Magnetic Resonance Spin-lattice Relaxation Times Research Articles

Structural geometry and molecular dynamics of hybrid organic–inorganic [NH3(CH2)6NH3]CdCl4 crystals close to phase transition temperatures

Organic–inorganic hybrid perovskites are highly promising for application in various electrochemical devices, such as batteries and fuel cells. The physical properties of crystals of the organic–inorganic hybrid perovskite [NH3(CH2)6NH3]CdCl4 are important for their future application. Therefore, these crystals were grown and their phase transition temperatures TC1 = 337 K and TC2 = 472 K were determined using powder X-ray diffraction and differential scanning calorimetry. We observed that the crystallographic surroundings of 1H and 13C in the cation near TC1 did not show significant changes, whereas those of 113Cd in the anion changed significantly. The change in the coordination geometry of C1 around Cd near TC1 changed the N–H···Cl bond by connecting with the 1H of NH3. The nuclear magnetic resonance spin-lattice relaxation time T1ρ suggested that the 1H energy transfer processes in phases III and II were easier, whereas the 13C energy transfer processes were easier in phase III than in phase II. The results of the current study for [NH3(CH2)nNH3]CdCl4 (n = 6) were compared with those for n = 2, 3, 4, and 5 obtained from previous studies. The features of the length of CH2 and even–odd number of carbons in the diammonium chain are expected to facilitate potential applications in the future.

Molecular motion of imidazolium cations and phase transitions in (ImH)2KCo(CN)6 with double perovskite structure

Temperature dependence of 1H nuclear magnetic resonance spin–lattice relaxation time T1 of (ImH)2KCo(CN)6, ImH+: imidazolium cation, is reported. The asymmetric T1 minimum observed in intermediate-temperature phase between 198 and 112 K is analyzed assuming the Cole-Davidson distribution of correlation time. Temperature dependences of 14N nuclear quadrupole resonance frequencies and spin–lattice relaxation times are also reported. The phase transitions at 198 and 112 K are shown to be of second and first order, respectively. The 14N relaxation is shown to be governed by the electric field gradient fluctuation due to the in-plane reorientational motion of the imidazolium cations.

Characterization of Ion Association and Solvation in NaPF6 Carbonate Electrolytes

In this work we aimed to determine the ions association and solvation in electrolytes comprised of NaPF6 in binary EC:PC, and ternary 5EC:3PC:2DEC carbonate solvents. We studied their local and bulk transport properties through multi-Nuclear (1H, 19F and 23Na) Magnetic Resonance spin-lattice relaxation time (T1 ) and self-diffusion coefficient (D) measurements. These studies were complemented with ionic conductivity and solution viscosity measurements. Results show several interesting features. Firstly, the Walden Product – which relates the molar conductivity and viscosity through a temperature-dependent constant – showed a dependence on temperature (see Figure 1). This suggested the dissociation of NaPF6 is dependent on temperature and is lower for the less polar electrolytes. Secondly, the sodium ions are preferentially solvated by PC molecules over EC. Thirdly, significant cation-anion association was observed for the less polar electrolytes. Lastly, this association was independent of temperature indicating that the temperature dependence of the Walden Product is primarily due to that of the viscosity. These results can aid in the improvement of ‘beyond lithium ion’ electrolytes for energy storage applications. Figure 1

Transport studies of NaPF6 carbonate solvents-based sodium ion electrolytes

Characterizing fundamental ion transport is necessary for the modification and development of electrolytes. In this study, we investigated the local and bulk transport properties of 1 M NaPF6 in the binary EC:PC (1:1), and ternary 5EC:3PC:2DEC (5:3:2) solvents, as well as their 2 wt.% FEC modulated versions. We determined 1H, 19F and 23Na Nuclear Magnetic Resonance (NMR) spin-lattice relaxation time (T1), self-diffusion coefficient (D), linewidth and chemical shift. These were complimented with density, viscosity and ionic conductivity measurements, all as a function of temperature. Both the viscosity and ionic conductivity displayed VFT behavior and their Walden products showed a dependence on temperature. This coupled with their having the lower Walden products indicated less salt dissociation in the 5EC:3PC:2DEC and 5EC:3PC:2DEC + 2 wt.% electrolytes. NMR D19F and 23Na values for these electrolytes were also similar at 298K indicating cation-anion association, but diverged with increasing temperatures, with that for the anion being greater than the cation. The addition of the FEC provides greater local dynamics for the individual solvents in the less polar electrolyte but appeared to impede that of the ions.

Clinical assessment of adenosine stress and rest cardiac magnetic resonance T1 mapping for detecting ischemic and infarcted myocardium

Cardiac magnetic resonance (CMR) spin-lattice relaxation time (T1) may be influenced by pathologic conditions due to changes in myocardial water content. We aimed to validate the principle and investigate T1 mapping at rest and adenosine stress to differentiate ischemic and infarcted myocardium from controls. Patients with suspected coronary artery disease who underwent CMR were prospectively recruited. Native rest and adenosine stress T1 maps were obtained using standard modified Look-Locker Inversion-Recovery technique. Among 181 patients included, T1 values were measured from three groups. In the control group, 72 patients showed myocardium with a T1 profile of 1,039 ± 75 ms at rest and a significant increase during stress (4.79 ± 3.14%, p < 0.001). While the ischemic (51 patients) and infarcted (58 patients) groups showed elevated resting T1 compared to controls (1,040 ± 90 ms for ischemic; 1,239 ± 121 ms for infarcted, p < 0.001), neither of which presented significant T1 reactivity (1.38 ± 3.02% for ischemic; 1.55 ± 5.25% for infarcted). We concluded that adenosine stress and rest T1 mapping may be useful to differentiate normal, ischemic and infarcted myocardium.

Motional freedom of dimethylammonium ions in a cyanoelpasolite, [(CH3)2NH2]2KCo(CN)6, which exhibits phase transition associated with a distinct change in dielectric property.

The temperature dependence of the 1H nuclear magnetic resonance spin-lattice relaxation time T1 of [(CH3)2NH2]2KCo(CN)6 and the partially deuterated analogue [(CD3)2NH2]2KCo(CN)6 has been reported. A change in the molecular motion of organic cations through the first-order phase transition at Tc = 246 K has been discussed. Although this first-order transition has been reported as an isosymmetric transition, in which the space group of the crystal does not change, the number of the 14N nuclear quadrupole resonance line changed from three in the high-temperature phase to twelve in the low-temperature phase indicating the lowering of crystal symmetry.

Investigating the Internal Processes of Thermo-Electrochemical Cells Using in-Situ Magnetic Resonance Imaging

Thermo-electrochemical cells are emerging as a promising and efficient method to convert waste heat to electricity. In addition to developing novel redox electrolyte materials to increase the energy densities, it is crucial for future developments to understand the internal chemical and thermodynamical processes happening inside the thermo-electrochemical cell while it is under operating conditions. Although the temperature and concentration distributions between the electrodes and the variations of the convection currents with cell orientation have been modelled mathematically, it has been a challenge to study these internal processes using experimental techniques. Magnetic resonance imaging (MRI) is a non-invasive and well-established diagnosis technique in medical research to investigate physiology, but it also has the capability to study spatial properties of materials, chemical and thermodynamic processes in energy storage devices using different image contrast options including spin density, relaxation time and velocity [1]. The goals of this study are to develop an MRI method to study the internal chemical and thermodynamic processes within a functioning thermo-electrochemical cell, and understanding how these are affected by the electrolyte, the cell geometry and the orientation of the thermal gradient. A cobalt redox couple based electrolyte, which has been shown to be a promising candidate for thermo-electrochemical cells [2], was used for the in-situ experiments inside specialised cells custom built for MRI experiments, and temperature and concentration dependent 1H nuclear magnetic resonance spin-lattice relaxation time (T1) measurements were used to interpret the 1H T1 weighted MRI images of the working cells.

Imaging of nuclear magnetic resonance spin-lattice relaxation activation energy in cartilage.

Samples of human and bovine cartilage have been examined using magnetic resonance imaging to determine the proton nuclear magnetic resonance spin–lattice relaxation time, T1, as a function of depth within through the cartilage tissue. T1 was measured at five to seven temperatures between 8 and 38°C. From this, it is shown that the T1 relaxation time is well described by Arrhenius-type behaviour and the activation energy of the relaxation process is quantified. The activation energy within the cartilage is approximately 11 ± 2 kJ mol−1 with this notably being less than that for both pure water (16.6 ± 0.4 kJ mol−1) and the phosphate-buffered solution in which the cartilage was immersed (14.7 ± 1.0 kJ mol−1). It is shown that this activation energy increases as a function of depth in the cartilage. It is known that cartilage composition varies with depth, and hence, these results have been interpreted in terms of the structure within the cartilage tissue and the association of the water with the macromolecular constituents of the cartilage.

NMR Characterization of the Na+ Ion Transport in Mixed Ionic Liquids Electrolytes

In the market of medium- and large- scale energy supplies, sodium (Na+) ion batteries are considered a possible alternative to current technologies. This is because sodium is naturally more abundant and cheaper than the ubiquitous lithium. Room temperature Na+ batteries typically employ organic solvents such as propylene carbonate. However, room temperature ionic liquids – while more viscous – are safer, and because of this attention is being paid towards their partial or complete replacement of organic solvents. Towards this endeavor, we investigated ions dynamics in mixtures of varying concentrations of sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) or sodium bis(fluorosulfonyl)imide (NaFSI) salts, combined with the 1-butyl-1-methylpyrrolidinium (PYR1,4) FSI and PYR1,4 TFSI ionic liquids. We used variable-temperature 1H and 23Na Nuclear Magnetic Resonance (NMR) spin-lattice relaxation times (T1) and self-diffusion coefficients (D) measurements to investigate the short- and long- range ions dynamics and a preview of or results are shown in Figure 1. Both T1 (1H and 23Na) and 1H D data show greater short- and long- range dynamics for mixtures containing the FSI salt or ionic liquid. A more comprehensive discussion will be presented. Figure 1. Variable-temperature Na+ spin-lattice relaxation times (left) and 1H self-diffusion coefficients (right) for mixtures of various mol% of NaFSI or NaTFSI salt with PYR1,4 based ILs. Figure 1

Nuclear magnetic resonance relaxometry of water in two and quasi-two dimensions

Molecular dynamics (MD) and Monte Carlo (MC) methods are used to determine the spin-pair correlation function G(*)(t) for the diffusion of bulk water in three dimensions (3D) and pore water in two dimensions (2D) and quasi-two dimensions (Q2D). The correlation function is required for the determination of the nuclear magnetic resonance spin-lattice and spin-spin relaxation times T(1) and T(2). It is shown that the analytic form of the powder-average correlation function, introduced by Sholl [Sholl, J. Phys. C: Solid State Phys. 7, 3378 (1974)] for the diffusion of spins on a 3D lattice, is of general validity. An analytic expression for G(*)(t) for a uniform spin fluid is derived in 2D. An analytic expression for the long-time behavior of G(*)(t) is derived for spins diffusing on 3D, 2D, and Q2D lattices. An analytic correction term, which accounts for spin pairs outside the scope of the numerical simulations, is derived for 3D and 2D and shown to improve the accuracy of the simulations. The contributions to T(1) due to translational and rotational motion obtained from the MD simulation of bulk water at 300 K are 7.4 s and 10±1 s, respectively, at 150 MHz, leading to an overall time of 4.3±0.4 s compared to the experimental value of 3.8 s. In Q2D systems, in which water is confined by alpha-quartz surfaces to thicknesses of 1-5 nm, T(1) for both translational and rotational relaxation is reduced due to the orientation and adsorption of spins at the surfaces. A method of parametrizing the MC lattice-diffusion simulations in 3D, 2D, and Q2D systems is presented. MC results for G(*)(t) for 3D and 2D systems are found to be consistent with an analytic uniform fluid model for t~/>40 ps. The value of TT(1) for translational diffusion obtained from the MC simulation of bulk water is found to be 4.8 s at 15 MHz. G(*)(t) obtained from MC simulations of Q2D systems, where water is confined by hard walls, is found to execute a distinct transition from 3D to 2D behavior. The T(1) is found to be similar to the 3D bulk water result at all pore thicknesses.

1H and 2H NMR Spin–Lattice Relaxation Probing Water: PEG Molecular Dynamics in Solution

Nuclear magnetic resonance spin-lattice relaxation times (T(1)) measurements were performed in aqueous solutions of poly(ethylene glycol) (PEG) of 6000 Da molecular mass to study the dynamical relation between PEG and water molecules at different solute concentrations. (1)H-T(1) experiments were carried on at a low magnetic field in the time domain (20 MHz) and at a high field (400 MHz) to obtain spectral resolution. Two contributing components were identified in each proton system, PEG and water, presenting values of T(1) with very different orders of magnitude. The approximate matching between the shorter (1)H-T(1) values associated with water and PEG has lead us to conclude that there exists a network of interactions (hydrogen bonds) between the solute and the solvent, which results in the presence of an ordered and dehydrated structure of PEG folded or self-assembled in equilibrium with a more flexible monomer structure. Dynamic light scattering results were consistent with the formation of PEG aggregates, showing a mean size between 40 and 100 nm.

Nuclear magnetic resonance relaxometry as a method of measuring translational diffusion coefficients in liquids

By application of the field-cycling technique, we measure the dispersion of the (1)H nuclear magnetic resonance (NMR) spin-lattice relaxation time T(1)(ω) for a series of molecular liquids. We demonstrate that such NMR relaxometry studies can be used for determining diffusion coefficients. A broad frequency range of 10 kHz-20 MHz is covered. By scanning T(1)(ω) one directly probes the spectral density of the diffusion processes. The value of the diffusion coefficient D can be determined from a linear dependence of the (1)H spin-lattice relaxation rate on the square root of the frequency at which it is measured. The power of this method lies in its simplicity, which allows one to determine D(T) independently of the diffusive model. The results obtained are in very good agreement with those of field gradient NMR methods.

Dynamics of Discotic Fluoroalkylated Triphenylene Molecules Studied by Proton NMR Relaxometry

The Larmor frequency and temperature dependence of the proton nuclear magnetic resonance (NMR) spin-lattice relaxation time was measured in the isotropic and columnar phases of both chain-end fluorinated triphenylene disk-like and fully hydrogenated molecules. In the columnar phase, the results are interpreted in terms of the collective motions, due to the deformations of the columns, and individual molecular translational self-diffusion displacements and rotations/reorientacions. In the isotropic phase, local molecular motions and order fluctuations as a pretransitional effect were considered. The activation energies of the molecular motions of the partially fluorinated molecule were found to be higher than those corresponding to the hydrocarbon homologue. Our findings show a clear difference in the relaxation dispersion between the two liquid crystals homologues. In particular it is observed that the columnar undulations have a much stronger contribution to the relaxation rate in the low frequency regime in the case of the fully hydrogenated triphenylene. The effect of fluorination of the pheripheral chain enhances the columnar mesophase's stability.

Spectroscopic and photopolymerization studies of benzyl methacrylate/poly(benzyl methacrylate) two-component system

The influence of the viscosity of a two-component system on its molecular dynamics (on the basis of hypersonic wave velocity and attenuation coefficient) and photopolymeriza- tion kinetics was studied. The system investigated represented the solution of poly(benzyl methacrylate), PBzMA (MW ¼ 70000) in its monomer, benzyl methacrylate (BzMA). The viscosity of the system was varied by adding various amounts of the poly- mer to the monomer (10-50 wt %). The molecular dynamics in the neat BzMA was studied by the proton Nuclear Magnetic Resonance (NMR) spin-lattice relaxation time measurements and the wide-line 1 H NMR spectroscopy in a wide range of tem- perature. Information on the local dynamics in liquid BzMA above its melting temperature was gained from the high-resolu- tion 1 H and 13 C NMR spectra. The hypersonic wave velocity and the attenuation coefficient were investigated in the appropriate temperature range related to a viscoelastic relaxation process by the Brillouin light scattering method. The kinetic measure- ments have demonstrated that the photopolymerization rate rapidly increases and the monomer conversion decreases with increasing polymer-to-monomer ratio; this effect has been noted in the whole range of polymer concentration and reaction temperature studied. V C 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1336-1348, 2010

1H NMR Relaxation Studies of Proton-Conducting Imidazolium Salts of Dicarboxylic Acids

The proton nuclear magnetic resonance spin–lattice relaxation time has been successfully used to evidence the reorientation of the imidazolium rings in carboxylic acid salts of imidazole: imidazolium oxalate, imidazolium adipate monohydrate and diimidazolium suberate. The process occurs in the temperature range between 325 K and the melting point of the salts (523, 379 and 360 K) and is described by a single activation energy of 20, 48 and 66 kJ/mol for imidazolium oxalate, imidazolium adipate monohydrate and diimidazolium suberate, respectively. The studied salts are considered as possible candidate materials for fuel-cell membranes.

Rotational dynamics of water and benzene controlled by anion field in ionic liquids: 1-butyl-3-methylimidazolium chloride and hexafluorophosphate

The rotational correlation time (tau(2R)) is determined for D(2)O (polar) and C(6)D(6) (apolar) in 1-butyl-3-methylimidazolium chloride ([bmim][Cl]) and hexafluorophosphate ([bmim][PF(6)]) by measuring (2)H (D) nuclear magnetic resonance spin-lattice relaxation time (T(1)) in the temperature range from -20 to 110 degrees C. The tau(2R) ratio of water to benzene (tau(WB)) was used as a measure of solute-solvent attraction. tau(WB) is 0.73 and 0.52 in [bmim][Cl] and [bmim][PF(6)], respectively, whereas the molecular volume ratio is as small as 0.11. The slowdown of the water dynamics compared to the benzene dynamics in ionic liquids is interpreted by the Coulombic attractive interaction between the polar water molecule and the anion. As for the anion effect, the rotational dynamics of water solvated by Cl(-) is slower than that solvated by PF(6) (-), whereas the rotational dynamics of benzene is similar in the two ionic liquids. This is interpreted as an indication of the stronger solvation by the anion with a larger surface charge density. The slowdown of the water dynamics via Coulombic solvation is actually significant only at water concentrations lower than approximately 9 mol dm(-3) at room temperature, and it is indistinguishable at temperatures above approximately 100 degrees C. The quadrupolar coupling constants determined for D(2)O and C(6)D(6) in the ionic liquids were smaller by a factor of 2-3 than those in the pure liquid state.

Change in Diffusional Water Permeability of Spinach Leaf Cell Membrane determined by Nuclear Magnetic Resonance Relaxation Time

The nuclear magnetic resonance spin–lattice relaxation times of spinach leaf protoplasts suspended in 0·6 M mannitol solution were measured in the presence ( T 1s ) and absence ( T 1 f ) of Mn 2+ ions to determine the water exchange time λ and water permeability coefficient P d of membrane. For the spinach stored for 1 d at 2 °C, λ was determined to be 76·4 m s and P d was 49 μm s −1 . As to the spinach stored for 6 d, λ was 52·7 ms and P d was 71 u s −1 . Values of P d obtained in this study had the same order of magnitude as literature values. Both the decrease in λ and the increase in P d with storage time showed that cell membrane became freely permeable state with storage duration. It was thought that the aquaporin of PM28A, a major intrinsic protein of the spinach leaf plasma membrane, was not a major cause of the increase in water permeability of cell membrane after harvest because the decrease in the expression of PM28A was observed in 1 d.

Experimental and theoretical study of proton spin-lattice relaxation in H2-Ar gas mixtures: critical examination of the XC(fit) potential energy surface.

Proton nuclear magnetic resonance spin-lattice relaxation time measurements have been carried out at 500 MHz proton Larmor frequency on two hydrogen-argon gas mixtures with 1.90% and 3.93% hydrogen at four different temperatures in the range 225 K < T < 337 K and at two different number densities. The results for different hydrogen mole percentages have been extrapolated to infinite dilution to obtain the contributions to the overall relaxation times arising from the hydrogen-argon interaction. The extrapolated relaxation times fall in the reciprocal regime in which relaxation times are inversely proportional to the density. Relaxation times have also been calculated using quantum mechanical close-coupled computations based on the H2-Ar XC(fit) potential energy surface obtained by Bissonnette et al. [J. Chem. Phys. 105, 2639 (1996)]. Significant differences found between the experimental and theoretical results indicate that the short-range anisotropy of the XC(fit) potential surface is too weak. The reciprocal regime is shown to have a much higher sensitivity to changes in the anisotropic component of the intermolecular potential energy surface.

Characterization of cation-pi interactions in aqueous solution using deuterium nuclear magnetic resonance spectroscopy.

Chemical interactions of aromatic organic contaminants control their fate, transport, and toxicity in the environment. Recent molecular modeling studies have suggested that strong interactions can occur between the pi electrons of aromatic molecules and metal cations in aqueous solutions and/or on mineral surfaces, and that such interactions may be important in some environmental systems. However, spectroscopic evidence for these so-called cation-pi interactions has been extremely limited to date. In this paper, cation-pi interactions in aqueous salt solutions were characterized via 2H nuclear magnetic resonance (NMR) spin-lattice relaxation times (T1) and calculations of molecular correlation times (tau(c)) for a series of perdeuterated (d6-benzene) benzene-cation complexes. The T1 values for d6-benzene decreased with increasing concentrations of LiCl, NaCl, KCl, RbCl, CsCl, and AgNO3, with the largest effects observed in the AgNO3 and CsCl solutions. Upon normalizing tau(c) values by solution viscosity effects, an overall affinity trend of Ag+ >> Cs+ > K+ > Rb+ > Na+ > Li+ was derived for the d6-benzene-cation complexes. The ability of Ag+ to complex d6-benzene was significantly reduced upon addition of NH3, which strongly coordinates Ag+ at high pH. Results with d6-benzene, d8-naphthalene, d2-dichloromethane, and d12-cyclohexane in 0.1 M methanolic salt solutions confirmed that spin-lattice relaxation rates are characterizing cation-pi interactions. The relatively strong cation-pi bonding observed between Ag+ and aromatic hydrocarbons probably results from covalent interactions between the aromatic pi electrons and the d orbitals of Ag+, in addition to the normal electrostatic interaction.

Molecular Dynamics in a Chiral Smectic Liquid Crystal Studied by 13C-Nuclear Magnetic Resonance Spin-Lattice Relaxation Time Observation

In order to study molecular rotation around its long axis in chiral smectic phases, the temperature dependence of the spin-lattice relaxation time T1 is investigated in an antiferroelectric liquid crystal, (S)-4-(1-methylheptyloxy-carbonyl)phenyl 4'-octyloxybiphenyl-4-carboxylate (S-MHPOBC) by means of solid-state 13C-NMR spectroscopy. The spin-lattice relaxation time T1 of the aromatic core part has a minimum at about 95°C in its temperature dependent curve. Since T1 minimum occurs under a condition of ω0τc∼1, the correlation time τc of molecular motion should be of the order of 2π/ω0∼10-8 s here, with the Larmor frequency ω0 of 100 MHz in the present measurements. The temperature dependence of T1 reveals that the activation energy in the SmCA* phase is larger than that in SmA for most of the carbon nuclei of the aromatic core part. It suggests that the molecular rotation is hindered in the SmCA* phase, in which the large spontaneous polarization exists in a layer.