Strong, Localized, and Directional Hydrogen Bonds Fluidize Ionic Liquids

Abstract

Hydrogen bonds are very important in chemistry and biology. [1][2][3] The properties of liquids and solutions consisting purely of neutral molecules are characteristically determined by the strength and number of hydrogen bonds. When water freezes to form ice, each water molecule forms four strong hydrogen bonds to its neighbors in tetrahedral fashion giving a periodical H-bond network.[4] In nonpolar solvents peptides retain their helical secondary structure up to very high temperature as a result of intramolecular H bonds.[5] Nucleic acids when neutralized in aqueous electrolyte solutions build the famous double-helical structure on the basis of strong two-and threefold hydrogen bonds between base pairs. [6] What all these important structures have in common is that they are stabilized by hydrogen bonds; they usually become more rigid and less flexible with increasing strength and number of H bonds.In this study we show that the opposite behavior can be found for ionic liquids (ILs), which are composed solely of ions rather than neutral molecules. ILs constitute a remarkably promising class of technologically useful and fundamentally interesting materials. [7][8][9][10][11][12] Herein we show that strong and directional H bonds formed between cations and anions destroy the charge symmetry and thus can fluidize ionic liquids. H bonds introduce "defects" into the Coulomb network of ILs and increase the dynamics of the cations and anions, resulting in decreased melting points and reduced viscosities. Thus the properties of ILs can be altered by adjusting the ratio between Coulomb forces and van der Waals interactions represented by H bonds. This possibility is demonstrated by FTIR measurements of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C 2 mim]-[NTf 2 ] (1) and 1-ethyl-2,3-dimethylimidazolium bis(trifluoro methylsulfonyl)imide [C 2 C 1 mim][NTf 2 ] (2), wherein characteristic H-bond contributions can be switched off by methylation at C(2).Recently, we presented low-frequency vibrational spectra of imidazolium-based ionic liquids in the range between 30 and 300 cm À1 obtained by far-infrared spectroscopy.[13] We could show that the absorptions at wavenumbers above 150 cm À1 can be assigned to intramolecular bending and wagging modes of cations and anions in the ionic liquid. The contributions below 150 cm À1 were assigned to the intermolecular interactions between cations and anions that describe the bending and stretching vibrational modes of hydrogen bonds. This assignment was supported by DFT calculations which gave wavenumbers for the bending and stretching modes of ion pairs and ion-pair aggregates in this frequency region. Further proof of the intermolecular interactions came from a nearly linear relation between the average binding energies of calculated IL aggregates and the measured wavenumbers for maxima of the low-frequency vibrational bands for a series of ionic liquids containing the same imidazolium cation but different anions. Although this assignment is supported by recent THz...

Effects of C(2) Methylation on Thermal Behavior and Interionic Interactions in Imidazolium-Based Ionic Liquids with Highly Symmetric Anions

“…In the real liquid, a distribution of several cation/anion configurations will be present simultaneously [43], and the sketch must be seen as one example of what the data suggest exists in larger amounts. in computational studies [4,5,7,28,29,38]. This suggests that the above findings represent a rather universal phenomenon.…”

“…The list of investigated anions includes bis(trifluoromethylsulfonyl) imide [4][5][6][7], nitrate [8], alkyl sulfates [5,9], dicyanamide [5,10], thiocyanate [5], acetate [11,12], hydrogen sulfate [13,14], and trifluoromethanesulfate [15,16]. On the other hand, interactions with highly symmetrical anions have been studied less.…”

“…The impact of C(2) methylation on the viscosity of imidazolium ionic liquids has been studied extensively, leading to several theories. Fumino's defect theory [7] suggests that the absence of a strong and directional hydrogen bond at C(2) produces a reduced disruption of order. On the other hand, Hunt's reduced entropy theory [28] argues that the methyl group at the C(2) position blocks low energy configurations.…”

Controlled self-assembly of amphiphiles in ionic liquids and the formation of ionogels by molecular tuning of cohesive energies

“…Imidazolium-based ILs are reported to form ionic hydrogen bonds of the form C(2)-H y A À (A, anion), and bonds formed between 1-ethyl-3-methyl-imidazolium and TFSA anion were reported by Fumino et al 36 Meanwhile, ILs with TFSA anions were reported to possess a significantly smaller cohesive energy density than water. 13 The observation that 1 and 2 are molecularly dispersed in C 4 mimTFSA without forming self-assemblies can be understood by the insufficient difference in cohesive energy between 1, 2 and C 4 mimTFSA.…”

“…It should also be noted that ILs can be amphiphilic 35 and capable of forming ionic hydrogen bonds. 36 We describe herein the formation of bilayer membranes in ILs for a series of aqueous-bilayer-forming cationic glutamate amphiphiles. The intermolecular interactions required for self-assembly in ILs vary depending on the chemical structure of ILs, and these results provide a general guideline to design ordered molecular self-assembly systems in ILs.…”