A diverse range of 4-dimethylaminopyridinium (DMAP) bis(trifluoromethylsulfonyl)-amide ionic liquids with specific functionalities (alkyl, alkoxy, hydroxyalkyl and benzyl) were designed, characterized and compared with their pyridinium analogs in terms of their physical and radiolytic properties. The influence of the dimethylamino group on ionic liquid structure was investigated by X-ray diffraction and molecular dynamics simulations. The influence of the electron-donating ability of the dimethylamino-substituted cation is evident in the differences in the electronic density of states between the DMAP and pyridinium ILs. This leads to substantial changes in the radical transients observed in pulse radiolysis of the neat ILs. It was found that the DMAP salts were higher melting, more viscous and less conducting than their pyridinium analogs. However, the DMAP salts exhibited higher thermal stabilities and could therefore be useful for high-temperature applications.

High-energy X-ray scattering experiments and molecular dynamics (MD) simulations were carried out on ionic liquids (ILs) consisting of 1-alkyl-3-methylimidazolium and 1-alkyl-1-methylpyrrolidinium cations. These cations were paired with bis(trifluoromethylsulfonyl)amide anions and identical alkyl tails were used for both cationic species. The goal of this work is to investigate how the nanoscale structure of the ionic liquid changes with the length and with the degree of branching of the alkyl tail, for ILs having a common anion. We investigate spatial correlations in the intermolecular region, focusing on the intrinsic charge-charge interactions that characterize all ionic liquids, as well as the nanoscale domain segregation that is present in IL species with significant nonpolar components.

In this study, we investigate the temperature dependence of low-frequency spectra in the frequency range of 0.3-200 cm-1 for ionic liquids (ILs) whose cations possess two systematically different cyclic groups, using femtosecond Raman-induced Kerr effect spectroscopy. The target ILs are bis(trifluoromethylsulfonyl)amide [NTf2]- salts of 1-cyclohexylmethyl-1-methylpyrrolidinium [CHxmMPyrr]+, 1-cyclohexylmethyl-3-methylimidazolium [CHxmMIm]+, N-cyclohexylmethylpyridinium [CHxmPy]+, 1-benzyl-1-methylpyrrolidinium [BzMPyrr]+, 1-benzyl-3-methylimidazolium [BzMIm]+, and N-benzylpyridinium [BzPy]+ cations. The aim of this study is to better understand the effects of aromaticity in the cations' constituent groups on the temperature-dependent low-frequency spectral features of the ILs. The low-frequency spectra of these ILs are temperature dependent, but the temperature-dependent spectrum of [CHxmMPyrr][NTf2] is different from that of other ILs. While [CHxmMPyrr][NTf2] shows spectral changes with temperature in the low-frequency region below 50 cm-1, the other ILs also show spectral changes in the high-frequency region above 80 cm-1 (above 50 cm-1 in the case of [BzMPyrr][NTf2]). We conclude that the spectral change in the low-frequency region is due to both the cation and anion, while the change in the high-frequency region is attributed to the red shift of the aromatic ring librations. On the basis of the plots of the first moment of the spectra vs. temperature, we found that the first moment of the low-frequency spectrum of the IL whose cation does not have an aromatic ring is less temperature dependent than that of the other ILs. However, the intrinsic first moment, the first moment at 0 K, of the low-frequency spectrum is governed by the absence or presence of a charged aromatic group, while a neutral aromatic group does not have much influence on determining the intrinsic first moment.

A comprehensive study of multi-nuclear (1H, 2H, and 19F) magnetic resonance self-diffusion coefficients (D) and spin-lattice relaxation times (T1) was done as functions of both temperature and pressure on selectively deuterated 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (EMIM TFSA) ionic liquid isotopologues and related ionic liquids. While variable temperature studies provided useful insight, pressure dependent studies were more elucidating as discussed below. First, stronger electric field gradients were indicated in the alkyl chain region compared to the imidazolium ring of EMIM TFSA as determined from variable temperature 2H T1. Second and most significantly, variable pressure measurements of the EMIM and TFSA diffusion revealed independent motions for the cations and anions, with that of the TFSA anions being slowed much more by increasing pressure than for the EMIM cations. Activation volumes were determined from the pressure dependent D data and values of 28.8 ± 2.5 cm3/mol and 14.6 ± 1.3 cm3/mol were obtained for TFSA and EMIM respectively. Typically, smaller activation volumes support less restriction in motion. One possible explanation for this difference in behavior between EMIM and TFSA is that increasing pressure limits the conformational mobility of the TFSA. Third, variable pressure 2H T1 data shows two T1 regimes, for the MD3 and D2 isotopologues between 0.1-100 and 100-250 MPa respectively. These two regimes could represent dissimilar arrangements for the EMIM cation. Activation volumes determined from the pressure dependent 2H T1 data were 21 and 25 (0-100 MPa) and 11 and 12 (100-250 MPa) cm3/mol for the MD3 and D2 isotopologues, respectively. These and other details will be discussed.

Multinuclear ((1)H, (2)H, and (19)F) magnetic resonance spectroscopy techniques as functions of temperature and pressure were applied to the study of selectively deuterated 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (EMIM TFSA) ionic liquid isotopologues and related ionic liquids. For EMIM TFSA, temperature-dependent (2)H T1 data indicate stronger electric field gradients in the alkyl chain region compared to the imidazolium ring. Most significantly, the pressure dependences of the EMIM and TFSA self-diffusion coefficients revealed that the displacements of the cations and anions are independent, with diffusion of the TFSA anions being slowed much more by increasing pressure than for the EMIM cations, as shown by their respective activation volumes (28.8 ± 2.5 cm(3)/mol for TFSA vs 14.6 ± 1.3 cm(3)/mol for EMIM). Increasing pressure may lower the mobility of the TFSA anion by hindering its interconversion between trans and cis conformers, a process that is coupled to diffusion according to published molecular dynamics simulations. Measured activation volumes (ΔV(‡)) for ion self-diffusion in EMIM bis(fluoromethylsulfonyl)amide and EMIM tetrafluoroborate support this hypothesis. In addition, (2)H T1 data suggest increased ordering with increasing pressure, with two T1 regimes observed for the MD3 and D2 isotopologues between 0.1-100 and 100-250 MPa, respectively. The activation volumes for T1 were 21 and 25 cm(3)/mol (0-100 MPa) and 11 and 12 cm(3)/mol (100-250 MPa) for the MD3 and D2 isotopologues, respectively.

We have critically investigated the low-frequency spectra of six ionic liquids (ILs) consisting of systematically different cations having benzyl moieties or comparable-sized saturated cyclohexylmethyl groups, by means of femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES). The target ionic liquids are bis(trifluoromethylsulfonyl)amide ([NTf2](-)) salts of the 1-benzyl-3-methylimidazolium ([BzMIm](+)), 1-benzyl-1-methylpyrrolidinium ([BzMPyrr](+)), 1-benzylpyridinium ([BzPy](+)), 1-cyclohexylmethyl-3-methylimidazolium ([CHxmMIm](+)), 1-cyclohexylmethyl-1-methylpyrrolidinium ([CHxmMPyrr](+)), and 1-cyclohexylmethylpyridinium ([CHxmPy](+)) cations. The primary purpose of this study is to clarify the effects of charged and neutral aromatic moieties on the low-frequency spectrum and bulk properties such as liquid density, surface tension, shear viscosity, glass transition temperature, and melting point. We found that ILs with benzyl groups have larger surface tensions than those with the same cation bearing the cyclohexylmethyl group. The trend in the glass transition temperatures, comparing ILs having the same side group, is pyridinium > imidazolium > pyrrolidinium. The effects of a single aromatic moiety on the shear viscosity are inconclusive, although the viscosities of the ILs with aromatic moieties on both the cation and the benzyl group, i.e., [BzMIm][NTf2] and [BzPy][NTf2], are substantially lower than those of the other ILs at room temperature, as a consequence of their higher fragilities. In the low-frequency Kerr spectra in the frequency range of approximately 0.1 to 200 cm(-1) measured by fs-RIKES, the ILs possessing two aromatic groups show the largest relative intensity of the nuclear response to the electronic response. Both the charged and neutral aromatic rings show signals due to the ring libration; the neutral one appears at a lower frequency than the charged one. The relationship between the first moment of the broad low-frequency spectrum band and the bulk parameter consisting of the square root of the surface tension divided by the liquid density is obeyed by the cyclohexylmethyl derivatives whether the cation is aromatic or not, but not by the ILs with the neutral aromatic benzyl group. Quantum chemistry calculations have been also performed to understand the vibrational modes of the ionic species in the ILs.

Binary mixtures of ionic liquids (ILs) were prepared and characterized to obtain ILs with viscosity and conductivity properties optimized for use in energy storage devices such as supercapacitors. The bis(trifluoromethylsulfonyl)amide anion (NTf2 -) was the common anion for both ILs in the mixture. Ethoxyethyl was the common substituent on the solute and solvent, as ether substituents have been shown to produce low viscosity ILs. N-Ethoxyethyl-N-methylpyrrolidinium NTf2 was used as the solvent for all of the IL mixtures. The solutes were either monocationic or dicationic and made up 10% of the binary mixture by weight. We report here on the physical properties of the individual ILs and their binary mixtures.

We report the synthesis, characterization and cellulose dissolution properties of a series of ionic liquids bearing alkyl and hydroxyl-functionalized imidazolium and pyrrolidinium cations, and dialkyl phosphate anions. Ionic liquids that dissolve cellulose have anions that are excellent hydrogen bond acceptors. Based on this study, it is clear that the cation also plays a role. Cellulose dissolution was achieved in the imidazolium ionic liquids but not in the pyrrolidinium ionic liquids. Thermogravimetric analysis results show that most of the ILs are thermally stable up to 240 degrees C and thus are more suitable solvents for high temperature applications than most organic solvents. Imidazolium ILs are less viscous and slightly more thermally stable than the pyrrolidinium ILs of the same chain length.

The need for improved energy storage capability in supercapacitors has prompted the use of ionic liquids as alternatives to acetonitrile based solvents. Despite the advantages of supercapacitors, their use in transportation is not favorable because of the deterioration of organic solvents with increasing temperatures and their high vapor pressures. We report here on the synthesis and characterization of ionic liquids optimized for use in supercapacitors. The cations were designed to bear ether substituents which are known to lower the viscosity of ILs. ILs and binary mixtures of ILs based on pyrrolidinium, imidazolium and dicationic ammonium cations bearing ether substituents and bis(trifluoromethylsulfonyl)amide [NTf2 -] anions have been synthesized, purified and characterized. The halide salts were synthesized by reaction of the amine with 2-bromoethylethylether. They were then converted to the NTf2 salts after purification. The structures of the salts were confirmed using H-1 and C-13 Nuclear Magnetic Resonance spectroscopy. The dicationic ILs were used as the solutes in the binary IL mixyures. The binary IL mixtures have been analyzed for their transport properties using NMR. The work at BNL was supported by the BNL Office of Educational Programs and the DOE Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences under contract # DE-AC02-98CH10886.

X-ray scattering and molecular dynamics simulations have been carried out to investigate structural differences and similarities in the condensed phase between pyrrolidinium-based ionic liquids paired with the bis(trifluoromethylsulfonyl)amide (NTf2(-)) anion where the cationic tail is linear, branched, or cyclic. This is important in light of the charge and polarity type alternations that have recently been shown to be present in the case of liquids with cations of moderately long linear tails. For this study, we have chosen to use the 1-alkyl-1-methylpyrrolidinium, Pyrr(1,n(+)) with n = 5 or 7, as systems with linear tails, 1-(2-ethylhexyl)-1-methylpyrrolidinium, Pyrr(1,EtHx(+)), as a system with a branched tail, and 1-(cyclohexylmethyl)-1-methylpyrrolidinium, Pyrr(1,ChxMe(+)), as a system with a cyclic tail. We put these results into context by comparing these data with recently published results for the Pyrr(1,n(+))/NTf2(-) ionic liquids with n = 4, 6, 8, and 10.1,2 General methods for interpreting the structure function S(q) in terms of q-dependent natural partitionings are described. This allows for an in-depth analysis of the scattering data based on molecular dynamics (MD) trajectories that highlight the effect of modifying the cationic tail.