An investigation of the low-frequency (i.e., less than 5 THz), inter-molecular dynamics of three imidazolium-based ionic liquids-1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4mim][NTf2]), 1-butyl-3-methylimidazolium dicyanamide ([C4mim][DCA]), and 1-ethyl-3-methylimidazolium dicyanamide ([C2mim][DCA])-is presented using two-dimensional (2D) Raman-THz spectroscopy combined with molecular dynamics (MD) simulations. By observing an echo in the 2D Raman-THz response, the experimental results indicate that the substitution of a small [DCA]- anion with a larger [NTf2]- one leads to a substantial increase in the structural inhomogeneity of the low-frequency modes of the system. These findings are corroborated by MD simulations, comparing the experimentally observed echo decay times to those of a computed velocity echo. The comparison suggests that the echo decay time reflects the instantaneous amount of structural order related to the charge alternation network, which is enhanced for the ionic liquid with the larger anion.
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