Showing papers on "Relaxation (NMR) published in 2009"
TL;DR: The basics of T2* relaxation, T1*-weighted sequences, and their clinical applications are presented, followed by the principles, techniques, and clinical uses of four T2*,-based applications, including SW imaging, perfusion MR imaging, functional MR Imaging, and iron overload imaging.
Abstract: T2* relaxation refers to decay of transverse magnetization caused by a combination of spin-spin relaxation and magnetic fi eld inhomogeneity. T2* relaxation is seen only with gradient-echo (GRE) imaging because transverse relaxation caused by magnetic fi eld inhomogeneities is eliminated by the 180° pulse at spin-echo imaging. T2* relaxation is one of the main determinants of image contrast with GRE sequences and forms the basis for many magnetic resonance (MR) applications, such as susceptibility-weighted (SW) imaging, perfusion MR imaging, and functional MR imaging. GRE sequences can be made predominantly T2* weighted by using a low fl ip angle, long echo time, and long repetition time. GRE sequences with T2*-based contrast are used to depict hemorrhage, calcifi cation, and iron deposition in various tissues and lesions. SW imaging uses phase information in addition to T2*-based contrast to exploit the magnetic susceptibility differences of the blood and of iron and calcifi cation in various tissues. Perfusion MR imaging exploits the signal intensity decrease that occurs with the passage of a high concentration of gadopentetate dimeglumine through the microvasculature. Change in oxygen saturation during specifi c tasks changes the local T2*, which leads to the blood oxygen level–dependent effect seen at functional MR imaging. The basics of T2* relaxation, T2*-weighted sequences, and their clinical applications are presented, followed by the principles, techniques, and clinical uses of four T2*-based applications, including SW imaging, perfusion MR imaging, functional MR imaging, and iron overload imaging.
588 citations
TL;DR: In this paper, an ultra-sensitive atomic magnetometer using optically-pumped potassium atoms operating in spin exchange relaxation free (SERF) regime was described, achieving a magnetic field sensitivity of 160 aT/Hz$ 1/2}$ in a gradiometer arrangement with a measurement volume of 0.45 cm$^3$ and energy resolution per unit time of $44 hbar.
Abstract: We describe an ultra-sensitive atomic magnetometer using optically-pumped potassium atoms operating in spin-exchange relaxation free (SERF) regime. We demonstrate magnetic field sensitivity of 160 aT/Hz$^{1/2}$ in a gradiometer arrangement with a measurement volume of 0.45 cm$^3$ and energy resolution per unit time of $44 \hbar$. As an example of a new application enabled by such a magnetometer we describe measurements of weak remnant rock magnetization as a function of temperature with a sensitivity on the order of 10$^{-10}$ emu/cm$^3$/Hz$^{1/2}$ and temperatures up to 420$^\circ$C.
554 citations
TL;DR: Simple theoretical approaches used for understanding typical SCM behavior and some rational synthetic strategies to obtain SCM materials are summarized together with representative examples of SCMs previously reported.
Abstract: Slow relaxation of the magnetization (i.e., "magnet-like" behavior) in materials composed of magnetically isolated chains was observed for the first time in 2001. This type of behavior was predicted in the 1960s by Glauber in a chain of ferromagnetically coupled Ising spins (the so-called Glauber dynamics). In 2002, this new class of nanomagnets was named single-chain magnets (SCMs) by analogy to single-molecule magnets that are isolated molecules displaying related superparamagnetic properties. A long-range order occurs only at T = 0 K in any pure one-dimensional (1D) system, and thus such systems remain in their paramagnetic state at any finite temperature. Nevertheless, the combined action of large uniaxial anisotropy and intrachain magnetic interactions between high-spin magnetic units of the 1D arrangement promotes long relaxation times for the magnetization reversal with decreasing temperature, and finally at significantly low temperatures, the material can behave as a magnet. In this Forum Article, we summarize simple theoretical approaches used for understanding typical SCM behavior and some rational synthetic strategies to obtain SCM materials together with representative examples of SCMs previously reported.
353 citations
TL;DR: In this article, the authors demonstrate that 1,1-disubstituted 2,3,4,5-tetraphenyl siloles and their polymers exhibit the opposite behaviors.
Abstract: Aggregation generally quenches the light emissions of chromophoric molecules. In this review, we demonstrate that 1,1-disubstituted 2,3,4,5-tetraphenyl siloles and 2,5-difunctionalized siloles as well as their polymers exhibit the opposite behaviors. Instead of quenching, aggregation has greatly boosted their photoluminescence quantum yields by up to two orders of magnitude, turning them from faint fluorophores into strong emitters. Such “abnormal” phenomenon of “aggregation-induced emission (AIE)” is attributed to restricted intramolecular rotations of the peripheral phenyl rings against the central silole core, which block the nonradiative channel via the rotational energy relaxation processes and effectively populate the radiative decay of the excitons. Utilizing such a novel effect, siloles and their polymers find an array of applications as: sensors for chemicals, explosives, pH, and biomacromolecules (proteins, DNAs and RNAs), indicators for determining CMC and monitoring layer-by-layer self-assembling, biocompatible fluorogens for cell imaging, visualizing agent for DNA gel electrophoresis, biolabels for immunoassay, stimuli-responsive organic nanomaterials, magnetic fluorescent nanoparticles for potential bio-imaging and -separation, and outstanding materials for efficient OLEDs and PV cells.
302 citations
TL;DR: There are different types of maghemite particles whose relaxation characteristics are suited to a specific MRI application, and the relaxation induced by ferritin in aqueous solutions has been demonstrated to be caused by the exchange of protons between bulk water protons and the surface of the ferrihydrite crystal.
Abstract: Nanometric crystals of maghemite are known to exhibit superparamagnetism. Because of the significance of their magnetic moment, maghemite nanoparticles are exceptional contrast agents and are used for magnetic resonance imaging (of the liver, spleen, lymph nodes), for magnetic resonance angiography and for molecular and cellular imaging. The relaxivity of these agents depends on their size, saturation magnetization and magnetic field and also on their degree of clustering. There are different types of maghemite particles whose relaxation characteristics are suited to a specific MRI application. The relaxation induced by maghemite particles is caused by the diffusion of water protons in the inhomogeneous field surrounding the particles. This is well described by a theoretical model that takes magnetite crystal anisotropy and Neel relaxation into account. Another type of superparamagnetic compound is ferritin, the iron-storing protein: it contains a superparamagnetic ferrihydrite core. Even if the resulting magnetic moment of ferritin is far smaller than for magnetite nanoparticles, its massive presence in different organs darkens T2-weighted MR images, allowing the noninvasive estimation of iron content, thanks to MRI. The relaxation induced by ferritin in aqueous solutions has been demonstrated to be caused by the exchange of protons between bulk water protons and the surface of the ferrihydrite crystal. However, in vivo, the relaxation properties of ferritin are still unexplained, probably because of protein clustering. Copyright © 2009 John Wiley & Sons, Inc.
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284 citations
TL;DR: Three types of biosensors that employ different biosensing principles, magnetic materials, and instrumentation are covered, which consist of magnetic relaxation switch assay-sensors, which are based on the effects magnetic particles exert on water proton relaxation rates.
Abstract: Many types of biosensors employ magnetic nanoparticles (diameter = 5–300 nm) or magnetic particles (diameter = 300–5,000 nm) which have been surface functionalized to recognize specific molecular targets. Here we cover three types of biosensors that employ different biosensing principles, magnetic materials, and instrumentation. The first type consists of magnetic relaxation switch assay-sensors, which are based on the effects magnetic particles exert on water proton relaxation rates. The second type consists of magnetic particle relaxation sensors, which determine the relaxation of the magnetic moment within the magnetic particle. The third type is magnetoresistive sensors, which detect the presence of magnetic particles on the surface of electronic devices that are sensitive to changes in magnetic fields on their surface. Recent improvements in the design of magnetic nanoparticles (and magnetic particles), together with improvements in instrumentation, suggest that magnetic material-based biosensors may become widely used in the future.
282 citations
TL;DR: Of greatest interest though, is an intense low frequency (sub-alpha) relaxation that is in accordance with recent simulations that have reported mesoscopic structure arising from aggregates or clusters--structure that explains the anomalous and inconveniently high viscosities of these liquids.
Abstract: We have measured the intermolecular dynamics of the 1,3-dialkylimidazolium-based room-temperature ionic liquids (RTILs) [emim][BF4], [emim][DCA], and [bmim][DCA] at 25 °C from below 1 GHz to 10 THz by ultrafast optical Kerr effect (OKE) spectroscopy and dielectric relaxation spectroscopy (DRS) augmented by time-domain terahertz and far-infrared FTIR spectroscopy. This concerted approach allows a more detailed analysis to be made of the relatively featureless terahertz region, where the higher frequency diffusional modes are strongly overlapped with librations and intermolecular vibrations. Of greatest interest though, is an intense low frequency (sub-α) relaxation that we show is in accordance with recent simulations that have reported mesoscopic structure arising from aggregates or clusters—structure that explains the anomalous and inconveniently high viscosities of these liquids.
255 citations
TL;DR: Results of ac magnetic susceptibility measurements performed on the trigonal prismatic complex U(Ph(2)BPz(2))(3) demonstrate the presence of slow magnetic relaxation under zero applied dc field and suggest a general strategy for identifying further uranium(III)-based single-molecule magnets by concentrating the ligand-field contributions above and below the equatorial plane of an axially symmetric coordination complex.
Abstract: Results of ac magnetic susceptibility measurements performed on the trigonal prismatic complex U(Ph2BPz2)3 demonstrate the presence of slow magnetic relaxation under zero applied dc field. Analysis of both the temperature and frequency dependence of the ac susceptibility indicate a temperature regime (T > ∼3 K) where Arrhenius behavior dominates the relaxation processes, leading to a spin relaxation barrier of Ueff = 20 cm−1. The dc-field dependence of the relaxation time was studied to reveal evidence of quantum tunneling processes occurring at lower temperatures. The results suggest a general strategy for identifying further uranium(III)-based single-molecule magnets by concentrating the ligand-field contributions above and below the equatorial plane of an axially symmetric coordination complex.
252 citations
TL;DR: A simple approach to including KBr powder in hydrated samples, such as biological membrane samples, hydrated amyloid fibrils, and hydrated microcrystalline proteins, that allows direct assessment of the effects of frictional and radio-frequency heating under experimentally relevant conditions.
239 citations
TL;DR: The exchange of water hydroxyl hydrogen bonds between anions and water oxygens is observed directly with ultrafast 2D IR vibrational echo chemical exchange spectroscopy (CES) and the growth of the chemical exchange peaks yields the time dependence of anion–water hydroxym hydrogen bond switching under thermal equilibrium conditions.
Abstract: The exchange of water hydroxyl hydrogen bonds between anions and water oxygens is observed directly with ultrafast 2D IR vibrational echo chemical exchange spectroscopy (CES). The OD hydroxyl stretch of dilute HOD in H2O in concentrated (5.5 M) aqueous solutions of sodium tetrafluoroborate (NaBF4) displays a spectrum with a broad water-like band (hydroxyl bound to water oxygen) and a resolved, blue shifted band (hydroxyl bound to BF4−). At short time (200 fs), the 2D IR vibrational echo spectrum has 4 peaks, 2 on the diagonal and 2 off-diagonal. The 2 diagonal peaks are the 0–1 transitions of the water-like band and the hydroxyl-anion band. Vibrational echo emissions at the 1–2 transition frequencies give rise to 2 off-diagonal peaks. On a picosecond time scale, additional off-diagonal peaks grow in. These new peaks arise from chemical exchange between water hydroxyls bound to anions and hydroxyls bound to water oxygens. The growth of the chemical exchange peaks yields the time dependence of anion–water hydroxyl hydrogen bond switching under thermal equilibrium conditions as Taw = 7 ± 1 ps. Pump-probe measurements of the orientational relaxation rates and vibrational lifetimes are used in the CES data analysis. The pump-probe measurements are shown to have the correct functional form for a system undergoing exchange.
225 citations
TL;DR: The findings indicate that the procedure provides an accurate estimate of the relative statistical weights of the different conformations populated by alpha-synuclein in its natively unfolded state.
Abstract: Natively unfolded proteins present a challenge for structure determination because they populate highly heterogeneous ensembles of conformations. A useful source of structural information about these states is provided by paramagnetic relaxation enhancement measurements by nuclear magnetic resonance spectroscopy, from which long-range interatomic distances can be estimated. Here we describe a method for using such distances as restraints in molecular dynamics simulations to obtain a mapping of the free energy landscapes of natively unfolded proteins. We demonstrate the method in the case of α-synuclein and validate the results by a comparison with electron transfer measurements. Our findings indicate that our procedure provides an accurate estimate of the relative statistical weights of the different conformations populated by α-synuclein in its natively unfolded state.
TL;DR: A systematic global mapping of water motions in the hydration layer around a model protein of apomyoglobin in both native and molten globule states is reported and a solvation speed and an angular speed are defined to quantify the water-network rigidity and local protein flexibility.
Abstract: Protein surface hydration is fundamental to its structural stability and flexibility, and water-protein fluctuations are essential to biological function. Here, we report a systematic global mapping of water motions in the hydration layer around a model protein of apomyoglobin in both native and molten globule states. With site-directed mutagenesis, we use intrinsic tryptophan as a local optical probe to scan the protein surface one at a time with single-site specificity. With femtosecond resolution, we examined 16 mutants in two states and observed two types of water-network relaxation with distinct energy and time distributions. The first water motion results from the local collective hydrogen-bond network relaxation and occurs in a few picoseconds. The initial hindered motions, observed in bulk water in femtoseconds, are highly suppressed and drastically slow down due to structured water-network collectivity in the layer. The second water-network relaxation unambiguously results from the lateral cooperative rearrangements in the inner hydration shell and occurs in tens to hundreds of picoseconds. Significantly, this longtime dynamics is the coupled interfacial water-protein motions and is the direct measurement of such cooperative fluctuations. These local protein motions, although highly constrained, are necessary to assist the longtime water-network relaxation. A series of correlations of hydrating water dynamics and coupled fluctuations with local protein's chemical and structural properties were observed. These results are significant and reveal various water behaviors in the hydration layer with wide heterogeneity. We defined a solvation speed and an angular speed to quantify the water-network rigidity and local protein flexibility, respectively. We also observed that the dynamic hydration layer extends to more than 10 A. Finally, from native to molten globule states, the hydration water networks loosen up, and the protein locally becomes more flexible with larger global plasticity and partial unfolding.
TL;DR: Evaluation of the temperature dependence of the static permittivities, effective dipole moments, volumes of rotation, activation energies, and relaxation times derived from the dielectric data indicates that the low frequency process cannot be solely due to rotational diffusion of the dipolar imidazolium cations, but must also include other contributions, probably from cooperative motions.
Abstract: Dielectric spectra were measured for eight, mostly imidazolium-based, room temperature ionic liquids (RTILs) over a wide range of frequencies (0.2≤ν/GHz≤89) and temperatures (5≤θ/°C≤65). Detailed analysis of the spectra shows that the dominant low frequency process centred at ca. 0.06 to 10 GHz (depending on the salt and the temperature) is better described using a symmetrically broadened Cole-Cole model rather than the asymmetric Cole-Davidson models used previously. Evaluation of the temperature dependence of the static permittivities, effective dipole moments, volumes of rotation, activation energies, and relaxation times derived from the dielectric data indicates that the low frequency process cannot be solely due to rotational diffusion of the dipolar imidazolium cations, as has been thought, but must also include other contributions, probably from cooperative motions. Analysis of the Debye process observed at higher frequencies for these RTILs is not undertaken because it overlaps with even faster processes that lie outside the range of the present instrumentation.
TL;DR: This analysis indicates that the antiferromagnetic order imprinted in the initial state vanishes exponentially and the corresponding relaxation time exhibits a minimum at the critical point, in contrast to the usual notion of critical slowing down, from which a maximum is expected.
Abstract: We study the unitary time evolution of antiferromagnetic order in anisotropic Heisenberg chains that are initially prepared in a pure quantum state far from equilibrium. Our analysis indicates that the antiferromagnetic order imprinted in the initial state vanishes exponentially. Depending on the anisotropy parameter, oscillatory or nonoscillatory relaxation dynamics is observed. Furthermore, the corresponding relaxation time exhibits a minimum at the critical point, in contrast to the usual notion of critical slowing down, from which a maximum is expected.
TL;DR: In this article, the effect of annealing on the bending ductility, the isoconfigurational elastic constants, the structure and the thermal stability of bulk metallic glasses was examined.
TL;DR: In this article, the effect of plasticizer on dielectric and electrical properties of an ionically conducting polymer nanocomposites PEO 25 -NaI+5.5 ¼wt.% PEG 200 with different value of x has been investigated.
TL;DR: In this paper, the effect of the size of the lens-shaped quantum dot and optical intensity on the absorption coefficient and the refractive indices was investigated, and it was found that AC and RI are strongly affected by the optical intensity and the size.
TL;DR: This work studies hydrated model membranes, consisting of stacked bilayers of 1,2-dioleoyl-sn-glycero-3-phosphocholine lipids, using terahertz time-domain and infrared spectroscopy to elucidate how the interplay between the model membrane and water molecules results in different water dynamics.
TL;DR: A discrete dysprosium cubane has been prepared and structurally characterized and slow relaxation of magnetization in this complex is observed, which may stimulate further investigations into the dynamics of magnetized lanthanide clusters with different topologies.
Abstract: A discrete dysprosium cubane has been prepared and structurally characterized. Slow relaxation of magnetization in this complex is observed, which may stimulate further investigations into the dynamics of magnetization in lanthanide clusters with different topologies.
TL;DR: ExPRESS is used to investigate fundamental differences in the mechanism of echo formation in deuteron MAS and quadrupole CPMG experiments, and to illustrate significant differences between these techniques in the context of high spin quadrupolar nuclei.
TL;DR: In this article, a study on the magnetic properties of naked and silica-coated Fe₃O₄nanoparticles with sizes between 5 and 110 nm was presented, and their efficiency as heating agents was assessed through specific power absorption measurements as a function of particle size and shape.
TL;DR: In this article, the authors describe the generalization of the Bloch equation in terms of Caputo fractional derivatives of order α (0 < α < 1) for a single spin system in a static magnetic field at resonance.
Abstract: Nuclear magnetic resonance (NMR) is a physical phenomenon widely used in chemistry, medicine, and engineering to study complex materials. NMR is governed by the Bloch equation, which relates a macroscopic model of magnetization to applied radiofrequency, gradient and static magnetic fields. Simple models of materials are well described by the classical first order dynamics of precession and relaxation inherent in the vector form of the Bloch equation. Fractional order generalization of the Bloch equation presents an opportunity to extend its use to describe a wider range of experimental situations involving heterogeneous, porous, or composite materials. Here we describe the generalization of the Bloch equation in terms of Caputo fractional derivatives of order α (0 < α < 1) for a single spin system in a static magnetic field at resonance. The results are expressed in terms of the Mittag–Leffler function—a generalized exponential function that converges to the classical case when α = 1. © 2009 Wiley Periodicals, Inc. Concepts Magn Reson Part A 34A: 16–23, 2009.
TL;DR: A nonaromatic core-shell structural model of nanodiamond particles has been proposed and it is proposed that the unpaired electrons are mostly located in the disordered shell, at distances between 0.4 and 1 nm from the surface.
Abstract: The structure of synthetic nanodiamond has been characterized by 13C nuclear magnetic resonance (NMR) spectral editing combined with measurements of long-range 1H−13C dipolar couplings and 13C relaxation times. The surface layer of these ∼4.8-nm diameter carbon particles consists mostly of sp3-hybridized C that is protonated or bonded to OH groups, while sp2-hybridized carbon makes up less than 1% of the material. The surface protons surprisingly resonate at 3.8 ppm, but their direct bonding to carbon is proved by fast dipolar dephasing under homonuclear decoupling. Long-range 1H−13C distance measurements, based on 13C{1H} dipolar dephasing by surface protons, show that seven carbon layers, in a shell of 0.63 nm thickness that contains ∼60% of all carbons, predominantly resonate more than +8 ppm from the 37-ppm peak of bulk diamond (i.e., within the 45−80 ppm range). Nitrogen detected in 15N NMR spectra is mostly not protonated and can account for some of the high-frequency shift of carbon. The location o...
TL;DR: The steady-state and ultrafast vibrational spectroscopy of dilute HOD in aqueous solutions of sodium bromide is studied, finding that spectral diffusion is very complicated, with processes occurring on multiple time scales.
Abstract: We study theoretically the steady-state and ultrafast vibrational spectroscopy, in the OD-stretch region, of dilute HOD in aqueous solutions of sodium bromide. Based on electronic-structure calculations on clusters containing salt ions and water, we develop new spectroscopic maps that enable us to undertake this study. We calculate OD-stretch absorption line shapes as a function of salt concentration, finding good agreement with experiment. We provide molecular-level understandings of the monotonic (as a function of concentration) blueshift, and nonmonotonic line width. We also calculate the frequency time-correlation function, as measured by spectral diffusion experiments. Here again we obtain good agreement with experiment, finding that at the highest salt concentration spectral diffusion slows down by a factor of 3 or 4 (compared to pure water). For longer times than can be accessed experimentally, we find that spectral diffusion is very complicated, with processes occurring on multiple time scales. We argue that from 6 to 40 ps, relaxation involves anionic solvation shell rearrangements. Finally, we consider our findings within the general context of the Hofmeister series, concluding that this series must reflect only local ordering of water molecules.
TL;DR: It is shown experimentally and theoretically that slow relaxation of the magnetization can still be observed in the AF phase, with a maximum of the relaxation time close to the AF-paramagnetic phase transition.
Abstract: A system presenting an antiferromagnetic (AF) phase of Single-Chain Magnets (SCMs) is studied at low temperatures. The phase diagram is first discussed to specify the magnitude of the interchain couplings. Then, we show experimentally and theoretically that slow relaxation of the magnetization can still be observed in the AF phase, with a maximum of the relaxation time close to the AF-paramagnetic phase transition. This counter intuitive result shows that materials presenting such an ordered state can be used to design high temperature magnets using SCM-based materials.
TL;DR: In this article, the authors studied the longest relaxation times of polymer solutions of semi-flexible T4 DNA and flexible 18 M molar mass polyacrylamide (PAAm) in dilute and semi-dilute concentration range.
Abstract: The longest relaxation times of polymer solutions of semi-flexible T4 DNA and flexible 18 M molar mass polyacrylamide (PAAm) in dilute and semi-dilute concentration range are studied by the polymer extension relaxation of stretched single DNA molecules and by the stress relaxation of PAAm solutions measurements. For both polymer solutions, the longest relaxation time normalized by the value at infinite dilution with the same solvent viscosity τ/τ0 increases with increasing concentration. In the dilute regime, the longest relaxation time increases just slightly with increasing concentration as τ/τ0=[1+cA−2(cA)1.5+2(cA)2] as well as the empirical relation of τ/τ0=exp(cA) up to c∼3c∗ with A≈0.5[η], where c∗ is the overlap concentration, in accord with the theory and previous experiments. For the semi-dilute solutions, the scaling of τ/τ0 with concentration shows two different exponents in two concentration regions, corresponding to the unentangled and entangled regimes. The exponents are consistent with thos...
TL;DR: The steady state of a finite XX chain coupled at its boundaries to quantum reservoirs made of free spins that interact one after the other with the chain is studied and it is shown that the steady state can be described by a generalized Gibbs state.
Abstract: We study the steady state of a finite XX chain coupled at its boundaries to quantum reservoirs made of free spins that interact one after the other with the chain. The two-point correlations are calculated exactly, and it is shown that the steady state is completely characterized by the magnetization profile and the associated current. Except at the boundary sites, the magnetization is given by the average of the reservoirs' magnetizations. The steady-state current, proportional to the difference in the reservoirs' magnetizations, shows a nonmonotonic behavior with respect to the system-reservoir coupling strength, with an optimal current state for a finite value of the coupling. Moreover, we show that the steady state can be described by a generalized Gibbs state.
TL;DR: Two types of 2D IR vibrational echo experiments are used to examine the influence that charged species have on water hydrogen-bond dynamics, and both types of measurements show the slowing of hydrogen- bond network structural evolution with an increasing salt concentration.
Abstract: Water is ubiquitous in nature, but it exists as pure water infrequently. From the ocean to biology, water molecules interact with a wide variety of dissolved species. Many of these species are charged. In the ocean, water interacts with dissolved salts. In biological systems, water interacts with dissolved salts as well as charged amino acids, the zwitterionic head groups of membranes, and other biological groups that carry charges. Water plays a central role in a vast number of chemical processes because of its dynamic hydrogen-bond network. A water molecule can form up to four hydrogen bonds in an approximately tetrahedral arrangement. These hydrogen bonds are continually being broken, and new bonds are being formed on a picosecond time scale. The ability of the hydrogen-bond network of water to rapidly reconfigure enables water to accommodate and facilitate chemical processes. Therefore, the influence of charged species on water hydrogen-bond dynamics is important. Recent advances in ultrafast coherent infrared spectroscopy have greatly expanded our understanding of water dynamics. Two-dimensional infrared (2D IR) vibrational echo spectroscopy is providing new observables that yield direct information on the fast dynamics of molecules in their ground electronic state under thermal equilibrium conditions. The 2D IR vibrational echoes are akin to 2D nuclear magnetic resonance (NMR) but operate on time scales that are many orders of magnitude shorter. In a 2D IR vibrational echo experiment (see the Conspectus figure), three IR pulses are tuned to the vibrational frequency of interest, which in this case is the frequency of the hydroxyl stretching mode of water. The first two pulses "label" the initial molecular structures by their vibrational frequencies. The system evolves between pulses two and three, and the third pulse stimulates the emission of the vibrational echo pulse, which is the signal. The vibrational echo pulse is heterodyne, detected by combining it with another pulse, the local oscillator. Heterodyne detection provides phase and amplitude information, which are both necessary to perform the two Fourier transforms that take the data from the time domain to a two-dimensional frequency domain spectrum. The time dependence of a series of 2D IR vibrational echo spectra provides direct information on system dynamics. Here, we use two types of 2D IR vibrational echo experiments to examine the influence that charged species have on water hydrogen-bond dynamics. Solutions of NaBr and NaBF(4) are studied. The NaBr solutions are studied as a function of the concentration using vibrational echo measurements of spectral diffusion and polarization-selective IR pump-probe measurements of orientational relaxation. Both types of measurements show the slowing of hydrogen-bond network structural evolution with an increasing salt concentration. NaBF(4) is studied using vibrational echo chemical-exchange spectroscopy. In these experiments, it is possible to directly observe the chemical exchange of water molecules switching their hydrogen-bond partners between BF(4)(-) and other water molecules. The results demonstrate that water interacting with ions has slower hydrogen-bond dynamics than pure water, but the slowing is a factor of 3 or 4 rather than orders of magnitude.
TL;DR: A noncollinear frustrated spin structure of the bitriangular cluster is revealed and a stretched-exponential temperature dependence typical of a Berezinskii-Kosterlitz-Thouless model points to a quasi-2D XY behavior.
Abstract: Octahedral Co2+ centers have been connected by mu(3)-OH and mu(2)-OH2 units forming [Co-4] clusters which are linked by pyrazine forming a two-dimensional network The two-dimensional layers are bridged by oxybisbenzoate (OBA) ligands giving rise to a three-dimensional structure The [Co-4] clusters bond with the pyrazine and the OBA results in a body-centered arrangement of the clusters, which has been observed for the first time Magnetic studies reveal a noncollinear frustrated spin structure of the bitriangular cluster, resulting in a net magnetic moment of 14 mu B per cluster For T > 32 K, the correlation length of the cluster moments shows a stretched-exponential temperature dependence typical of a Berezinskii-Kosterlitz-Thouless model, which points to a quasi-2D XY behavior At lower temperature and down to 14 K, the compound behaves as a soft ferromagnet and a slow relaxation is observed, with an energy barrier of ca 500 K Then, on further cooling, a hysteretic behavior takes place with a coercive field that reaches 5 Tat 4 K The slow relaxation is assigned to the creation/annihilation of vortex-antivortex pairs, which are the elementary excitations of a 2D XY spin system
TL;DR: Formation of the poor ionic-conducting LiTiO2 at the surface of the particles explains why micro-anatase Li(x) TiO2 is not able to reach the theoretical maximum capacity at room temperature, and why this theoretical maximumcapacity reached in nano-sized materials cannot be (dis)charged at high rates.
Abstract: Upon lithium insertion in the pristine TiO2 anatase phase the theoretical maximum of LiTiO2 can be reached in crystallite sizes less than ∼10 nm, whereas bulk compositions appear limited to Lix≈0.6TiO2 at room temperature. Both X-ray absorption spectroscopy (XAS) and ab initio calculations have been applied to probe the electronic structure of the newly formed LiTiO2 phase. These results indicate that a large majority of the Li-2s electrons reside at the Ti-3d(t2g)/4s hybridized site. About 10% of these electrons are transferred to non-localized states which makes this compound a good electronic conductor. Ionic conductivity is probed by nuclear magnetic resonance (NMR) relaxation experiments indicating relatively small hopping rates between the Li-ion sites in LiTiO2. Formation of the poor ionic-conducting LiTiO2 at the surface of the particles explains why micro-anatase LixTiO2 is not able to reach the theoretical maximum capacity at room temperature, and why this theoretical maximum capacity reached in nano-sized materials cannot be (dis)charged at high rates.