Translational and Rotational Diffusion of Glycerol by Means of Field Cycling 1H NMR Relaxometry

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.

Computing the frequency-dependent NMR relaxation of 1H nuclei in liquid water

We present a computational framework for reliably determining the frequency-dependent intermolecular and intramolecular nuclear magnetic resonance (NMR) dipole-dipole relaxation rates of spin 1/2 nuclei from Molecular Dynamics (MD) simulations. This approach avoids the alterations caused by the well-known finite-size effects of translational diffusion. Moreover, a procedure is derived to control and correct for effects caused by fixed distance-sampling cutoffs and periodic boundary conditions. By construction, this approach is capable of accurately predicting the correct low-frequency scaling behavior of the intermolecular NMR dipole-dipole relaxation rate and thus allows for the reliable calculation of the frequency-dependent relaxation rate over many orders of magnitude. Our approach is based on the utilization of the theory of Hwang and Freed for the intermolecular dipole-dipole correlation function and its corresponding spectral density [L.-P. Hwang and J. H. Freed, J. Chem. Phys. 63, 4017-4025 (1975)] and its combination with data from MD simulations. The deviations from the Hwang and Freed theory caused by periodic boundary conditions and sampling distance cutoffs are quantified by means of random walker Monte Carlo simulations. An expression based on the Hwang and Freed theory is also suggested for correcting those effects. As a proof of principle, our approach is demonstrated by computing the frequency-dependent intermolecular and intramolecular dipolar NMR relaxation rates of 1H nuclei in liquid water at 273 and 298 K based on the simulations of the TIP4P/2005 model. Our calculations are suggesting that the intermolecular contribution to the 1H NMR relaxation rate of the TIP4P/2005 model in the extreme narrowing limit has previously been substantially underestimated.

Diffusion of Water Molecules on the Surface of Silica Nanoparticles─Insights from Nuclear Magnetic Resonance Relaxometry

1H spin-lattice nuclear magnetic resonance (NMR) relaxation experiments have been performed for water dispersions of functionalized silica nanoparticles of diameters of 25 and 45 nm. The experiments have been performed in a broad frequency range spanning 3 orders of magnitude, from 10 kHz to 10 MHz, versus temperature, from 313 to 263 K. On the basis of the data, two-dimensional translation diffusion (diffusion close to the nanoparticle surface within a layer of the order of a few diameters of water molecules) has been revealed. The translational correlation times as well as the residence life times on the nanoparticle surface have been determined. It has turned out that the residence lifetime is temperature-independent and is on the order of 5 × 10-6 s for the smaller nanoparticles and by about a factor of 3 longer for the larger ones. The translational correlation time for the case of 25 nm nanoparticles is also temperature-independent and yields about 6 × 10-7 s, while for the dispersion of the larger nanoparticles, the correlation times changed from about 8 × 10-7 s at 313 K to about 1.2 × 10-6 s at 263 K. In addition to the quantitative characterization of the two-dimensional translation diffusion, correlation times associated with bound water molecules have been determined. The studies have also given insights into the population of the bound and diffusing water on the surface water fractions.

Molecular Dynamics of Ionic Liquids from Fast-Field Cycling NMR and Molecular Dynamics Simulations

Understanding the connection between the molecular structure of ionic liquids and their properties is of paramount importance for practical applications. However, this connection can only be established if a broad range of physicochemical properties on different length and time scales is already available. Even then, the interpretation of the results often remains ambiguous due to the natural limits of experimental approaches. Here we use fast-field cycling (FFC) to access both translational and rotational dynamics of ionic liquids. These combined with a comprehensive physicochemical characterization and MD simulations provide a toolkit to give insight into the mechanisms of molecular mechanics. The FFC results are consistent with the computer simulation and conventional physicochemical approaches. We show that curling of the side chains around the positively charged cationic core is essential for the properties of ether-functionalized ionic liquids, and we demonstrate that neither geometry nor polarity alone are sufficient to explain the macroscopic properties.

A Relation between the Formation of a Hydrogen-Bond Network and a Time-Scale Separation of Translation and Rotation in Molecular Liquids

We study the relation between the translational and rotational motions of liquids, which is anticipated in the framework of the Stokes-Einstein-Debye (SED) treatment. For this purpose, we exploit the fact that ¹H field-cycling nuclear magnetic resonance relaxometry and molecular dynamics simulations provide access to both modes of motion. The experimental and computational findings are fully consistent and show that the time-scale separation between translation and rotation increases from the van der Waals liquid o-terphenyl over ethylene glycol to the hydrogen-bonded liquid glycerol, indicating an increasing degree of breakdown of the SED relation. Furthermore, the simulation results for two ethylene glycol models with different molecular conformations indicate that the translation is more retarded than the rotation when the density of intermolecular hydrogen bonds increases. We conclude that an increasing connectivity of a hydrogen-bond network leads to an increasing time-scale separation and, thus, to a stronger SED violation.

Relationship between Translational and Rotational Dynamics of Alkyltriethylammonium-Based Ionic Liquids

¹H spin-lattice relaxation experiments have been performed for a series of ionic liquids including bis(trifluoromethanesulfonyl)imide anion and cations of a varying alkyl chain length: triethylhexylammonium, triethyloctylammonium, decyltriethylammonium, dodecyltriethylammonium, triethyltetradecylammonium, and hexadecyltriethylammonium. The relaxation studies were carried out in abroad frequency range covering three orders of magnitude, from 10 kHz to 10 MHz, versus temperature. On the basis of a thorough, quantitative analysis of this reach data set, parameters characterizing the relative, cation-cation, translation diffusion (relative diffusion coefficients and translational correlation times), and rotational motion of the cation (rotational correlation times) were determined. Relationships between these quantities and their dependence on the alkyl chain length were discussed in comparison to analogous properties of molecular liquids. It was shown, among other findings, that the ratio between the translational and rotational correlation times is smaller than for molecular liquids and considerably dependent on temperature. Moreover, a comparison of relative and self-diffusion coefficients indicate correlated translational dynamics of the cations.

Solvation Dynamics of Wet Ethaline: Water is the Magic Component

The past two decades witnessed the development of a new type of solvent system, named deep eutectic solvents, which have become increasingly investigated because they offer new and potentially favorable properties, such as wide tunability in electrochemical, mechanical, and transport properties. Deep eutectic solvent (DES) systems are composed of at least one main solvent and an additional component that is meant to interrupt the original solvent/solvent interactions, thereby introducing lower melting points relative to each individual component. Ethaline (a 1:2 mol % mixture of choline chloride and ethylene glycol) is one of the most promising DES systems. However, it is also known to be very hygroscopic, which is a constant concern because water absorption during the use of ethaline alters its properties. Within this work, we demonstrate that modest amounts of water addition (1-10%) to ethaline are of little concern for practical use and can even lead to performance improvements, such as accelerated relaxation and solvation. In contrast, very small amounts of <1% of water lead to additional slowing of the solvent response. Thus, we suggest that the attempt to dry ethaline below 1% moisture is rather counterproductive if one attempts to achieve effective solvation and charge transport properties from DESs. This study investigates the effect of water content on the diffusional relaxation dynamics of ethaline. A set of independent spectroscopic experiments and computational simulations are aimed to provide insight into the solvent response of the DES system using femtosecond time-resolved absorption spectroscopy (fs-TA), broadband dielectric spectroscopy (BDS), nuclear magnetic resonance (NMR) diffusometry and broadband relaxometry, and molecular dynamics simulations (MDS) on ethaline with 0, 0.1, 1, 10, and 28.5 wt % added water. For dry ethaline, we identify choline chloride as the rate-limiting solvation component in ethaline. However, the role of the solvent components changes gradually as water is added. We provide quantitative solvent relaxation rates using the different presented time-resolved spectroscopic techniques and find remarkable agreement between them. Based on the solvent relaxation rates and combined with MDS, we develop a molecular understanding of the individual solvent components and their interactions in dry and wet ethaline with varying amounts of water content.

Water dynamics in eggs by means of Nuclear Magnetic Resonance relaxometry

¹H Nuclear Magnetic Resonance relaxometry has been applied to reveal dynamical properties of water molecules embedded into egg yolk and white of three species: turkey, chicken and quail. Two fractions of water molecules, referred to as confined-water and free-water fractions, have been revealed. It has been demonstrated that translation diffusion of the confined-water fraction is three-dimensional. The dynamics of the confined-water has been quantitatively described in terms of diffusion coefficients and rotational correlation times. The parameters have been compared for egg yolk and white for all the species. In addition to these quantities, the number of the confined-water molecules per unit volume has been provided for all cases. The obtained parameters provide insight into the dynamics of water in eggs of different origin and allow to identify similarities and differences between them in connection to the structure of the network formed by the macromolecular fraction of egg yolk and white.

Insights into the translational and rotational dynamics of cations and anions in protic ionic liquids by means of NMR fast-field-cycling relaxometry

Understanding the translational and rotational dynamics of cations and anions in hydrogen bonded protic ionic liquids (PIls) is still a challenge. In this study, we determine self-diffusion coefficients and rotational correlation times of both ions in triethylammonium based PILs by means of NMR Fast-Field-Cycling (FFC) relaxometry. Global fits of 1H and 19F nuclear magnetic relaxation dispersion (NMRD) curves allowed proper separation into intra and inter molecular relaxation rates for both NMR sensitive nuclei and thus a reliable description of translational and rotational motion for both ions individually. The diffusion coefficients of the cations are in the order of 6 × 10-11 m2 s-1 at room temperature and about 50 per cent larger than those of the anions. The diffusion coefficients of cations and anions in both PILs were compared with those we derived from applying an universal dispersion power law and those known from pulsed field gradient (PFG) NMR studies. Considering the Nernst-Einstein equation, molar conductivities were calculated from cationic and anionic diffusion coefficients and related to directly measured molar conductivities, allowing the determination of the degree of dissociation. The rotational correlation times τR ranging from 50 ps up to 2 ns as a function of temperature were compared with those obtained from high-field NMR quadrupolar relaxation time measurements addressing explicitly the rotation of the NH vector and giving insights into the acidic proton mobility. The Stokes-Einstein and Stokes-Einstein-Debye relations were applied to relate the diffusion coefficients and rotational correlation times to the macroscopic bulk viscosity. The results were also discussed with respect to the archetypical PIL ethylammonium nitrate.

Dynamics of Ionic Liquids in Confinement by Means of NMR Relaxometry-EMIM-FSI in a Silica Matrix as an Example

¹H and ¹⁹F spin–lattice relaxation studies for 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide in bulk and mesoporous MCM-41 silica matrix confinement were performed under varying temperatures in a broad range of magnetic fields, corresponding to ¹H resonance frequency from 5Hz to 30MHz.A thorough analysis of the relaxation data revealed a three-dimensional translation diffusion of the ions in the bulk liquid and two-dimensional diffusion in the vicinity of the confining walls in the confinement. Parameters describing the translation dynamics were determined and compared. The rotational motion of both kinds of ions in the confinement was described by two correlation times that might be attributed to anisotropic reorientation of these species.

1H spin-lattice NMR relaxation in the presence of residual dipolar interactions - Dipolar relaxation enhancement

A model of spin-lattice relaxation for spin-1/2 nuclei in the presence of a residual dipole-dipole coupling has been presented. For slow dynamics the model predicts a bi-exponential relaxation at low frequencies, when the residual dipole-dipole interaction dominates the Zeeman coupling. Moreover, according to the model a frequency-specific relaxation enhancement, referred to as Dipolar Relaxation Enhancement (DRE) in analogy to the Quadrupole Relaxation Enhancement (QRE) is expected. The frequency position of the relaxation maximum is determined by the amplitude of the residual dipole-dipole interaction. Experimental examples of relaxation properties that might be attributed to the DRE are presented. The DRE effect has the potential to be exploited, in analogy to QRE, as a unique source of information about molecular dynamics and structure.

Slow dynamics of solid proteins - Nuclear magnetic resonance relaxometry versus dielectric spectroscopy

¹H Nuclear Magnetic Resonance (NMR) relaxometry and Dielectric Spectroscopy (DS) have been exploited to investigate the dynamics of solid proteins. The experiments have been carried out in the frequency range of about 10 kHz-40 MHz for NMR relaxometry and 10^-2Hz-20 MHz for DS. The data sets have been analyzed in terms of theoretical models allowing for a comparison of the correlation times revealed by NMR relaxometry and DS. The ¹H spin-lattice relaxation profiles have been decomposed into relaxation contributions associated with ¹H-¹H and ¹H-¹⁴N dipole - dipole interactions. The ¹H-¹H relaxation contribution has been interpreted in terms of three dynamical processes of time scales of 10^-6s, 10^-7s and 10^-8s. It has turned out that the correlation times do not differ much among proteins and they are only weakly dependent on temperature. The analysis of DS relaxation spectra has also revealed three motional processes characterized by correlation times that considerably depend on temperature in contrast to those obtained from the ¹H relaxation. This finding suggest that for solid proteins there is a contribution to the ¹H spin-lattice relaxation associated with a kind of motion that is not probed in DS as it does not lead to a reorientation of the electric dipole moment.

Dielectric relaxation and proton field-cycling NMR relaxometry study of dimethyl sulfoxide/glycerol mixtures down to glass-forming temperatures

Mixtures of glycerol and dimethyl sulfoxide (DMSO) are studied by dielectric spectroscopy (DS) and by 1H field-cycling (FC) NMR relaxometry in the entire concentration range and down to glass-forming temperatures (170-323 K). Molecular dynamics is accessed for 0 < xDMSO ≤ 0.64, at higher concentration phase separation occurs. The FC technique provides the frequency dependence of the spin-lattice relaxation rate which is transformed to the susceptibility representation and thus allows comparing NMR and DS results. The DS spectra virtually do not change with xDMSO and T, only the relaxation times become shorter. This is in contrast to the non-associated mixture toluene/quinaldine for which strong spectral changes occur. The FC relaxation spectra of glycerol in solution with DMSO or (deuterated) DMSO-d6 display a bimodal structure with a high-frequency part reflecting rotational and a low-frequency part reflecting translational dynamics. Regarding the rotational contribution in the glycerol/DMSO-d6 mixtures, no spectral change with xDMSO and T is observed. Yet, the non-deuterated mixture reveals a broader relaxation spectrum. Time constants τrot(T) probed by the two techniques complement each, a range 10-11 s < τ < 10 s is covered. The glass transition temperature Tg(xDMSO) is determined, yielding Tg = 149.5 ± 1 K of pure DMSO by extrapolation. Analysing the low-frequency FC NMR spectra allows to determine the diffusion coefficient Dtrans. Its logarithm shows a linear xDMSO-dependence as does lg τrot. The ratio Dtrans/Drot is independent of xDMSO and its low value indicates large separation of translation and rotation. The corresponding unphysically small hydrodynamic radius indicates strong failure of Stokes-Einstein-Debye relation. Such anomaly is taken as characteristics of a 3d hydrogen-bonded network. We conclude, although DMSO is an aprotic liquid the molecule is continuously incorporated in the hydrogen network of glycerol. Both molecules display common dynamics, i.e., no decoupling of the component dynamics is found in contrast to quinaldine/toluene with a similar Tg difference of its components.

Understanding the Nature of Nuclear Magnetic Resonance Relaxation by Means of Fast-Field-Cycling Relaxometry and Molecular Dynamics Simulations-The Validity of Relaxation Models

Fast-field-cycling relaxometry is a nuclear magnetic resonance method growing in popularity; yet, theoretical interpretation is limited to analytical models of uncertain accuracy. We present the first study calculating fast-field-cycling dipolar coupling directly from a molecular dynamics simulation trajectory. In principle, the frequency-resolved dispersion contains both rotational and translational diffusion information, among others. The present joint experimental/molecular dynamics study demonstrates that nuclear magnetic resonance properties calculated from the latter reproduce measured dispersion curves and temperature trends faithfully. Furthermore, molecular dynamics simulations can verify interpretation model assumptions by providing actual diffusion coefficients and correlation times.

Mechanism of Water Dynamics in Hyaluronic Dermal Fillers Revealed by Nuclear Magnetic Resonance Relaxometry

1 H spin-lattice nuclear magnetic resonance relaxation experiments were performed for five kinds of dermal fillers based on hyaluronic acid. The relaxation data were collected over a broad frequency range between 4 kHz and 40 MHz, at body temperature. Thanks to the frequency range encompassing four orders of magnitude, the dynamics of water confined in the polymeric matrix was revealed. It is demonstrated that translation diffusion of the confined water molecules exhibits a two-dimensional character and the diffusion process is slower than diffusion in bulk water by 3-4 orders of magnitude. As far as rotational dynamics of the confined water is concerned, it is shown that in all cases there is a water pool characterized by a rotational correlation time of about 4×10-9  s. In some of the dermal fillers a fraction of the confined water (about 10 %) forms a pool that exhibits considerably slower (by an order of magnitude) rotational dynamics. In addition, the water binding capacity of the dermal fillers was quantitatively compared.

Application of proton field-cycling NMR relaxometry for studying translational diffusion in simple liquids and polymer melts

With the availability of commercial field-cycling relaxometers together with progress of home-built instruments nuclear magnetic resonance relaxometry has gained new momentum as a method of investigating the dynamics in viscous liquids and polymer melts. The method provides the frequency dependence of the spin-lattice relaxation rate. In the case of protons, one distinguishes intramolecular and intermolecular relaxation pathways. Whereas the intramolecular contribution prevails at high frequencies and reflects rotational dynamics, the often ignored intermolecular relaxation contribution dominates at low-frequency and provides access to translational dynamics. A universal low-frequencies dispersion law holds which in pure systems allows determining the diffusion coefficient in a straightforward way. In addition, the rotational time constant is extracted from the high-frequency relaxation contribution. This is demonstrated for simple and ionic liquids and for polymer melts.

Peculiar relaxation dynamics of propylene carbonate derivatives

The aim of this work is to analyze in detail the effect of the alkyl chain length on the dynamics of glass-forming propylene carbonate (PC) derivatives. Examined samples are low-molecular weight derivatives of the PC structure, i.e., the 4-alkyl-1,3-dioxolan-2-one series, modified by changing the alkyl substituent from methyl to hexyl. The molecular dynamics (MD) has been analyzed based on experimental data collected from differential scanning calorimetry, broadband dielectric spectroscopy (BDS), X-ray diffraction (XRD), and nuclear magnetic resonance relaxometry measurements as well as MD simulations. The dielectric results show in samples with the propyl- or longer carbon chain the presence of slow Debye-like relaxation with features similar to those found in associative materials. Both XRD and MD reveal differences in the intermolecular structure between PC and 4-butyl-1,3-dioxolan-2-one liquids. Moreover, MD shows that the probability of finding one terminal carbon atom of the side chain of BPC in the vicinity of another carbon atom of the same type is much higher than in the case of PC. It suggests that there is a preference for longer hydrocarbon chains to set themselves close to each other. Consequently, the observed slow-mode peak may be caused by movement of aggregates maintained by van der Waals interactions. Reported herein, findings provide a new insight into the molecular origin of Debye-like relaxation.

Dynamics of Dimethylbutanols in Plastic Crystalline Phases by Field Cycling 1H NMR Relaxometry

2,2-Dimethylbutan-1-ol (2,2-DM-1-B), 3,3-dimethylbutan-1-ol (3,3-DM-1-B), and 3,3-dimethylbutan-2-ol (3,3-DM-2-B) show a rich solid-state polymorphism, which includes one or more plastic crystalline phases (also referred to as orientationally disordered crystalline (ODIC) phases) and glass of the liquid or ODIC phases. In this work, the dynamics of the three isomeric alcohols was investigated in the liquid and plastic crystalline phases by fast field cycling ¹H NMR relaxometry in the temperature range between 213 and 303 K. The analysis of the nuclear magnetic relaxation dispersion curves (i.e., longitudinal relaxation rate R₁ vs ¹H Larmor frequency) acquired for the different alcohols at different temperatures gave quantitative information on internal motions, overall molecular reorientations, and molecular self-diffusion. Self-diffusion coefficients were also determined in the liquid phase and in some ODIC phases of the samples from the trends of ¹H R₁ as a function of the frequency square root at low frequencies. Remarkable changes in the temperature trends of correlation times and self-diffusion coefficients were found at the transition between the liquid and the ODIC phase for 2,2-DM-1-B and 3,3-DM-1-B, and between ODIC phases for 3,3-DM-2-B, the latter sample showing a markedly different dynamic and phase behavior.

Dynamics of solid alanine by means of nuclear magnetic resonance relaxometry

¹H nuclear magnetic resonance relaxometry was applied to investigate the dynamics of l-alanine in the solid phase (powder). The experimental studies were carried out in a very broad frequency range, covering four orders of magnitude-from 4 kHz to 40 MHz (referring to the ¹H resonance frequency) in order to probe motional processes of much different time scales by a single experiment. To get access to the dynamics of different proton groups of alanine, the ¹H spin-lattice relaxation measurements were performed for non-deuterated and partially deuterated alanine. The experiments were carried out in the temperature range of 293 K-370 K (non-deuterated alanine) and 318 K-370 K (partially deuterated alanine). As a result of a thorough theoretical analysis of the extensive set of experimental results, three motional processes occurring on different time scales are identified and quantitatively described. The slowest process occurs on a time scale of μs and it is attributed to the collective dynamics of a 3D hydrogen bond network of alanine, while the intermediate, attributed to the dynamics of the NH₃ group, corresponds to the range of tenths of ns. The fast process describes the rotation of the CH₃ group.

Molecular dynamics of 1-ethyl-3-methylimidazolium triflate ionic liquid studied by 1H and 19F nuclear magnetic resonances

The molecular dynamics of an ionic liquid (IL) composed of a 1-ethyl-3-methylimidazolium cation and a triflate (trifluoromethanesulfonate) anion, abbreviated as [Emim][TfO], were studied by NMR spectroscopy. By measuring the temperature-dependent high-field 1H and ¹⁹F spin-lattice relaxation (SLR) rates, the frequency-dependent ¹H and ¹⁹F SLR dispersion curves using fast-field-cycling relaxometry, and the temperature-dependent 1H and ¹⁹F diffusion constants, and by utilizing the fact that the primary NMR-active nucleus on the Emim cation is ¹H, whereas on the TfO anion it is ¹⁹F, the cationic and anionic dynamics were studied separately. A single theoretical relaxation model successfully reproduced all the experimental data of both types of resonant nuclei by fitting all the data simultaneously with the same set of fit parameters. Upon cooling, [Emim][TfO] exhibited a supercooled liquid phase between T_SL = 256 K and the crystallization temperature T_Cr ≈ 227-222 K, as confirmed by differential scanning calorimetry (DSC) experiments. Theoretical analysis revealed that within the liquid and the supercooled liquid states of [Emim][TfO], the ¹H and ¹⁹F relaxation rates are affected by both the rotational and translational diffusional processes with no discontinuous change at T_SL. While the rotational diffusion is well described as an Arrhenius thermally activated process, the translational diffusion undergoes strong freezing dynamics that are well described by the Vogel-Fulcher model assuming a freezing temperature of T₀ = 157 K. The existence of the supercooled liquid region in the [Emim][TfO] IL should be taken into account when using this IL for a specific application.

Structure and dynamics of [NH2(CH3)2]3Sb2Cl9 by means of 1H NMR relaxometry - quadrupolar relaxation enhancement effects

¹H spin-lattice relaxation experiments have been performed for [NH₂(CH₃)₂]₃Sb₂Cl₉ (tris(dimethylammonium)nonachlorodiantimonate(iii)) in the temperature range of 253-313 K and a broad range of frequencies - from 4 kHz to 40 MHz. From the analysis of quadrupolar relaxation enhancement effects (quadrupolar peaks) associated with ¹⁴N nuclei, two lattice sites characterized by different electric field gradient tensors have been revealed. The ¹⁴N quadrupolar couplings and asymmetry parameters at these sites differ by a factor of about two. Three motional processes have been identified and attributed to the overall dynamics of the NH₂(CH₃)₂ cations (slow motion), dynamics of the NH₂ groups (intermediate motion) and methyl group rotation (fast motion). It has been shown that the slow dynamics is only weakly temperature dependent, while the intermediate and fast motional processes are characterized by activation energies of 2.92 kJ mol^-1 and 0.41 kJ mol^-1, respectively. The correlation time of the slow dynamics is of the order of μs, while the intermediate dynamics is faster by 2-3 orders of magnitude (depending on temperature). All correlation times have turned out to be independent of the position of the cations in the lattice (they are the same for both lattice sites). The analysis presented in this work is an example of the potential of the quadrupolar relaxation enhancement effects as a method revealing information on the dynamics and structure of solids.

Dynamics of ionic liquids in bulk and in confinement by means of (1)H NMR relaxometry - BMIM-OcSO4 in an SiO2 matrix as an example

(1)H nuclear magnetic resonance relaxometry is applied to reveal information on the translational and rotational dynamics of the ionic liquid: 1-butyl-3-methyl imidazoliumoctyl sulfate (BMIM-OcSO4) in bulk and in a confinement formed by a nanoporous SiO2 matrix. The experimental studies were performed in a very broad frequency range, from 8 kHz to 40 MHz (referring to the (1)H resonance frequency), in order to probe motional processes at very different time scales using a single experiment, and in the temperature range of 243-303 K. The relaxation results for BMIM-OcSO4 in bulk are interpreted in terms of three relaxation contributions: a term associated with the translational dynamics of the ions (it has been assumed that the translational dynamics of cations and anions can be described by one diffusion coefficient) and two terms associated with the rotational motion of the anion and the cation, respectively. The relationships between the obtained dynamic parameters (rotational correlation times and translational diffusion coefficients) are thoroughly discussed and used as a "reference" for the dynamics of BMIM-OcSO4 confined in an SiO2 matrix. Analysis of the corresponding relaxation data for the confined liquid shows that the confinement does not significantly affect the rotational dynamics, but it has a considerable impact on the translational motion. It is demonstrated that the relaxation term associated with the translational dynamics stems from two contributions: a contribution from a core (bulk-like) fraction of the liquid and from a fraction moving near the pore surface and therefore being for some time adsorbed on the pore walls. The translational diffusion coefficient for the last fraction is determined and several conclusions regarding the residence lifetime of the ions on the surface are drawn. Moreover, an additional motional process on the timescale of ns or shorter is revealed in the confinement.

Self-diffusion studies by intra- and inter-molecular spin-lattice relaxometry using field-cycling: Liquids, plastic crystals, porous media, and polymer segments

Field-cycling NMR relaxometry is a well-established technique for probing molecular dynamics in a frequency range from typically a few kHz up to several tens of MHz. For the interpretation of relaxometry data, it is quite often assumed that the spin-lattice relaxation process is of an intra-molecular nature so that rotational fluctuations dominate. However, dipolar interactions as the main type of couplings between protons and other dipolar species without quadrupole moments can imply appreciable inter-molecular contributions. These fluctuate due to translational displacements and to a lesser degree also by rotational reorientations in the short-range limit. The analysis of the inter-molecular proton spin-lattice relaxation rate thus permits one to evaluate self-diffusion variables such as the diffusion coefficient or the mean square displacement on a time scale from nanoseconds to several hundreds of microseconds. Numerous applications to solvents, plastic crystals and polymers will be reviewed. The technique is of particular interest for polymer dynamics since inter-molecular spin-lattice relaxation diffusometry bridges the time scales of quasi-elastic neutron scattering and field-gradient NMR diffusometry. This is just the range where model-specific intra-coil mechanisms are assumed to occur. They are expected to reveal themselves by characteristic power laws for the time-dependence of the mean-square segment displacement. These can be favorably tested on this basis. Results reported in the literature will be compared with theoretical predictions. On the other hand, there is a second way for translational diffusion phenomena to affect the spin-lattice relaxation dispersion. If rotational diffusion of molecules is restricted, translational diffusion properties can be deduced even from molecular reorientation dynamics detected by intra-molecular spin-lattice relaxation. This sort of scenario will be relevant for adsorbates on surfaces or polymer segments under entanglement and chain connectivity constraints. Under such conditions, reorientations will be correlated with translational displacements leading to the so-called RMTD relaxation process (reorientation mediated by translational displacements). Applications to porous glasses, protein solutions, lipid bilayers, and clays will be discussed. Finally, we will address the intriguing fact that the various time limits of the segment mean-square displacement of polymers in some cases perfectly reproduce predictions of the tube/reptation model whereas the reorientation dynamics suggests strongly deviating power laws.

Dynamical properties of EMIM-SCN confined in a SiO2 matrix by means of 1H NMR relaxometry

NMR relaxometry gives deep insights into ionic dynamics in ionogels.

Investigation of dynamics in BMIM TFSA ionic liquid through variable temperature and pressure NMR relaxometry and diffusometry

A comprehensive variable temperature, pressure and frequency multinuclear (1H, 2H, and 19F) magnetic resonance study was undertaken on selectively deuterated 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (BMIM TFSA) ionic liquid isotopologues. This study builds on our earlier investigation of the effects of increasing alkyl chain length on diffusion and dynamics in imidazolium-based TFSA ionic liquids. Fast field cycling 1H T1 data revealed multiple modes of motion. Through calculation of diffusion coefficient (D) values and activation energies, the low- and high-field regimes were assigned to the translational and reorientation dynamics respectively. Variable-pressure 2H T1 measurements reveal site-dependent interactions in the cation with strengths in the order MD3 > CD3 > CD2, indicating dissimilarities in the electric field gradients along the alkyl chain, with the CD2 sites having the largest gradient. Additionally, the α saturation effect in T1 vs. P was observed for all three sites, suggesting significant reduction of the short-range rapid reorientational dynamics. This reduction was also deduced from the variable pressure 1H T1 data, which showed an approach to saturation for both the methyl and butyl group terminal methyl sites. Pressure-dependent D measurements show independent motions for both cations and anions, with the cations having greater D values over the entire pressure range.

Picosecond dynamic heterogeneity, hopping, and Johari-Goldstein relaxation in glass-forming liquids

We show that incoherent quasielastic neutron scattering from molecular liquids reveals a two-state dynamic heterogeneity on a 1 ps time scale, where molecules are either highly confined or are free to undergo relatively large excursions. Data ranging from deep in the glassy state to well above the melting point allows us to observe temperature-dependent population levels and exchange between these two states. A simple physical picture emerges from this data, combined with published work, that provides a mechanism for hopping and for the Johari-Goldstein (β_{JG}) relaxation, and allows us to accurately calculate the diffusion coefficient, D_{T}, and characteristic times for α, and β_{JG} relaxations from ps time scale neutron data.