Thiocyanate
as a Local Probe of Ultrafast Structure
and Dynamics in Imidazolium-Based Ionic Liquids: Water-Induced Heterogeneity
and Cation-Induced Ion Pairing
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Abstract
Ultrafast
two-dimensional infrared spectroscopy (2D-IR) of thiocyanate
([SCN]<sup>−</sup>) in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide
([C<sub>4</sub>C<sub>1</sub>im][NTf<sub>2</sub>]) and 1-butyl-2,3-dimethylimidazolium
bis(trifluoromethylsulfonyl)imide ([C<sub>4</sub>C<sub>1</sub>C<sub>1</sub><sup>2</sup>im][NTf<sub>2</sub>]) ionic liquids probes local structure and dynamics as a function
of the water content, solute counterion, and solute concentration.
The 2D-IR spectra of the water-saturated ionic liquids resolve two
distinct kinds of dynamics. This dynamical heterogeneity is explained
as two subensembles, one with and one without a water molecule in
the first solvation shell. When the countercation is K<sup>+</sup>, ion pairs between K<sup>+</sup> and [SCN]<sup>−</sup> that
persist for >100 ps are detected by long-lasting vibrational frequency
correlations. The observed dynamics are invariant to [SCN]<sup>−</sup>concentration, which indicates that the [SCN]<sup>−</sup> does
not cluster in ionic liquid solution. Taken together, these results
are consistent with a picture of thiocyanate as a local probe that
can interrogate ultrafast structure and dynamics at a small spatial
scale in ionic liquids