of increasing nanoscopic polar-apolar segregation. The calculated barriers for CO₂ do not show this dependence on alkyl chain length, consistent with molecular modeling of the CO₂ frequency.