6,7Li NMR study of ion mobility on the molecular scale in lithated imidazole complexes

Two model compounds, lithium imidazolium (LiIm) and lithium 2-undecylimidazolium (und-LiIm), were synthesized. These materials are chosen as models of potential lithium ion conductors for use as electrolytes in lithium batteries. Solid-state NMR was used to provide information on the microscopic interactions including ionic mobility and ring reorientations which govern the efficiency of conductivity. Lithium imidazolium was mixed with lithium methylsulfonate, generating a doped complex in which a doubly lithiated imidazole ring was inferred based on the ⁷Li NMR chemical shifts. Our research includes ^6,7Li variable temperature MAS NMR experiments at intermediate spinning speeds, relaxation studies to determine spin-lattice relaxation times (T₁) of lithium ion hopping, and 2D exchange spectroscopy to determine possible chemical exchange processes. The possibility of 2-site ring reorientation for the doubly lithiated imidazole ring was supported by exchange spectroscopy. Comparisons of spin-lattice relaxation times and corresponding activation energies of the lithium imidazolium and the doped complex point to a higher degree of mobility in the latter.Lithium 2-undecylimidazolium was prepared and exhibited a lower melting point than the parent lithium imidazolium, as expected. This small molecule was chosen as representative of a side-chain functionalized polyethylene-based material. ⁷Li MAS spectra show mainly the presence of the doubly lithiated imidazole ring in pure und-LiIm, and in the LiCH₃SO₃–und-LiIm mixture. The data clearly indicate local mobility of the lithium ions in the materials.     

7Li NMR analysis on perovskite structured Li0.15La0.28TaO3

The nanocrystalline perovskite material Li_0.15La_0.28TaO₃ has been synthesized by alkoxide-free Pechini type sol gel method. ⁷Li NMR measurements were carried out using a Bruker Avance 300 spectrometer at 116 MHz over the temperature range 150 to 400 K. Longitudinal spin-lattice relaxation times (T₁) measured by saturation recovery and longitudinal relaxation times in the rotating frame (T_1ρ) measured using the pulse sequence (π/2–spin lock τ acquisition) with lock radio-frequency field υ = 62.5 kHz and the T₂ relaxation time measured by Hahn echo are presented. The static Hahn-echo spectra show two different lithium sites in this perovskite oxide. Further, the relaxation measurements T₁ and T_1ρ show two different types of lithium cations with fast and slow dynamics.     

Ionic Pathways in Li13Si4 investigated by 6Li and 7Li solid state NMR experiments

Local environments and dynamics of lithium ions in the binary lithium silicide Li₁₃Si₄ have been studied by ⁶Li MAS-NMR, ⁷Li spin-lattice relaxation time and site-resolved ⁷Li 2D exchange NMR measurements as a function of mixing time. Variable temperature experiments result in distinct differences in activation energies characterizing the transfer rates between the different lithium sites. Based on this information, a comprehensive picture of the preferred ionic transfer pathways in this silicide has been developed. With respect to local mobility, the results of the present study suggests the ordering Li6/Li7>Li5>Li1>Li4 >Li2/Li3. Mobility within the z=0.5 plane is distinctly higher than within the z=0 plane, and the ionic transfer between the planes is most facile via Li1/Li5 exchange. The lithium ionic mobility can be rationalized on the basis of the type of the coordinating silicide anions and the lithium-lithium distances within the structure. Lithium ions strongly interacting with the isolated Si⁴⁻ anions have distinctly lower mobility than those the coordination of which is dominated by Si₂⁶⁻ dumbbells.     

7Li NMR study of the mobility of lithium ions in one-dimensional channels of Nb3Se4

The temperature dependence of the spin-lattice relaxation time T ₁ and the ⁷Li NMR spectra of the Li_0.7Nb₃Se₄ intercalation compound with one-dimensional channel structure have been studied. It is found that the temperature dependence of T ₁ exhibits two relaxation minima, and the quadrupole splitting in the Li NMR spectra shows an anomalous temperature behavior. The inference is drawn that the observed effects are associated with the high-rate diffusive motion of lithium ions along one-dimensional channels and the interchannel transitions.     

Clustering and polymerization of Li15C60

The structural and electronic properties of lithium intercalated fullerides (of which Li₁₅C₆₀ is the most representative) are still puzzling and unclear. Above 520 K, x-ray/neutron diffraction shows an fcc phase in which the 15 Li atoms clusterize in the octahedral interstices. However, at lower temperatures, a change in the crystalline symmetry and also in the electronic properties takes place as observed from ¹³C, ⁷Li/⁶Li NMR and x-ray diffraction measurements. X-ray diffraction data suggest the presence of two different stable structures: a tetragonal monomeric and an orthorhombic polymerised phase. Detailed ¹³C magic angle spinning NMR experiments in the latter phase indicate sp ³ bondings among the carbon atoms, whereas the relative (sp ²/sp ³) intensities, together with x-ray data, suggest the C₆₀ polymerization to be a [2+2] cycloaddition. Multiple quantum NMR experiments on ⁷Li confirm the presence of lithium clusters, as observed by x-ray diffraction in the high temperature phase, also at lower temperatures. However, the inferred cluster size is significantly smaller than that suggested by the stoichiometry. The distortion in the low-T structure of L₁₅C₆₀ is supposed to induce the migration of Li atoms from octahedral to tetrahedral voids, thus accounting for the lower number of Li atoms in the clusters. Further evidence of this scenario is obtained also from preliminary measurements of line shapes and T ₁ relaxation times, which exhibit a multiexponential recovery with very different constants that are hardly compatible with a single family of Li atom sites.     

Low-molecular weight lanthanide contrast agents: Evaluation of susceptibility and dipolar effects in red blood cell suspensions

Red blood cell (RBC) suspensions, containing low-molecular weight (LMW) dysprosium (Dy) and gadolinium (Gd) chelates, were selected as a two-compartment system for the evaluation of the magnetic dipolar and susceptibility contributions to the transverse (T₂) relaxation of solvent water protons. The influence of RBC geometry and degree of metal chelate compartmentalization on T₂ was investigated by variation of the osmolality and hematocrit (HC), respectively. The T₂-relaxation ability of Dy-chelates was markedly improved in RBC suspensions, in comparison to aqueous solutions, due to the presence of susceptibility effects that more than compensated for the low dipolar relaxation efficacy. Despite a smaller susceptibility effect, the Gd-chelates were still the most efficacious in shortening T₂ due to their comparatively larger dipolar relaxation contribution. The results obtained with the Dy-chelates allowed the evaluation of the relative contributions of susceptibility and dipolar mediated relaxation for the Gd-chelates. The RBC geometry and degree of compartmentalization influenced strongly the T₂ relaxation efficacy of Dy-chelates, as opposed to the Gd-chelates. Hemolysis eliminated the susceptibility effect, essentially removing the T₂ relaxation ability of Dy-chelates. The T₂ relaxation efficacy of Gd-chelates was improved by hemolysis due to enhancement of the dipolar relaxation. As a conclusion, RBC suspensions have clearly been shown to be a suitable ex vivo model with which to distinguish the different contrast mechanisms of LMW Dy- and Gd-based MRI contrast agents.     

Characterization of proton NMR relaxation times in normal and pathological tissues by correlation with other tissue parameters

To help understand which tissue parameters best account for the water proton NMR relaxation times, the longitudinal relaxation time (T₂), the transverse relaxation time (T₂), and the water content of 16 tissues from normal adult rats were measured at 10.7 MHz and 29°C. Regression analyses between the above and other tissue parameters were performed. These other tissue parameters included: the amounts of various organic and inorganic components, protein synthetic rate, oxygen consumption rate, and morphological composition. In addition, the differences in T₁, T₂, and water content values between normal liver and malignant tumor (Morris #7777 a transplantable hepatoma) were studied to help understand how a disease state can be detected and characterized by NMR spectroscopy. The results of this study and information from the literature allow the following generalizations to be made about tissue T₁ and T₂ values: (1) Each normal tissue has rather consistent and characteristic T₁ and T₂ relaxation times which are always shorter than the T₁ and T₂ of bulk water; (2) tissues with higher water content tend to have longer T₁ relaxation times; (3) tissue T₂ values are not, however, as well correlated with water content as T₁ values; (4) tissues with shorter T₁ values have higher calculated hydration fractions, greater amounts of rough endoplasmic reticulum, and a greater rate of protein synthetic activity; (5) tissues with higher lipid content, associated with intracellular non-membrane bounded lipid droplets, tend to have longer T₂ values; (6) tissues with greater overall surface area, whether in the form of cellular membranes or intracellular or extracellular fibrillar macromolecules, tend to have shorter T₂ values; (7) the differences between T₁ and T₂ values between tumor and normal tissues correlated with differences in the volume fraction (amounts) of extracellular fluid volumes and in the amounts of membrane and fibrillar surface area in the cells. The above generalizations should be useful in predicting T₁ and T₂ changes associated with specific tissue pathologies.     

The first in situ 7Li NMR study of the reversible lithium insertion mechanism in disorganised carbons

We study a disordered carbon designed to be used as negative electrode in a secondary lithium battery. We perform in situ magnetic resonance, i.e. we run several electrochemical cycles inside the magnet while recording NMR spectra. We obtain simultaneous ⁷Li NMR and electrochemistry data and may thus analyse: the detailed chronology of the events, the Knight shift, the line width, the relaxation time T₁. This experimental work allows us to evidence a quasi metallic lithium, less diffusive than in metal and in graphite. The quasi metallic lithium is responsible for the very good reversible capacity, at a very low voltage. We indicate a model for the insertion/extraction of lithium in the fibers. The disordered carbon under study exhibits better qualities than graphite's for secondary battery applications. Only one drawback remains: a 0.5 V hysteresis; NMR shows that it occurs when the lasts quasi metal lithium are to be extracted. We evaluate the energy barrier.     

A nuclear magnetic resonance study of the structural properties and molecular motions of Li2KH(SO4)2 and LiKSO4 single crystals

The structural properties and relaxation mechanisms of Li₂KH(SO₄)₂ crystals were determined using the temperature dependences of NMR spectra and the spin-lattice relaxation times (T₁) of their ¹H, ⁷Li, and ³⁹K nuclei. The results obtained were compared with the previously reported physical properties of LiKSO₄ crystals. The substitution of the potassium ions with protons in the LiKSO₄ crystals were variations in the phase transition temperatures, and the non-appearance of ferroelastic properties. The ⁷Li T₁ for the Li₂KH(SO₄)₂ crystals was much shorter than the ⁷Li T₁ for the LiKSO₄ crystals, and these findings indicate that the presence of the protons in Li₂KH(SO₄)₂ causes the Li ions to move with greater freedom.     

What causes the hyperintense T2-weighting and increased short T2 signal in the corticospinal tract?

The corticospinal tract (CST) appears hyperintense on both T₂-weighted images and myelin water maps. Here, an extended multiecho T₂ relaxation sequence with echoes out to 1120 ms was used to characterize the longer T₂ times present in the CST. The T₂ distribution from the CST was compared to other white matter structures in 14 healthy subjects. The intra-/extracellular T₂ peak of the CST was broadened relative to other white matter structures and often split into two distinct peaks. In the CST, it appeared that the intracellular and extracellular water environments had unique T₂ times, causing the intracellular water peak to be pushed down into the myelin water T₂ regime and the extracellular peak to be pushed up to longer T₂ times. The conventional myelin water T₂ limits of 5-40 ms resulted in an artificial increase in myelin water fraction (MWF), causing the CST to be bright on myelin water images. When the upper limit for MWF was decreased to 25 ms, the CST regions exhibited MWF values similar to those found for adjacent anterior and posterior regions. The CST has unique magnetic resonance characteristics, which should be taken into consideration when being examined, especially when compared to pathological tissue.     

Simultaneous measurement of total water content and myelin water fraction in brain at 3 T using a T2 relaxation based method

PurposeThis work demonstrates the in vivo application of a T₂ relaxation based total water content (TWC) measurement technique at 3 T in healthy human brain, and evaluates accuracy using simulations that model brain tissue. The benefit of using T₂ relaxation is that it provides simultaneous measurements of myelin water fraction, which correlates to myelin content.MethodsT₂ relaxation data was collected from 10 healthy human subjects with a gradient and spin echo (GRASE) sequence, along with inversion recovery for T₁ mapping. Voxel-wise T₂ distributions were calculated by fitting the T₂ relaxation data with a non-negative least squares algorithm incorporating B₁⁺ inhomogeneity corrections. TWC was the sum of the signals in the T₂ distribution, corrected for T₁ relaxation and receiver coil inhomogeneity, relative to either an external water standard or cerebrospinal fluid (CSF). Simulations were performed to determine theoretical errors in TWC.ResultsTWC values measured in healthy human brain relative to both external and CSF standards agreed with literature values. Simulations demonstrated that TWC could be measured to within 3–4% accuracy.ConclusionIn vivo TWC measurement using T₂ relaxation at 3 T works well and provides a valuable tool for studying neurological diseases with both myelin and water changes.     

Dynamic polarization of 7Li nuclei in solutions containing radical ions

Dynamic nuclear polarization parameters, obtained at 75 G, are reported for ⁷Li ions in collision with several radical anions and with one radical cation. All systems show large negative ⁷Li N.M.R. enhancements indicative of weak scalar relaxation. However, radical induced relaxation rates derived from ⁷Li T ₁ measurements suggest stronger complexing of lithium ions with radical anions than with the radical cation as would be expected from simple coulombic considerations. Translational modulation of the dipolar interaction best accounts for proton and radical cation dipolar relaxation rates while rotational modulation best accounts for the corresponding radical anion rates; this supports the interpretation above. A model for the lithium radical collision is proposed which implies that, for radical anions, scalar coupling is only apparently weak and that the low ⁷Li scalar relaxation rates observed result from scalar correlation times (τ_c = 10^-8-10^-9 s) longer than any yet observed by this technique. The model predicts that, for certain ranges of τ_c, increasing strength of complex formation should lead to smaller scalar relaxation rates and more negative enhancements, in contrast with the behaviour of fluorocarbons where the reverse was true. The predicted dependence of enhancement upon τ_c also suggests that ⁷Li enhancements should be extremely sensitive to variations in the chemical properties of the system.     

Estimation of water content and water mobility in the nucleus and cytoplasm of Xenopus laevis oocytes by NMR microscopy

NMR microscopy is a noninvasive approach for studying cell structure and properties. Spatially resolved measurement of the relaxation times T₁ and T₂ provided information on the water proton spin density and water mobility in different parts of Xenopus laevis oocytes. The spin-lattice relaxation time T₁ was determined using a saturation-recovery sequence and the common spin-echo sequence with increasing repetition times, while the transverse relaxation time T₂ was measured by means of the spin-echo sequence with varying echo times. From the relaxation times, the mole fractions of possible reorientational correlation times τ_c for different types of intracellular water were calculated according to a simple two-phase model. The values for T₁, T₂, and proton spin density (i.e., water content) are: nucleus ⪢ animal cytoplasm > vegetal cytoplasm. Based on the estimation of τ_c, nearly 90% of the nuclear water and 74.4% of the water of the animal pole was considered as free mobile water, whereas 55.5% of the water of the vegetal pole appeared as bound water.     

非晶锂离子导体B2O3-0.7Li2O-0.7LiCl-xAl2O3晶化初期的核磁共振研究

通过真空密封热处理、避免了样品晶化后吸水引起的误差,采用脉冲法在293K和77K测量了晶化过程初期三种非晶锂离子导体B₂O₃-0.7Li₂O-0.7LiCl-xAl₂O₃(x=0.15,0.10和0.05)的⁷Li核磁共振谱。发现在低温(77K)只有固相锂离子对应的自旋-自旋弛豫时间T₂=87μs,严格按高斯函数衰减。在室温下固相锂离子对应的T_2s=127μs,仍是高斯型;但液相锂离子对应的T₂却按洛仑兹函数衰减。这反映出锂离子导体的固-液二相性。三种非晶B₂O₃-0.7Li₂O-0.7LiCl-xAl₂O₃(x=0.15,0.10和0.05)分别在热处理温度401,388和381℃附近,其液相锂离子对应的T_2l都剧增,其吸收谱线宽都变窄。由此再次验证了非晶母体与微晶之间的两相界面效应的物理图象。 关键词：     

Lithium motion in Li0.33 TiS2 and Li0.94 TiS2

We have studied the motion of lithium ions in Li_xTiS₂ (x = 0.33, 0.94) using pulsed NMR techniques. The temperature dependences of the spin lattice relaxation in the rotating frame (T_1?) suggest comparable activation energies for lithium ion diffusion for both samples, 3370 K, but an appreciably longer hopping time for the x = 0.94 sample. Low temperature values of T₂ agree with calculated and measured second moments for both materials.     

7Li NMR study on Li+ ionic diffusion and phase transition in LixCoO2

The temperature dependence of the spin-lattice relaxation time, T₁ and the line width of the ⁷Li nucleus were measured in delithiated Li_xCoO₂ (x = 0.6, 0.8, 1.0). Two different relaxation behaviors were observed in the temperature dependence of T₁^− 1 in a x = 0.8 sample. These would have arisen from inequivalent Li sites in two coexisting phases; an original hexagonal (HEX-I) and a modified hexagonal (HEX-II) phase in the x = 0.8 sample. We analyzed using a phenomenological non Debye-type relaxation model. Motional narrowing in the line width was observed in each sample, the result revealing that Li⁺ ions begin to move at low temperature in samples with less Li content. It was found that the activation energy associating with Li⁺ ion hopping in the HEX-II phase is smaller than that in the HEX-I phase. These results show that the HEX-II phase produced in the Li deintercalation process would be suitable for Li⁺ ionic diffusion in multi-phase Li_xCoO₂, and it is expected that this would enable fast ionic diffusion. Li⁺ ionic diffusion related to phase transition is discussed from ⁷Li NMR results.     

Ionic lithium conductivity in sulphur base glasses. 7Li NMR study of xLi2S−(1−x)GeS2 system

The lithium mobility in glasses of composition xLi₂S?(1?x)GeS₂ has been followed by c.w. (for x = 0.3; 0.4 and 0.5) and pulsed NMR (for x = 0.3 and 0.5) between ? 150 and + 280°C.The second moment of the resonance line of ⁷Li(0.89–1.51 G²) is proportional to the molar fraction of Li₂S, which may be correlated to an homogeneous Li nuclei distribution.The resonance line profiles and their thermal evolution seem to show that some Li⁺ ions do not participate in the conduction.Thermal variation of the spin-lattice relaxation time T₁, shows a strongly asymmetrical shape, if one considers In $\text{T}{}^{\mathrm{?1}}{}_1\text{= ?(T}{}^{\mathrm{?1}}\text{)}$ on both sides of the observed maxima. This behaviour may be explained by a distribution of the Li correlation times τ corresponding to different jump distances between various possible sites. The Cole-Davidson model allows the best agreement between experimental and theoretical values of the correlation times.The activation energies deduced from this model are close to those obtained from conductivity measurements (0.43 eV by NMR, 0.56 eV by conductivity determinations for x = 0.50), they may be correlated to the longer Li⁺ jumps in the vitreous matrix.     

Temperature behavior of the photovoltaic and pyroelectric responses of Sn2P2S6 semiconductor ferroelectric films

The electric response of Sn₂P₂S₆ semiconductor ferroelectric films to focused laser radiation (λ = 6328 Å) with a modulation frequency of 24 Hz is studied experimentally. It is shown that the signal involves an initial spike of current (an unsteady component) followed by the relaxation decrease to a certain value (a quasisteady component). The most significant changes in the shape and amplitude of the film response is observed near the Sn₂P₂S₆ monocrystal phase transition point of T _C = 66°C. The behavior of the unsteady component of the response is related to the behavior of the spontaneous polarization. This component decreases with increasing temperature. The quasi-steady component reaches its maximum at T = C _C. It is revealed that the behavior of relaxation processes corresponding to the decreasing response undergoes a change near the phase transition temperature.     

Quadrupole interactions in lithium borodeuteride

Polycrystalline samples of lithium borohydride and borodeuteride, LiBH₄ and LiBD₄, are studied by ²H, ⁷Li, and ^10,11B NMR in 7.04 T and 9.35 T magnetic fields in the temperature range 116–580 K. The ^10,11B NMR line shape of the orthorhombic phase of LiBH₄ and LiBD₄ suggests that first-order quadrupole interaction takes place. The quadrupole coupling constant (QCC) χ _q and asymmetry parameter η of the electric field gradient tensor for ¹¹B are described by linear temperature dependences: χ _q (¹¹B) = 177 ? 0.24T and η = 0.043 + 0.0014T. The electric field gradient at the positions of boron nuclei is created by external charges, primarily lithium cations. In the range of 388–391 K, the ⁷Li NMR line shape reflects the coexistence of two phase modifications of LiBH₄ and LiBD₄ and the occurrence of a reversible first-order phase transition. In the temperature range of 390–530 K, the ⁷Li NMR line shape represents a first-order quadrupole perturbed spectrum with zero asymmetry parameter and a weakly temperature dependent ⁷Li QCC. The spin-lattice relaxation time and the NMR line shape of ²H are interpreted in terms of the reorientation of the BD ₄ ^? anion about their proper symmetry axes C₂ and C₃.