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Molecular dynamics (MD) simulations have recently shown that ionic liquids (ILs) based on the 1-alkyl-3-methylimidazolium cation ([C_nmim]⁺) with alkyl chains C₄ and longer are nanostructurally organized with polar regions arising from charge ordering and nonpolar regions arising from alkyl chain segregation. The goal of this project is to exam the role of nanostructural organization in the intermolecular dynamics of imidazolium ILs using optical Kerr effect (OKE) spectroscopy. During the first year of this grant three papers were published.

1. Nanostructural Organization and Anion Effects in the Optical Kerr Effect Spectra of Binary Ionic Liquid Mixtures. The effect of anions on the OKE spectra of binary ionic liquid mixtures at 295 K was studied with one mixture comprising the [C₅mim]⁺ cation and the anions PF₆^- and CF₃CO₂^- (TFA^-), and another mixture comprising the [C₅mim]⁺ cation and the anions Br^- and bis{(trifluoromethane)sulfonyl}amide (NTf₂^-). The OKE spectra of the mixtures were then compared with the calculated mole-fraction weighted sum of the normalized OKE spectra of the neat liquids.  In contrast to the OKE spectra of [C₅mim]Br/[C₅mim][NTf₂] mixtures, the OKE spectra of [C₅mim][PF₆]/[C₅mim][TFA] mixtures are nonadditive.  The effect of the relative sizes of the anions on the OKE spectra of binary mixtures can be explained if we assume mixtures are nanostructurally organized into nonpolar regions and ionic networks, as found previously for neat ILs. For anions that are nearly the same in size (PF₆^- and TFA^-), we postulate that the ionic networks are characterized by random co-networks, whereas for anions that differ greatly in size (Br^- and NTf₂^-), the ionic networks are characterized by block co-networks. Nonadditivity of the OKE spectra is the norm for most molecular liquids, even for non-associating molecular liquids.  That the OKE spectra of ILs can exhibit additivity or nonadditivity depending upon the relative sizes of the anions is yet another example of the unique properties of ILs and provides further insight into the nature of the heterogeneity in their structure and dynamics.

2. Intermolecular Vibrational Motions of Solute Molecules Confined in the Nonpolar Domains of Ionic Liquids. Because of microphase separation between nonpolar and polar domains in ILs, solute-solvent interactions can be varied and quite complex.  At low concentrations, solute molecules are found in the domains for which the affinity is the greatest.  Nonpolar molecules, such as n-hexane, tend to reside in the nonpolar domains and are excluded from the ionic networks because of the cohesive energy of the charged groups, whereas associating solutes, such as water, reside mainly in the ionic networks, forming strong hydrogen bonds with the charged part of the ions. On the other hand, dipolar molecules, such as acetonitrile, interact with nonpolar domains as well as the charged head groups in the ionic networks.  At high concentrations, the presence of solute molecule eventually leads to disruption of the nanostructural organization and, in the case of water, to the evolution of micellar structures.  In this study we addressed the following question about the dynamics of solute molecules in ILs.  Are the intermolecular vibrational motions of nonpolar molecules confined in the nonpolar domains formed by tail aggregation in ILs the same as in an alkane solvent?    To address this question, we compared the OKE spectrum of a dilute solution of CS₂ in [C₅mim][NTf₂] at 295 K to that of CS₂ in n-pentane at a mole fraction x_CS2 = 0.05 at 295 K. Based on multicomponent line shape analysis of the OKE spectrum of the CS₂/IL mixture, we find that the CS₂ and [C₅mim][NTf₂] contributions are separable and that the intermolecular spectrum of CS₂ in [C₅mim][NTf₂] is similar to that of CS₂ in n-pentane. This comparative study of the OKE spectra of dilute CS₂/[C₅mim][NTf₂] and CS₂/n-pentane solutions strongly suggests that the intermolecular vibrational motions of CS₂ in the IL are almost the same as that of CS₂ in n-pentane.  At first glance, this result is not surprising given that CS₂ molecules tend to reside in the nonpolar domains of the IL. However, to our knowledge, this work provides for the first time information about the intermolecular dynamics of a nonpolar solute molecule in the nonpolar domains of ILs.

3. Effect of Cation Symmetry and Alkyl Chain Length on the Structure and Intermolecular Dynamics of 1,3-Dialkylimidazolium Bis(trifluoromethanesulfonyl)amide Ionic Liquids. In this study, the structure and intermolecular dynamics of [C_nmim][NTf₂] with n = 2-5 were compared to that of 1,3-dialkylimidazolium bis{(trifluoromethane)sulfonyl}amides [(C_n)₂im][NTf₂] with n = 2-5.  The structures of these ILs were studied by small-wide angle X-ray scattering (SWAXS) and their intermolecular dynamics by OKE spectroscopy. This study was done in collaboration with Richard A. Bartsch's group at Texas Tech University, who synthesized the ILs, and Alessandro Triolo and Olga Russina of Istituto per i Processi Chimico-Fisici-CNR, Messina, Italy, who performed the SWAXS measurements.  The existence of a low-Q (momentum transfer) peak in the SWAXS data that depends both in amplitude and position on n is the signature of the occurrence of nanoscale structural heterogeneities whose sizes depend on the alkyl chain length. The OKE measurements indicate that the intermolecular dynamics of ILs with symmetric [(C_n)₂im]⁺ cations are higher in frequency than that of ILs with asymmetric [C_nmim]⁺ cations.  These results suggest that the local structure of ILs with symmetric cations is more solid-like than that of ILs with asymmetric cations.  Further evidence for this difference is that the width of the low-Q peak in the SWAXS data is narrower in [(C₅)₂im][NTf₂] than in [C₅mim][NTf₂].  Moreover, the structure and intermolecular dynamics of the ILs with ethyl-substituted cations appear to be quite different than that of other ILs within a given series.  This difference is evidenced by a change in the dependence of the first spectral moment and width of intermolecular part of the OKE spectra on alkyl chain length in going from n = 2 to n = 3. This change is consistent with the SWAXS data which indicates that alkyl chain segregation may be occurring for n = 3.