Showing papers on "Relaxation (NMR) published in 2001"
TL;DR: Determinations of the relaxation parameters of blood are reported, employing in vitro samples that are well representative of human blood in situ, and the Gaussian component, AR*, should be recognized in accurate modeling of MRI phenomena that depend upon the magnetic state of blood.
Abstract: Accurate knowledge of the magnetic properties of human blood is required for the precise modeling of functional and vascular flow-related MRI. Herein are reported determinations of the relaxation parameters of blood, employing in vitro samples that are well representative of human blood in situ. The envelope of the blood (1)H(2)O free-induction decay signal magnitude during the first 100 msec following a spin echo at time TE is well- described empirically by an expression of the form, S(t) = S(o). exp[-R(*)(2). (t - TE) - AR*. (t - TE)(2)]. The relaxation parameters AR* and R(*)(2) increase as a function of the square of the susceptibility difference between red blood cell and plasma and depend on the spin-echo time. The Gaussian component, AR*, should be recognized in accurate modeling of MRI phenomena that depend upon the magnetic state of blood. The magnetic susceptibility difference between fully deoxygenated and fully oxygenated red blood cells at 37 degrees C is 0.27 ppm, as determined independently by MR and superconducting quantum interference device (SQUID) measurements. This value agrees well with the 1936 report of Pauling and Coryell (Proc Natl Acad Sci USA 1936;22:210-216), but is substantially larger than that frequently used in MRI literature. Magn Reson Med 45:533-542, 2001.
454 citations
TL;DR: A novel imaging technique is described that yields all of the observable properties of the binary spin‐bath model for magnetization transfer and is demonstrated for in vivo studies of the human head.
Abstract: We describe a novel imaging technique that yields all of the observable properties of the binary spin-bath model for magnetization transfer (MT) and demonstrate this method for in vivo studies of the human head. Based on a new model of the steady-state behavior of the magnetization during a pulsed MT-weighted imaging sequence, this approach yields parametric images of the fractional size of the restricted pool, the magnetization exchange rate, the T(2) of the restricted pool, as well as the relaxation times in the free pool. Validated experimentally on agar gels and samples of uncooked beef, we demonstrate the method's application on two normal subjects and a patient with multiple sclerosis.
380 citations
TL;DR: These results demonstrate that a transfer of electron angular momentum between the magnetic layers leads to additional relaxation torques.
Abstract: Unique features of the Gilbert damping in magnetic multilayers were investigated by ferromagnetic resonance (FMR) using magnetic single and double layer structures prepared by molecular beam epitaxy. The FMR linewidth for the Fe films in the double layer structures was larger than the FMR linewidth in the single Fe films having the same thickness. The additional FMR linewidth scaled inversely with the film thickness, and increased linearly with increasing microwave frequency. These results demonstrate that a transfer of electron angular momentum between the magnetic layers leads to additional relaxation torques.
327 citations
TL;DR: In this paper, a series of molecular dynamics simulations of aqueous NaCl and KCl solutions at different concentrations, ranging from 0 M to 4.5 M, were performed to investigate the effects of ion concentration on the single-particle, pair, and collective dynamical properties of an electrolyte solution.
Abstract: We have performed a series of molecular dynamics simulations of aqueous NaCl and KCl solutions at different concentrations, ranging from 0 M to 4.5 M, to investigate the effects of ion concentration on the single-particle, pair, and collective dynamical properties of aqueous electrolyte solutions. The SPC/E model is used for water and the ions are modeled as charged Lennard-Jones particles. The single-particle dynamics is investigated by calculating the self-diffusion coefficients of ions and water molecules and also the orientational relaxation times. The pair dynamics is studied by evaluating the ion–water residence and water–water hydrogen bond time correlation functions. The relaxation of relative velocity autocorrelation function and the cross velocity correlation function of two hydrogen bonded water molecules are also investigated at varying ion concentration. Finally, we explore the collective dynamical properties by calculating the frequency dependent dielectric function and conductivity. It is found that the self and relative diffusion coeffcients decrease and the orientational relaxation times increase with ion concentration. The residence times of water molecules near ions and also the structural relaxation time of water–water hydrogen bonds show an increasing trend as the ion concentration is increased. The dielectric relaxation time is found to decrease with ion concentration for the solutions investigated here. The static conductivity of concentrated solutions shows significant departure from the Nernst–Einstein behavior due to formation of ion pairs. With an increase of frequency, the conductivity first increases substantially and then decreases at very high frequency. The initial increase of conductivity is attributed to the disruption of ion pairs on application of high frequency electric fields.
301 citations
TL;DR: The methods presented here give a powerful analysis of the surface microdynamics of confined liquids, which can be applied to the study of oil-bearing rocks.
Abstract: Proton nuclear spin-relaxation studies on water- or oil-saturated granular packings and limestone rocks allow estimating surface molecular dynamical parameters. Measurements were performed at various conditions of temperature, magnetic field strengths, and pore size. We show by low field NMR relaxation that changing the amount of surface paramagnetic impurities leads to striking different pore-size dependences of the relaxation times T1 and T2 of liquids in pores. These dependences are well supported by surface-limited or diffusion-limited relaxation models. Surface relaxivity parameters rho(1) and rho(2) are deduced from the pore-size dependence in the surface-limited regime. We evidence the frequency and temperature dependence of the surface relaxivity rho(1) by field cycling NMR relaxation and relevant theoretical models. The typical frequency dependence found allows an experimental separation of the surface and bulk microdynamics in porous media. Several surface dynamical parameters, such as diffusion coefficients, activation energies, time of residence, and coefficient of surface affinity, were therefore determined. The methods presented here give a powerful analysis of the surface microdynamics of confined liquids, which can be applied to the study of oil-bearing rocks.
240 citations
TL;DR: The CE equation is shown to be identical to these theories in the respective limits and appropriate parameter relationships are derived for spherical particles, and the model reproduces the essential relaxation characteristics in the intermediate range.
Abstract: Chemical exchange (CE) theory is compared with two theories of T2-shortening caused by microscopic magnetic centers: inner- and outer-sphere relaxation theory (long-echo limit) and mean gradient diffusion theory (short-echo limit). The CE equation is shown to be identical to these theories in the respective limits and appropriate parameter relationships are derived for spherical particles. The theories are then compared with computer simulations of spherical particles and with a recent general theory, with good agreement in the asymptotic regions. The CE model also reproduces the essential relaxation characteristics in the intermediate range. Finally, good agreement of a CE model with simulations for magnetized cylinders is also demonstrated. The discussion is limited to weakly magnetized particles such that the maximum phase shift during an echo interval is less than one radian, permitting the use of the Luz-Meiboom CE equation. Published 2001 Wiley-Liss, Inc.
220 citations
TL;DR: The van der Waals liquid ortho-terphenyl has been used as a model system in the study of the glass transition and extensive experiments have been undertaken to monitor the onset of structural relaxation on microscopic time and length scales.
Abstract: The van der Waals liquid ortho-terphenyl has long been used as a model system in the study of the glass transition. Motivated by mode-coupling theory, extensive experiments have been undertaken to monitor the onset of structural relaxation on microscopic time- and lengthscales. Using in particular quasielastic neutron scattering, the decay of density and tagged-particle correlations has been measured as a function of temperature, pressure and wavenumber. A consistent picture is developed, in which the mode-coupling singularity appears as a change of transport mechanism in the moderately viscous liquid, at temperatures far above the conventional (caloric) glass transition.
216 citations
TL;DR: In this article, the van der Waals liquid orthoterphenyl has been used as a model system in the study of the glass transition and the mode-coupling singularity appears as a change of transport mechanism in the moderately viscous liquid, at temperatures far above the conventional (caloric) glass transition.
Abstract: The van der Waals liquid orthoterphenyl has long been used as a model system in the study of the glass transition. Motivated by mode-coupling theory, extensive experiments have been undertaken to monitor the onset of structural relaxation on microscopic time and length scales. Using in particular quasielastic neutron scattering, the decay of density and tagged-particle correlations has been measured as a function of temperature, pressure and wave number. A consistent picture is developped in which the mode-coupling singularity appears as a change of transport mechanism in the moderately viscous liquid, at temperatures far above the conventional (caloric) glass transition.
190 citations
TL;DR: Cardiovascular magnetic resonance creates images from atomic nuclei with uneven spin using radio waves in the presence of a magnetic field to create tissue characterization, thus creating a powerful clinical tool.
Abstract: Cardiovascular magnetic resonance (CMR) creates images from atomic nuclei with uneven spin using radio waves in the presence of a magnetic field. Full details of the physical principles can be found elsewhere [1]. For clinical purposes, MR is performed using hydrogen-1, which is abundant in water and fat. Radiofrequency waves excite the area of interest to create tissue magnetization, which decays (relaxation) and after a short period is induced to release energy as a radio signal. These echoes are converted using Fourier transformation into images of spatially resolved radio signals. Relaxation is quantified in spatially orthogonal directions as T1 and T2, which allows tissue characterization, thus creating a powerful clinical tool. A CMR scanner consists of a superconducting magnet, a radiofrequency transmitter and receiver connected to radio aerials, and gradient coils driven by powerful pulses of electricity to create transient magnetic fields. The imaging computer triggers to the electrocardiogram and runs scanning sequences that coordinate the complex processes.
188 citations
TL;DR: The interfacial Ni spins are discussed in terms of the "uncompensated spins" at AFM/FM interfaces long held responsible for coercivity increases and exchange bias and a direct correlation between their number and the size of the coercivity is found.
Abstract: Using x-ray absorption spectromicroscopy we have imaged the uncompensated spins induced at the surface of antiferromagnetic (AFM) NiO(100) by deposition of ferromagnetic (FM) Co. These spins align parallel to the AFM spins in NiO(100) and align the FM spins in Co. The uncompensated interfacial spins arise from an ultrathin CoNiOx layer that is formed upon Co deposition through reduction of the NiO surface. The interfacial Ni spins are discussed in terms of the "uncompensated spins" at AFM/FM interfaces long held responsible for coercivity increases and exchange bias. We find a direct correlation between their number and the size of the coercivity.
188 citations
TL;DR: The magnetic properties of the perovskite system have been investigated by ac susceptibility and dc magnetization measurements, including relaxation and aging studies as discussed by the authors, and the overall results show evidence for the gradual appearance of a cluster glass behavior.
Abstract: The dynamic magnetic properties of the distorted perovskite system ${\mathrm{La}}_{0.7\ensuremath{-}x}{\mathrm{Y}}_{x}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}$ $(0l~xl~0.15)$ have been investigated by ac susceptibility and dc magnetization measurements, including relaxation and aging studies. All investigated samples display a metal-insulator transition. As yttrium is added in the compounds the overall results show evidence for the gradual appearance of a cluster glass behavior. For the $x=0.15$ sample, magnetization measurements as a function of time at various temperatures show that the magnetic relaxation is maximum at a given temperature, well below the ferromagnetic transition. This maximum coincides in temperature with a frequency-dependent feature in the imaginary part of the ac susceptibility, associated with a freezing process. This is interpreted as due to ferromagnetic clusters, which grow with decreasing temperature down to a temperature at which they freeze due to severe intercluster frustration.
TL;DR: In a stray field of a constant gradient g, it is found that unrestricted diffusion leads to nonexponential signal decay versus echo number N, but within experimental error the diffusion attenuation is still only a function of g(2)Dt(3)(E)N, where t(E) is the echo spacing.
TL;DR: Fast-field-cycling (FFC) as discussed by the authors has been used in nuclear magnetic resonance (NMR) experiments since the early days of NMR and has been successfully used during the last years to obtain information on the molecular dynamics, structure and molecular order.
Abstract: Magnetic field cycling in nuclear magnetic resonance (NMR) experiments has been used since the early days of NMR. Originally such time-dependent magnetic field experiments were motivated to study cross relaxation, spin system thermodynamics and indirect detection of quadrupolar resonance. The first apparatus used mechanical or pneumatic systems to shoot the sample between two magnets, the typical “flying time” being a few hundreds of milliseconds. As a natural evolution of the experimental technique and the need to extend its application to samples with higher relaxation rates, faster magnetic field switching devices were developed during the last years. Special electric networks combined with sophisticated air core magnets allowed one to switch magnetic fields between zero and fields of the order of 0.5 T in a few milliseconds. Today we refer to this new generation of instruments as “fast-field-cycling” devices. The technique has been successfully used during the last years to obtain information on the molecular dynamics and order in different materials, ranging from organic solids, metals, polymers, liquid crystals, porous media to biological systems. At present it is also turning to be an important tool for the design of contrast agents for magnetic resonance imaging. Fast field cycling was mainly oriented toT
1 relaxometry as a unique technique offering a dynamic window of several decades, ranging from few kilohertz to several megahertz. However, there exist less conventional applications of the technique that can also provide relevant information concerning molecular dynamics, structure and molecular order. In this article we will briefly deal with basic aspects of the technique, its evolution, present-day relevant applications and the last improvements concerning specialized instrumentation.
TL;DR: The aging dynamics of colloidal suspensions of Laponite, a synthetic clay, is investigated using dynamic light scattering (DLS) and viscometry after a quench into the glassy phase and it is revealed that there are two modes of relaxation.
Abstract: The aging dynamics of colloidal suspensions of Laponite, a synthetic clay, is investigated using dynamic light scattering (DLS) and viscometry after a quench into the glassy phase. DLS allows to follow the diffusion of Laponite particles and reveals that there are two modes of relaxation. The fast mode corresponds to a rapid diffusion of particles within ``cages'' formed by the neighboring particles. The slow mode corresponds to escape from the cages: its average relaxation time increases exponentially fast with the age of the glass. In addition, the slow mode has a broad distribution of relaxation times, its distribution becoming larger as the system ages. Measuring the concomitant increase of viscosity as the system ages, we can relate the slowing down of the particle dynamics to the viscosity.
TL;DR: It is shown that the complex relaxation properties of the methyl spin system do not complicate extraction of accurate exchange parameters as long as care is taken to ensure that appropriate magnetization modes are interchanged in the middle of the constant-time CPMG period.
Abstract: A relaxation dispersion-based NMR experiment is presented for the measurement and quantitation of μs−ms dynamic processes at methyl side-chain positions in proteins The experiment measures the exchange contribution to the 13C line widths of methyl groups using a constant-time CPMG scheme The effects of cross-correlated spin relaxation between dipole−dipole and dipole−CSA interactions as well as the effects of scalar coupling responsible for mixing of magnetization modes during the course of the experiment have been investigated in detail both theoretically and through simulations It is shown that the complex relaxation properties of the methyl spin system do not complicate extraction of accurate exchange parameters as long as care is taken to ensure that appropriate magnetization modes are interchanged in the middle of the constant-time CPMG period An application involving the measurement of relaxation dispersion profiles of methionine residues in a Leu99Ala substitution of T4 lysozyme is presented A
01 Jan 2001
TL;DR: In this article, two homologous organometallic compounds, Pd(2-thpy)2 and Pt(2 -thpy)-pyridinate, were chosen for case studies of photophysical properties of the lowest excited states.
Abstract: Two homologous organometallic compounds, Pd(2-thpy)2 and Pt(2-thpy)2 (with 2-thpy-=2-(2-thienyl)-pyridinate, structure formulae in Fig. 1), are chosen for case studies of photophysical properties of the lowest excited states. The triplets of these two representative compounds are marked by differences of nearly two orders of magnitude in metal/MLCT (metalto-ligand charge transfer) character. Determination of detailed photophysical properties of both compounds is possible, since highly resolved spectra are obtained when the compounds are dissolved in an n-octane matrix (Shpol’skii matrix), when the measurements are carried out at low temperature (typically at T = 1.3 K), and when modern techniques of laser spectroscopy are applied. In addition, methods of time-resolution and of microwave double-resonance, such as optically detected magnetic resonance (ODMR), microwave recovery, and phosphorescence microwave-double resonance (PMDR) are used. In particular, it is shown that with increasing metal character of the triplet, that is when Pd(2-thpy)2 is compared to Pt(2-thpy)2, many properties change characteristically and in part by orders of magnitude. For example, the following properties will be addressed: Transition probabilities, emission decay times, zero-field splittings (zfs), processes of spin-lattice relaxation (slr), intersystem crossing rates, intrastate relaxation rates, excited state binding properties as compared to those of the electronic ground state, anharmonicity effects, metal-mediated ligand-ligand coupling or spatial extensions of the excited state wavefunctions. Moreover, we focus on spin-selectivity in the vibrational satellite structures of the emission spectra as identified by the complementary methods of time-resolved emission and PMDR spectroscopy. We also discuss radiative deactivation processes, such as spin-vibronic Herzberg-Teller and Frank-Condon activities. Further, we specify sub-picosecond relaxation paths on the basis of micro-second time resolution by applying for the first time the method of time-resolved excitation spectroscopy to transition metal complexes. It is further demonstrated that the size of zero-field splitting of the triplet state can be used as an ordering parameter, that reflects the metal participation in the lowest triplet. Thus, one can relate Pd(2-thpy)2 and Pt(2-thpy)2 to a larger number of other compounds, such as [Rh(bpy)3]3+, [Pt(bpy)2]2+, Pt(qol)2, [Pt(mnt)2]2-, [Ru(bpy)3]2+, [Os(phen)3]2+, [Os(bpy)3]2+, etc. (compare Fig. 1 and Table 11) and one obtains a series that demonstrates chemical tunability of photophysical properties. — For several specific subjects, we present the basic background information in order to make the paper more easily readable, also for non-specialists.
TL;DR: The complex permittivity of ethylammonium nitrate has been measured as a function of frequency between 3 MHz and 40 GHz at eight temperatures between 288.15 and 353.15 K as mentioned in this paper.
Abstract: The complex permittivity of ethylammonium nitrate has been measured as a function of frequency between 3 MHz and 40 GHz at eight temperatures between 288.15 and 353.15 K. The spectra are well represented by a sum of a conductivity term and a relaxation spectral function that reflects an unsymmetrical relaxation time distribution. Parameter values are given for the Cole−Davidson term and the Kohlrausch−Williams−Watts model. Molecular mechanisms in conformance with an unsymmetrical relaxation time distribution are discussed. The dominant relaxation process with a relaxation frequency in the accessible range can be explained by the formation of a small amount of dipolar ion complexes. The values for the extrapolated high-frequency permittivity indicate a further relaxation process, well above the frequency range of measurements, which is likely to reflect modes of motions of the cation and anion lattices relative to one another.
TL;DR: In this paper, the authors measured the slow relaxation time of SDS micelles, as measured by pressure-jump and temperature-jump techniques, in the range of 10 4 ‐10 1 s depending on the surfactant concentration.
TL;DR: Time-resolved fluorescence spectroscopy was used to investigate relaxation of electronic excitations in films of π-conjugated polymer 1 in the ps time domain this paper, where the position of the fluorescence band and its width were measured as a function of time and excitation energy.
Abstract: Time-resolved fluorescence spectroscopy is used to investigate relaxation of electronic excitations in films of π-conjugated polymer 1 in the ps time domain. The position of the fluorescence band and its width are measured as a function of time and excitation energy. Both low (15 K) and room-temperature behavior are investigated. For high energy excitation, the fluorescence band shows a continuous red shift with time. The energy associated with the maximum of the fluorescence band E is proportional to log(t), with t being the time after excitation. For excitation in the tail of the lowest absorption band, the fluorescence remains stationary and selective excitation of a subset of chromophoric chain segments is possible. At intermediate excitation energy the time required for the excitations to make their first jump depends on the excitation energy and is longer at lower energy. At low temperature and high energy excitation the fluorescence bands are found to narrow with time, while for low energy excitati...
TL;DR: It is found in model systems and tissue that hydrogen exchange from NH and OH groups to water dominates the low frequency water T1ρ dispersion, in the context of the model used to interpret the relaxation data.
Abstract: Magnetic relaxation has been used extensively to study and characterize biological tissues. In particular, spin-lattice relaxation in the rotating frame (T1ρ) of water in protein solutions has been demonstrated to be sensitive to macromolecular weight and composition. However, the nature of the contribution from low frequency processes to water relaxation remains unclear. We have examined this problem by studying the water T1ρ dispersion in peptide solutions (14N- and 15N-labeled), glycosaminoglycan solutions, and samples of bovine articular cartilage before and after proteoglycan degradation. We find in model systems and tissue that hydrogen exchange from NH and OH groups to water dominates the low frequency water T1ρ dispersion, in the context of the model used to interpret the relaxation data. Further, low frequency dispersion changes are correlated with loss of proteoglycan from the extra-cellular matrix of articular cartilage. This finding has significance for the noninvasive detection of matrix degradation.
TL;DR: In this article, the phase change of the diffusion particle from discrete ion-pair to individual ion was attributed to the decomposition of the EMI-BF4 ion pair at temperatures above 335 K.
TL;DR: In this article, the Johari-Goldstein β process of organic glass formers by one-dimensional and two-dimensional (2D) 2H nuclear magnetic resonance (NMR) was studied.
Abstract: We study the Johari–Goldstein β process of organic glass formers by one- (1D) and two-dimensional (2D) 2H nuclear magnetic resonance (NMR). In particular, we compare systems with pronounced secondary relaxation in dielectric spectroscopy, namely toluene-d5 and polybutadiene-d6 (PB), with compounds which do not exhibit a distinct β peak, i.e., glycerol-d5 and polystyrene-d3 (PS). Choosing large interpulse delays in the applied echo pulse sequences we increase the sensitivity on small angle rotational jumps. This way, we are able to probe clearly the β process of toluene and PB in the line shape of 1D 2H NMR spectra and in the orientational correlation functions of 2D 2H NMR in time domain which is not possible when using the conventional techniques. Below the glass transition temperature Tg, the secondary relaxation of both glass formers is caused by a highly restricted reorientation of essentially all molecules. Comparing our results with simulations we estimate that the reorientation of most toluene mole...
TL;DR: The helix-coil transition of a synthetic alpha-helical peptide (the D-Arg peptide), Ac-YGG(KAAAA)(3)-CO-D-Arg-CONH(2), was studied by static far-UV circular dichroism and time-resolved infrared spectroscopy coupled with the laser-induced temperature-jump technique for rapid relaxation initiation.
Abstract: The helix-coil transition of a synthetic alpha-helical peptide (the D-Arg peptide), Ac-YGG(KAAAA)(3)-CO-D-Arg-CONH(2), was studied by static far-UV circular dichroism (CD) and time-resolved infrared spectroscopy coupled with the laser-induced temperature-jump technique for rapid relaxation initiation. Equilibrium thermal unfolding measurements of the D-Arg peptide monitored by CD spectroscopy reveal an apparent two-state helix-coil transition, with a thermal melting temperature around 10 degrees C. Time-resolved infrared (IR) measurements following a laser-induced temperature jump, however, reveal biphasic (or multiphasic) relaxation kinetics. The fast phase rises within the 20 ns response time of the detection system. The slow phase has a decay lifetime of approximately 140 ns at 300 K and exhibits monotonic temperature dependence with an apparent activation energy around 15.5 kcal/mol.
TL;DR: Values are consistent with independent estimates indicating that this procedure provides a useful first-order description of complex internal motions in macromolecules despite neglecting the coupling of overall and interdomain motions.
Abstract: The interpretation of NMR relaxation data for macromolecules possessing slow interdomain motions is considered. It is shown how the "extended model-free approach" can be used to analyze (15)N backbone relaxation data acquired at three different field strengths for Xenopus Ca(2+)-ligated calmodulin. This protein is comprised of two domains connected by two rigid helices joined by a flexible segment. It is possible to uniquely determine all "extended model-free" parameters without any a priori assumptions regarding their magnitudes by simultaneously least-squares fitting the relaxation data measured at two different magnetic fields. It is found that the two connecting helices (and consequently the domains) undergo slow motions relative to the conformation in which the two helices are parallel. The time scales and amplitudes of these "wobbling" motions are characterized by effective correlation times and squared-order parameters of approximately 3 ns and 0.7, respectively. These values are consistent with independent estimates indicating that this procedure provides a useful first-order description of complex internal motions in macromolecules despite neglecting the coupling of overall and interdomain motions.
TL;DR: Enhanced quantum relaxation in constant transverse fields allows the study of the relative magnitude of tunnel splittings in quantum dynamics at avoided level crossings of the energy spectrum of single Ho3+ ions.
Abstract: Staircaselike hysteresis loops of the magnetization of a LiY0.998Ho0.002F4 single crystal are observed at subkelvin temperatures and low field sweep rates. This behavior results from quantum dynamics at avoided level crossings of the energy spectrum of single Ho3+ ions in the presence of hyperfine interactions. Enhanced quantum relaxation in constant transverse fields allows the study of the relative magnitude of tunnel splittings. At faster sweep rates, nonequilibrated spin-phonon and spin-spin transitions, mediated by weak dipolar interactions, lead to magnetization oscillations and additional steps.
TL;DR: In this article, the origin of the magnetization relaxation processes was systematically examined in single-molecule magnets with the composition [Mn12O12(O2CR)16(H2O)x] (x = 3 or 4).
Abstract: Several single-molecule magnets with the composition [Mn12O12(O2CR)16(H2O)x] (x = 3 or 4) exhibit two out-of-phase ac magnetic susceptibility signals, one in the 4−7 K region and the other in the 2−3 K region. New Mn12 complexes were prepared and structurally characterized, and the origin of the two magnetization relaxation processes was systematically examined. Different crystallographic forms of a Mn12 complex with a given R substituent exist where the two forms have different compositions of solvent molecules of crystallization and this results in two different arrangements of bound H2O and carboxylate ligands for the two crystallographically different forms with the same R substituent. The X-ray structure of cubic crystals of [Mn12O12(O2CEt)16(H2O)3]· 4H2O (space group P1) (complex 2a) has been reported previously. The more prevalent needle-form of [Mn12O12(O2CEt)16(H2O)3] (complex 2b) crystallizes in the monoclinic space group P21/c, which at −170 °C has a = 16.462(7) A, b = 22.401(9) A, c = 20.766(...
TL;DR: In this article, the complex (electric) permittivity of aqueous solutions of d-glucose and d-fructose has been determined as a function of frequency υ between 300 kHz and 40 GHz.
Abstract: At some solute concentrations c between 1 and 5.4 mol/L, the complex (electric) permittivity of aqueous solutions of d-glucose and d-fructose has been determined as a function of frequency υ between 300 kHz and 40 GHz. The permittivity spectrum of the 5.4 mol/L d-fructose solution has been measured at six temperatures between 10 and 35 °C, and the other spectra have been taken at 25 °C. All dielectric spectra revealed one dispersion/dielectric loss region, which indicated a rather homogeneous relaxation of the solute and solvent dipole moments. Analytically, the measured spectra were represented by the Cole−Cole relaxation spectral function, which corresponds with a continuous, symmetrically bell shaped relaxation time distribution. The parameters of the spectral function are discussed to show that the monosaccharides exhibit unusual hydration properties. Particularly, when treated in terms of a wait-and-switch model of dipole reorientation, the principal dielectric relaxation time is indicative of the ex...
TL;DR: The magnetic properties of Fe2O3 nanoparticles (average diameter ∅≅3nm) in alumina have been investigated by magnetization measurements as discussed by the authors, which indicate a superparamagnetic behavior of interacting particles, which block with decreasing temperature (the zero-field-cooled susceptibility shows a maximum at T≅145 K) with a distribution of relaxation times.
TL;DR: A simple phenomenological model is developed to account for the observed dynamics of a gel composed of multilamellar vesicles and finds the same aging exponent mu = 0.78 for both techniques, suggesting that they probe similar physical processes, that is the relaxation of applied or internal stresses for rheology or MDLS, respectively.
Abstract: We use linear rheology and multispeckle dynamic light scattering (MDLS) to investigate the aging of a gel composed of multilamellar vesicles Light scattering data indicate rearrangement of the gel through an unusual ultraslow ballistic motion A dramatic slowdown of the dynamics with sample age t(w) is observed for both rheology and MDLS, the characteristic relaxation time scaling as t(mu)w We find the same aging exponent mu = 078 for both techniques, suggesting that they probe similar physical processes, that is the relaxation of applied or internal stresses for rheology or MDLS, respectively A simple phenomenological model is developed to account for the observed dynamics
TL;DR: A quantitative light induced excited spin state trapping (LIESST) effect was observed, and the HS --> LS relaxation in the range of temperatures 5-52.5 K was studied.
Abstract: {Fe(abpt)2[N(CN)2]2} (abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole) represents the first example of an iron(II) spin-crossover compound containing dicyanamide ligand, [N(CN)2]-, as a counterion. It shows an incomplete two-step spin transition with around 37% of HS molecules trapped in the low-temperature region when standard cooling or warming modes, i.e., 1−2 K min-1, were used. The temperature, T1/2 ≈ 86 K, at which 50% of the conversion takes place, is one of the lowest temperatures observed for an iron(II) spin-crossover compound. Quenching experiments at low temperatures have shown that the incomplete character of the conversion is a consequence of slow kinetics. The quenched HS state relaxes back to the LS state displaying noticeable deviation from a single-exponential law. The rate of relaxation was evaluated in the range of temperatures 10−60 K. In the upper limit of temperatures, where thermal activation predominates, the activation energy and the pre-exponential parameter were estimated a...