Relative Cation-Anion Diffusion in Alkyltriethylammonium-Based Ionic Liquids

Beyond isotropic reorientation: probing anisotropic and internal motions in ionic liquids with fast field cycling NMR relaxometry and MD simulations

We investigate the rotational and translational dynamics of ionic liquids (ILs) through a combined approach utilizing fast field cycling nuclear magnetic resonance (FFC NMR) relaxometry and molecular dynamics (MD) simulations. The ILs examined, [TEA][NTf2] and [C5Py][NTf2], were selected to explore differences arising from variations in ion shape and rigidity. FFC NMR relaxometry provides detailed spin-lattice relaxation rate data for both 1H on cations and 19F nuclei on anions across broad frequency and temperature ranges, enabling the characterization of ion-specific dynamics. To dissect the total relaxation rates into intramolecular and intermolecular contributions and to accurately interpret these data, advanced relaxation models were employed, accounting for isotropic, anisotropic, and internal rotational motions. The dynamics of the nearly spherical [TEA]+ cation were described using the Bloembergen-Purcell-Pound (BPP) model, while the elongated [C5Py]+ cation required a symmetric top model to capture anisotropic rotational behavior. Additionally, the [NTf2]- anion's rotational dynamics were modeled to include fast internal rotations of the CF3 groups. For both ILs, self-diffusion coefficients were also obtained in addition to rotational dynamics. Notably, the analysis explicitly considered heteronuclear intermolecular contributions, which were found to play a significant role in accurately capturing the relaxation behavior. Complementary MD simulations provided rotational correlation times and self-diffusion coefficients, which showed excellent agreement with experimental results, thereby validating the employed relaxation models. These findings contribute to a deeper understanding of IL dynamics, emphasizing the role of ion geometry and internal motions in data evaluation. Thereby, this work establishes a comprehensive framework for future studies on complex IL systems.

Cation-Cation, Cation-Anion, and Anion-Anion Translation Diffusion in Ionic Liquids─Insight from NMR Relaxometry

1H and 19F spin-lattice relaxation experiments have been performed for a series of ionic liquids: [HMIM][TFSI], [OMIM][TFSI], and [DMIM][TFSI] including the same anion and cations with progressively longer alkyl chains. The experiments were performed in a wide frequency range from 10 kHz to 10 MHz (referring to the 1H resonance frequency) versus temperature. This extensive data set has been analyzed in terms of a theoretical model including all relevant homonuclear (1H-1H and 19F-19F) and heteronuclear (1H-19F) relaxation pathways and linking the relaxation features to the relative translational diffusion between the ion pairs (cation-cation, cation-anion, and anion-anion). In addition to the comprehensive theoretical approach, closed-form expressions have been provided and applied to determine the diffusion coefficients from the slopes of the linear dependences of the relaxation rates on the square root of the resonance frequency. The combined experimental and theoretical studies have led to the determination of the complete set of diffusion coefficients, forming a consistent picture of the dynamical scenario. In addition to revealing the dynamical properties of the liquids and the influence of the subtle changes in the cation structure on the movement of both cations and anions, the theoretical means for exploiting Nuclear Magnetic Resonance relaxometry for ionic liquids have been provided.

Effect of Alkyl Chain Length on the Magnitude of Dynamically Correlated Molecules and Dynamical Crossover in Alkyltriethylammonium-Based Ionic Liquids

This study explores the impact of alkyl chain length on dynamic heterogeneity and dynamic crossover in alkyltriethylammonium-based ionic liquids ([TEA-R][TFSI]) with varying alkyl chain lengths (R = C6, C8, and C10). Using differential scanning calorimetry and broadband dielectric spectroscopy, we observed that these ionic liquids are excellent glassformers with notable ionic conductivity at room temperature. Furthermore, the number of dynamically correlated molecules at the glass transition temperature, reflecting the dynamic heterogeneity, is exceptionally small for TEA-R ILs and becomes more reduced with longer alkyl chains. Moreover, the temperature dependence of conductivity requires two Vogel-Fulcher-Tammann equations with distinct sets of fitting parameters for an accurate description. The crossover temperature Tb, indicating the transition to complex dynamics, increases with the alkyl chain length.

Dynamics of ionic liquid-polymer gel membranes-Insight from NMR relaxometry for [BMIM][BF4]-PVDF-HFP systems

1H spin-lattice relaxation experiments have been performed for ionic liquid-polymer gel membranes, including 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) and poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) with different proportions. The experiments have been performed in a broad range of resonance frequencies (from about 5 Hz to 40 MHz) vs temperature and complemented with analogous studies for [BMIM][BF4] in bulk as a reference. A model of the relaxation processes in the membranes has been proposed. The model includes two relaxation contributions. One of them corresponds to the concept of restricted, two-dimensional translation diffusion with a residence lifetime, while the second one has the form characteristic of polymers (mathematically similar to the limiting behavior of two-dimensional translation diffusion with a very long residence lifetime). The extensive dataset has been consistently interpreted in terms of the model, revealing two dynamical processes on the time scales of 10-7 s (for the second relaxation contribution) and 10-9 s (for the first one). The relationship of these relaxation contributions to the motion of the polymer or ionic liquid-polymer complexes and to the translation diffusion of BMIM cations in the matrix has been discussed.

Dynamics of ionic liquids by means of nuclear magnetic resonance relaxation - overview of theoretical approaches

This paper presents a comprehensive overview of the spin relaxation theory needed for exploring nuclear magnetic resonance (NMR) relaxometry to study the dynamical properties of ionic liquids. The term NMR relaxometry refers to relaxation experiments performed over a wide range of magnetic fields (resonance frequencies). In this way, dynamical processes occurring on timescales from milliseconds to nanoseconds can be studied, including translational and rotational dynamics of both types of ions (cations and anions). In order to take advantage of the remarkable experimental possibilities, appropriate theoretical models linking relaxation properties with ionic motion are needed. With the aim of providing such theoretical tools, 1H and 19F relaxation models for ionic liquids have been reviewed and their applications have been illustrated by several examples. The presented models are valid for an arbitrary magnetic field, include all relevant relaxation pathways and allow to extract detailed information about the translational and rotational dynamics of the ions. On the basis of the theoretical models, formulas allowing a straightforward determination of the translational diffusion coefficients of cations and anions from combined 1H and 19F relaxation studies have been derived and discussed in detail.

Translational Dynamics of Cations and Anions in Ionic Liquids from NMR Field Cycling Relaxometry: Highlighting the Importance of Heteronuclear Contributions

NMR field cycling relaxometry is a powerful method for determining the rotational and translational dynamics of ions, molecules, and dissolved particles. This is in particular true for ionic liquids (ILs) in which both ions carry NMR sensitive nuclei. In the IL triethylammonium bis(trifluoromethanesulfonyl)imide ([TEA][NTf2]), there are 1H nuclei at the [TEA]+ cations and 19F nuclei at the [NTf2]- anions. Moreover, the high viscosity of this IL leads to frequency-dependent relaxation rates, leaving the so-called extreme narrowing regime. Both the rotational and the translational dynamics of the constituents of ILs can be obtained by separating the contributions of intra- and intermolecular relaxation rates. In particular, the translational dynamics can be obtained separately by applying the so-called "low-frequency approach" (LFA), utilizing the fact that the change in the total relaxation rates at low frequencies results solely from translational motions. However, for systems containing multiple NMR active nuclei, heteronuclear interactions can also affect their relaxation rates. For [TEA][NTf2], the intermolecular relaxation rate is either the sum of 1H-1H cation-cation and 1H-19F cation-anion interactions or the sum of 19F-19F anion-anion and 19F-1H anion-cation interactions. Due to the lack of available experimental information, the 1H-19F heteronuclear intermolecular contribution has often been neglected in the past, assuming it to be negligible. Employing a suitable set of ILs and by making use of isotopic H/D substitution, we show that the 1H-19F heteronuclear intermolecular contribution in fact cannot be neglected and that the LFA cannot be applied to the total 1H and total 19F relaxation rates.

Dynamic of binary molecular systems-Advantages and limitations of NMR relaxometry

1H spin-lattice relaxation studies have been performed for binary systems, including glycerol as the first component and alanine, glycine, and aspartic acid (with different levels of deuteration) as the second one. The relaxation studies have been performed in the frequency range from 10 kHz to 10 MHz vs temperature. A theoretical framework, including all relevant 1H-1H and 1H-2H relaxation pathways, has been formulated. The theory has been exploited for a thorough interpretation of a large set of the experimental data. The importance of the 1H-2H relaxation contributions has been discussed, and the possibility of revealing dynamical properties of individual liquid components in binary liquids has been carefully investigated. As far as the dynamical properties of the specific binary liquids, chosen as an example, are considered, it has been shown that in the presence of the second component (alanine, glycine, and aspartic acid), both molecular fractions undergo dynamics similar to that of glycerol in bulk.

Three mechanisms of room temperature dynamic nuclear polarization occur simultaneously in an ionic liquid

For the first time, several mechanisms of dynamic nuclear polarization, namely Overhauser, solid effect and cross effect/thermal mixing, have been identified in an ionic liquid with a nitroxide radical at ambient temperatures.