Dipolar coupling and molecular vibrations in ionic liquids

Long-range correlation of intra-molecular and inter-molecular vibration in liquid CCl4

Experiments measuring the polarization dependence of hyper-Raman light scattering reveal long-range correlation of molecular vibrations in liquid CCl₄. The ν₃ and ν₁ + ν₄ intra-molecular vibrations at about 770 cm^-1 are strongly polarized transverse to the scattering wavevector. Weaker transverse polarization is exhibited by the ν₁, ν₂, and ν₄ intra-molecular vibrations and by the inter-molecular collision-induced band around 0 cm^-1. The observed polarization dependence is due to the correlation of the vibrations on molecules separated by about 200 nm. The strength of the observed correlation increases with the transition dipole moment for the vibration mode and is consistent with dipole-dipole coupling.

Quantitative Investigation of Ion Clusters in a Double Salt Ionic Liquid by Both Vibrational Spectroscopy and Molecular Dynamics Simulation

Vibrational spectroscopy and molecular dynamics simulations are powerful tools frequently used to elucidate interactions among ions in ionic liquid electrolyte solutions. We apply these techniques to characterize ionic interactions in mixtures of 1-butyl-1-methylpyrrolidinium trifluoromethansulfonate, [C₁C₄pyr][CF₃SO₃], and lithium trifluoromethanesulfonate, LiCF₃SO₃, namely, [Li]_0.091[C₁C₄pyr]_0.909[CF₃SO₃] and [Li]_0.167[C₁C₄pyr]_0.833[CF₃SO₃]. The computational and experimental data indicate that extensive, LiCF₃SO₃-rich regions exist within the solutions, and most of the anionic species that are composed of these domains are either [Li₂CF₃SO₃]⁺ or LiCF₃SO₃ moieties. The [Li]_0.167[C₁C₄pyr]_0.833[CF₃SO₃] system contains a larger number of [Li₂CF₃SO₃]⁺ and [Li₃CF₃SO₃]²⁺ species than [Li]_0.091[C₁C₄pyr]_0.909[CF₃SO₃], which may explain, in part, the reduction in ionic conductivity when LiCF₃SO₃ is added to [C₁C₄pyr][CF₃SO₃]. The charge-organized liquid structure inherent to [C₁C₄pyr][CF₃SO₃] supports the dynamic coupling of vibrationally induced dipole moments to form optical phonons. Consequently, intense, IR-active vibrational modes are split into transverse optical and longitudinal optical components. Band splitting is reduced when LiCF₃SO₃ is added to the ionic liquid, suggesting that ionically associated anions impede the ability of the ionic liquid to support optical phonons.

Effect of Ionic Composition on Physicochemical Properties of Mono-Ether Functional Ionic Liquids

Tunable properties prompt the development of different "tailor-made" functional ionic liquids (FILs) for specific tasks. FILs with an ether group are good solvents for many organic compounds and enzymatic reactions. However, ionic composition influences the solubility by affecting the physiochemical properties of these FILs. To address the structure effect, a series of novel FILs with a mono-ether group (ME) based on imidazole were prepared through cationic functionalization and anionic exchange reactions, and characterized by NMR, mass spectroscopy, and Thermogravimetric analysis (TGA). The effect of ionic composition (cationic structure and anions) on density, viscosity, ionic conductivity, electrochemical window, and thermal properties of these ME-FILs were systematically investigated. In general, the viscosity and heat capacity increases with the bigger cationic volume of ME-FILs; in particular, the 2-alkyl substitution of imidazolium enhances the viscosity remarkably, whereas the density and conductivity decrease on the condition of the same [NTf₂]^- anion; For these ME-FILs with the same cations, the density follows the order of [NTf₂]^- > [PF₆]^- > [BF₄]^-. The viscosity follows the order of [PF₆]^- > [BF₄]^- > [NTf₂]^-. Ion conductivity follows the order of [NTf₂]^- ≈ [BF₄]^- > [PF₆]^-. It is noted that the dynamic density has a good linear relationship with the temperature, and the slopes are the same for all ME-FILs. Furthermore, these ME-FILs have broad electrochemical windows and glass transition temperatures in addition to a cold crystallization and a melt temperature for ME-FIL7. Therefore, the cationic structure and counter anion affect the physicochemical properties of these ME-FILs together.