Dicationic versus monocationic ionic liquids: distinctive ionic dynamics and dynamical heterogeneity.

The dynamical properties of a dicationic ionic liquid (IL), 1,6-bis(3-methylimidazolium-1-yl)hexane bis(trifluoromethylsulfonyl)amide ([C(6)(MIm)(2)][NTf(2)](2)), compared to 1-methyl-3-propylimidazolium bis(trifluoromethylsulfonyl)amide ([C(3)MIm][NTf(2)]), as its monocationic imidazolic counterpart, are studied by molecular dynamics simulations. We investigate relaxation processes of the polarizability anisotropy of the system and collective dynamics of both the ILs with mean-squared displacement (MSD), non-Gaussian parameter, and the intermediate scattering functions. The analyses of librational dynamics show that the difference of the Kerr spectra between the ILs could be mainly ascribed to the distinctive angular momentum of [C(6)(MIm)(2)](2+) and [C(3)MIm](+) and related to the difference of relaxation behavior between [C(6)(MIm)(2)](2+) and [C(3)MIm](+). Also, it is indicated that the librational dynamics of [NTf(2)](-) indicate a common resonance-type sharp peak that corresponds to an intermolecular motion coupled to the vibrational mode intrinsic to [NTf(2)](-). In addition, it is exhibited from the total X-ray structure factors calculated for both of the ILs that the low-k peak at 0.20 Å(-1) appears for [C(6)(MIm)(2)][NTf(2)](2), while we do not see it for [C(3)MIm][NTf(2)]. We find that the contribution of the anion-cation and anion-anion correlations to the low-k peak is more significant than the cation-cation correlation. Therefore, it is suggested for [C(6)(MIm)(2)][NTf(2)](2) that dynamical heterogeneous behavior strongly correlates with structural variations or heterogeneity.