NMR Studies of Protic Ionic Liquids

Overbeck, Viviane; Ludwig, Ralf

doi:10.1016/bs.arnmr.2018.05.002

Cited by 11 publications

(11 citation statements)

References 166 publications

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“…The local structure of neat ILs has been extensively studied by different techniques, such as NMR, − IR spectroscopy, − X-ray scattering, − and molecular dynamic (MD) simulations. − The cationic group in a typical IL, such as 1-alkyl-3-methylimidazolium-based salts, usually contains the hydrophobic hydrocarbon tail and the hydrophilic cationic headgroup; thus, it is expected that the nonpolar part can aggregate to form the nanostructure or heterogeneous domains. , Both the X-ray scattering measurements and MD simulation showed evidence of the existence of nanoscale heterogeneities in neat ILs due to the segregation of the alkyl chains . As water molecules are added into the IL solution, the local structure of ILs is expected to be strongly affected due to the formation of hydrogen bond interactions between water and the cation/anion in the ILs. , Furthermore, NMR spectroscopy showed that the stacking of the imidazolium cationic ring is swollen or disrupted due to the presence of water clusters incorporated into the IL network .…”

Section: Introductionmentioning

confidence: 99%

Ultrafast Hydrogen Bond Exchanging between Water and Anions in Concentrated Ionic Liquid Aqueous Solutions

et al. 2019

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The mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4) ionic liquids (ILs) and water as a function of IL concentrations have been investigated by Fourier transform infrared (FTIR) spectroscopy and ultrafast two-dimensional IR (2D IR) spectroscopy. FTIR spectra of the mixtures resolve two different types of water species, one interacting with the BF4 – anions and the other associated with bulklike water molecules. These two water species are in a dynamic equilibrium through forming different hydrogen bonding configurations which are separated by more than 100 cm–1 in the IR spectra. The structural dynamics of the IL mixtures are further revealed by monitoring the vibrational relaxation dynamics of the OD stretching group of interfacial water molecules hydrogen bonded to BF4 – anions. With the increase of the IL bulk concentration, vibrational population and rotational dynamics of the interfacial water molecules can be described by a biexponential decay function and are strongly dependent on the IL concentrations. Furthermore, the ultrafast hydrogen bond exchanging between water and BF4 – anions in the ILs are also measured using 2D IR spectroscopy. The average hydrogen bond exchanging rate is determined to be 19 ± 4 ps, which is around 3 times slower than that in the NaBF4 electrolyte aqueous solution. The much slower hydrogen bond exchanging rate indicates that the local structure of ILs and water molecules are strongly mediated by the steric effect of the cationic group in the ILs, which is proposed to be responsible for the formation of the heterogeneous structure in the IL mixtures. By using SCN– as the anionic probe, the structural inhomogeneity in the IL solutions can be confirmed from the distinct rotational dynamics of the SCN–, which is segregated from the rotational dynamics of water molecules in the IL mixtures.

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Section: Introductionmentioning

confidence: 99%

Ultrafast Hydrogen Bond Exchanging between Water and Anions in Concentrated Ionic Liquid Aqueous Solutions

et al. 2019

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“…Due to fast intermolecular proton exchange, a single isolated 1 H NMR peak (Figure S1 and Table S2) emerged as an average of the peaks for molecular HAc and protonated TEA; , therefore, the chemical equilibrium of biphasic TEAA was analyzed qualitatively by IR spectroscopy. In Figure , the peaks in the range 1350–1450 cm –1 are attributed to both COO – and N + –H bands, the CO stretching mode band at 1510–1675 cm –1 is attributed to ionic HAc, and the C–O asymmetric stretching band at 1190–1320 cm –1 is attributed to molecular HAc. , The Gaussian fitting results of these absorption peaks are shown in Figure S2 and Table S7.…”

Section: Resultsmentioning

confidence: 99%

Phase and Chemical Equilibria of Biphasic Protic Ionic Liquid: Triethylamine–Acetic Acid

Zhang

Yao

2021

Ind. Eng. Chem. Res.

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Triethylamine−acetic acid, as a biphasic protic ionic liquid, typically contains both phase equilibrium and protontransferring chemical equilibrium, and these equilibria have been measured in this work. NMR and IR spectroscopy methods were applied to determine the neutral and ionic species in both lower and upper phases. At 298.15 K, the lower phase contained (25 ± 1) mol % molecular acetic acid, (10 ± 1) mol % molecular triethylamine, and (64 ± 1) mol % ionic species, while the upper phase contained (4.7 ± 0.2) mol % molecular acetic acid, (71.5 ± 0.5) mol % molecular triethylamine, and (24 ± 1) mol % ionic species. The temperature effect on the distributions of molecular and ionic species in biphasic triethylamine−acetic acid was also studied. The unique biphasic PIL containing both base-rich and acid-rich phases with different ionic concentrations (ionicity) should be of significant potential to be applied in industrial processes.

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“…9,11 To use the full potential of RTILs as solvents for the reaction, it is necessary to study the solvation of solutes dissolved in RTILs. 2 Since the rotational diffusion of a solute molecule is affected by its solvation, it has been common to study the solute solvation in RTILs by studying the rotational diffusion of fluorescent probes 12−14 and spin probes 15−17 dissolved in RTILs. NMR spectroscopy can also be used to study the solvation of solutes in RTILs.…”

Section: ■ Introductionmentioning

confidence: 99%

“…Recently, room-temperature ionic liquids (RTILs) have garnered a lot of academic and industrial interests. − The reason is that these organic salts, which commonly melt below 100 °C and are made up of a great variety of asymmetric organic cations and organic or inorganic anions, have tunable physicochemical properties. Due to their tunability, RTILs can be used in many applications such as energy production and storage, organic catalysis and synthesis, separation technology, and analytical chemistry .…”

Section: Introductionmentioning

confidence: 99%

“…Due to their tunability, RTILs can be used in many applications such as energy production and storage, organic catalysis and synthesis, separation technology, and analytical chemistry . RTILs are also seen as better alternatives to volatile organic solvents because of their negligible vapor pressure, low flammability, and their “green” nature. , To use the full potential of RTILs as solvents for the reaction, it is necessary to study the solvation of solutes dissolved in RTILs . Since the rotational diffusion of a solute molecule is affected by its solvation, it has been common to study the solute solvation in RTILs by studying the rotational diffusion of fluorescent probes − and spin probes − dissolved in RTILs.…”

Section: Introductionmentioning

confidence: 99%

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Rotation of a Charged Spin Probe in Room-Temperature Ionic Liquids

et al. 2021

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X-band electron paramagnetic resonance spectroscopy has been used to investigate the rotational diffusion of a stable, positively charged nitroxide 4-trimethylammonium-2,2,6,6-tetramethylpiperidine-1-oxyl iodide (Cat-1) in a series of 1-alkyl-3methylimidazolium tetrafluoroborate room-temperature ionic liquids (RTILs) having alkyl chain lengths from two to eight carbons. The rotation of Cat-1 is anisotropic with the preferential axis of rotation along the NO • moiety. The Stokes−Einstein−Debye law describes the mean rotational correlation time of Cat-1, assuming that the hydrodynamic radius is smaller than the van der Waals radius of the probe. This implies that the probe rotates freely, experiencing slip boundary condition, which is solvent-dependent. The rotational correlation time of Cat-1 in RTILs can very well be fitted to a power-law functionality with a singular temperature, which suggests that the apparent activation energy of rotation exhibits non-Arrhenius behavior. Compared to the rotation of perdeuterated 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl (pDTO), which is neutral, the rotation of Cat-1 is several times slower. The rotational anisotropy, the ratio of the rotational times of pDTO and Cat-1, and the apparent activation energy indicate the transition from a homogeneously globular structure to a spongelike structure when the alkyl chain has four carbons, which is also observed in molecular dynamics computational studies. For the first time, we have been able to show that the rotational correlation time of a solute molecule can be analyzed in terms of the Cohen−Turnbull free volume theory. The Cohen−Turnbull theory fully describes the rotation of Cat-1 in all ionic liquids in the measured temperature range.

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NMR Studies of Protic Ionic Liquids

Cited by 11 publications

References 166 publications

Ultrafast Hydrogen Bond Exchanging between Water and Anions in Concentrated Ionic Liquid Aqueous Solutions

Ultrafast Hydrogen Bond Exchanging between Water and Anions in Concentrated Ionic Liquid Aqueous Solutions

Phase and Chemical Equilibria of Biphasic Protic Ionic Liquid: Triethylamine–Acetic Acid

Rotation of a Charged Spin Probe in Room-Temperature Ionic Liquids

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