Study of nanostructural organization of ionic liquids by electron paramagnetic resonance spectroscopy

Effect of Charge on the Rotation of Prolate Nitroxide Spin Probes in Room-Temperature Ionic Liquids

We have studied the rotational diffusion of two prolate nitroxide probes, the doubly negatively charged peroxylamine disulfonate (Frémy's salt - FS) and neutral di-tert-butyl nitroxide (DTBN), in a series of 1-alkyl-3-methylimidazolium tetrafluoroborate room-temperature ionic liquids (RTILs) having alkyl chain lengths from two to eight carbons using electron paramagnetic resonance (EPR) spectroscopy. Though the size and shape of the probes are reasonably similar, they behave differently due to the charge difference. The rotation of FS is anisotropic, and the rotational anisotropy increases with the alkyl chain length of the cation, while the rotation of DTBN is isotropic. The hyperfine coupling constant of DTBN decreases as a function of the alkyl chain length and is proportional to the relative permittivity of ionic liquids. On the other hand, the hyperfine coupling constant of FS increases with increasing chain length. These behaviors indicate the location of each probe in RTILs. FS is likely located in the polar region near the network of charged imidazolium ions. DTBN molecules are predominately distributed in the nonpolar domains.

High-Pressure ESR Spectroscopy: On the Rotational Motion of Spin Probes in Pressurized Ionic Liquids

We report high-pressure (up to 50 MPa) ESR-spectroscopic investigations on the rotational correlation times of the nitroxide radicals 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPOL), and 4-amino-2,2,6,6-tetramethylpiperidine 1-oxyl (ATEMPO) in the ionic liquids 1-ethyl-3-methylimidazolium tetrafluoroborate (emimBF₄), 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF₆), 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF₄), 1-methyl-3-octylimidazolium tetrafluoroborate (omimBF₄), and 1-methyl-3-octylimidazolium hexafluorophosphate (omimPF₆). The activation volumes (38.5–56.6 ų) determined from pressure dependent rotational diffusion coefficients agree well with the pressure dependent viscosities of the ionic liquids. Experimentally, the fractional exponent of the generalized Stokes–Einstein–Debye relation is found to be close to one.

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

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-3-methylimidazolium 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.

Nanocage formation and structural anomalies in imidazolium ionic liquid glasses governed by alkyl chains of cations

Intriguing nanostructuring anomalies have been recently observed in imidazolium ionic liquids (ILs) near their glass transition points, where local density around a nanocaged solute progressively grows up with temperature. Herewith, we for the first time demonstrate experimentally and theoretically, that these anomalies are governed by alkyl chains of cations and crucially depend on their length. Electron Paramagnetic Resonance (EPR) spectroscopy on a series of ILs [C_nmim]BF₄ (n = 0-12) shows that only the chains with n = 3-10 favor anomaly. Moreover, remarkable even vs. odd n peculiarities were systematically observed. Finally, similar anomaly was for the first time observed for a non-IL glass of dibutyl phthalate, which structurally mimics cations of imidazolium ILs. Therefore, such anomalous density behavior in a glassy state nanocage goes far beyond ILs and proves to be a more general phenomenon, which can be structurally tuned and rationally adjusted for various potential applications in nanoscale materials.

Electron Paramagnetic Resonance Measurements of Four Nitroxide Probes in Supercooled Water Explained by Molecular Dynamics Simulations

Electron paramagnetic resonance (EPR) measurements of the rotational diffusion of small nitroxide probes have been demonstrated to be a powerful technique for experimentally investigating the properties of supercooled liquids, such as water. However, since only the rotational diffusion of the probe molecules is measured and EPR measurements are indirect, it is not clear what the relationship between the behavior of water and the probe molecule is. To address this, we have performed molecular dynamics simulations of four nitroxide probes in TIP4P-Ew and OPC water models to directly compare with EPR experiments and to determine the behavior of the water and the underlying microscopic coupling between the water and the probes. In all, 200 ns simulations were run for 23 temperatures between 253 and 283 K for all four probes with each water model for an aggregate of 36.8 μs of simulation time. Simulations for both water models systematically underestimated the rotational diffusion coefficients for both water and probes, though OPC simulations were generally in better agreement with the experiments than TIP4P-Ew simulations. Despite this, when the temperature dependence of the data was fit to a power law, fit parameters for TIP4P-Ew were generally in better agreement with the experiments than OPC. For probe molecules, the singular temperature was found to be T₀ = 226.5 ± 0.4 K from experiments, T₀ = 208 ± 2 K for OPC water, and T₀ = 215 ± 2 K for TIP4P-Ew water. While for water molecules, the singular temperature was found to be T₀ = 220.3 ± 0.2 K from experiments, T₀ = 208 ± 2 K for OPC water, and T₀ = 220 ± 1 K for TIP4P-Ew water. Systematic underestimation of the rotational diffusion coefficients was most pronounced at lower temperatures and was clearly observed in changes to the Arrhenius activation energy. Above the maximum density temperature of T_ρmax = 277 K, an activation energy of E_A ≈ 16.7 kJ/mol was observed for the probes from experiments, while OPC had E_A ≈ 15.2 kJ/mol and TIP4P-Ew had E_A ≈ 14.6 kJ/mol. Below the maximum density temperature, the activation energy jumped to E_A ≈ 32.5 kJ/mol for experiments but only E_A ≈ 23 kJ/mol for OPC and E_A ≈ 22 kJ/mol for TIP4P-Ew. In all cases, we saw good agreement between the behavior of the probe molecules and water. To understand why, we calculated the average number of hydrogen bonds between the probe molecules and water. From this, we were able to explain the rotational diffusion times for all of the probes. These results show that current molecular models are sufficient to capture physical phenomena observed with EPR and to help elucidate why the probes provide accurate insights into the behavior of supercooled water.

Structural Anomalies in Ionic Liquids near the Glass Transition Revealed by Pulse EPR

Unusual physical and chemical properties of ionic liquids (ILs) open up prospects for various applications. We report the first observation of density/rigidity heterogeneities in a series of ILs near the glass transition temperature ( T_g) by means of pulse electron paramagnetic resonance (EPR). Unprecedented suppression of molecular mobility is evidenced near the glass transition, which is assigned to unusual structural rearrangements of ILs on the nanometer scale. Indeed, pulse and continuous wave EPR clearly indicate the occurrence of heterogeneities near T_g, which exist in a rather broad temperature range of ∼50 K. The two types of local environments are evidenced, being drastically different by their stiffness. The more rigid one suppresses molecular mobility, whereas the softer one instead promotes diffusive molecular rotation. Such properties of ILs near T_g are of general importance; moreover, the observed density/rigidity heterogeneities controlled by temperature might be considered as a new type of tunable reaction nanoenvironment.