Intermolecular Dynamics of Room-Temperature Ionic Liquids:  Femtosecond Optical Kerr Effect Measurements on 1-Alkyl-3-methylimidazolium Bis((trifluoromethyl)sulfonyl)imides

Abstract

Using optically heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) with 40 fs laser pulses, the transient birefringence in the room-temperature ionic liquids (RTILs), 1-alkyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imides, [C n mim]NTf2 with n = 2, 4, 5, 6, 8, 10 (C2, C4, C5, C6, C8, C10), has been studied at room temperature and ambient pressure. Near zero delay, the OHD-RIKES response is dominated by the instantaneous electronic response. The nuclear response appears as a shoulder on the electronic response. Between 0 and 1 ps, the nuclear response is dominated by the intermolecular vibrational (nondiffusive) response. For C4, C5, C6, and C8, the 1/e time of the pseudo-exponential tail of the intermolecular response decreases with viscosity, in accord with the hydrodynamic model for vibrational dephasing. Superimposed on the OHD-RIKES response for C4, C5, and C6 is a coherent oscillation with a frequency of ∼140 cm-1. The intermolecular vibrational spectra for these RTILs obtained from the reduced OHD-RIKES data by using a Fourier transform procedure extend from 0 to 200 cm-1 and are bimodal with a low-frequency component at ∼22 cm-1 and a high-frequency component at ∼84 cm-1. The relative contribution of the high-frequency component to the total band increases in going from C2 to C5 and remains constant for C5, C6, and C8. The behavior of the reduced spectral densities is consistent with increasing order in the liquid. It is proposed that the 140 cm-1 oscillation arises from collective motions of locally ordered domains in the liquid.

Spotlight on ionic liquids

“…Spectral features with maxima near 15 cm −1 are often attributed to translational dynamics for both molecular solvents and ILs. 94,[96][97][98] If the cation in a series of ILs is held constant, changes in the low-frequency spectrum upon anion substitution illustrate the differences in the strength and specificity of anion-cation interactions, as shown in Fig. 7.…”

Dynamics of RTILs: A comparative dielectric and OKE study

“…This region has been intensively studied by OKE during the last decade, see Refs. [10,49,50,51,52] and literature cited therein. A pronounced peak at 1-3 THz is a general characteristic of liquids and well established as the librational motion, i.e.…”

Atom Substitution Effects in Ionic Liquids: A Microscopic View by Femtosecond Raman-Induced Kerr Effect Spectroscopy

“…Among these spectroscopic techniques, femtosecond RIKES can cover the frequency range of the intermolecular (interionic) vibrational band in molecular liquids including ILs. Quitevis and co-workers reported the first RIKES-based study of ILs in 2002 (Hyun et al, 2002). The Quitevis group extensively studied the detailed aspects for the microstructures of neat ILs (Xiao et al, 2009;Russina et al, 2010), as well as binary IL mixtures (Xiao et al, 2006;Xiao et al, 2007;Xiao et al, 2008).…”