Showing papers on "Relaxation (NMR) published in 2000"

Linear Viscoelasticity of Disordered Polystyrene−Polyisoprene Block Copolymer Based Layered-Silicate Nanocomposites

Abstract: The melt-state linear viscoelastic properties for a series of intercalated nanocomposites are examined. The nanocomposites are based on a short disordered polystyrene−polyisoprene diblock copolymer and varying amounts of dimethyldioctadecylammonium modified montmorillonite. The linear dynamic oscillatory moduli and the stress relaxation moduli are in quantitative agreement and suggest that at short times the relaxation of the nanocomposites is essentially unaffected by the presence of the layered-silicate. However, at long times (or equivalently low frequency), the hybrids exhibit dramatically altered viscoelastic behavior. Hybrids with silicate loadings in excess of 6.7 wt % exhibit pseudo-solidlike behavior, similar to that observed in previous studies of exfoliated end-tethered nanocomposites. On the basis of simple phenomenological arguments, the long time behavior is attributed to the presence of anisotropic stacks of silicate sheets randomly oriented and forming a percolated network structure that i...

Effects of ion atmosphere on hydrogen-bond dynamics in aqueous electrolyte solutions

Abstract: We have performed a series of molecular dynamics simulations of aqueous NaCl and KCl solutions at different concentrations to investigate the effects of ion atmosphere on the dynamics of water-water hydrogen bonds at room temperature. The average number of hydrogen bonds per water molecule decreases with increase of ion concentration. The dynamics of hydrogen-bond breaking is found to accelerate somewhat and that of hydrogen-bond structural relaxation, which occurs at a longer time scale, is found to slow down with increasing ion concentration for both NaCl and KCl solutions.

Structure property relations in BiFeO3/BaTiO3 solid solutions

Abstract: BiFeO3, when forming a solid solution with BaTiO3, shows structural transformations over the entire compositional range. Above 70 mole % of BiFeO3 the structure is rhombohedral and below 4 mole %, it is tetragonal. In between the structure is cubic. The ferroelectric TC decreases with increasing composition of BaTiO3 and a relatively small relaxation is observed. Impedance measurements showed a structural dependence and analysis of which has clearly shown that the capacitance observed is from the bulk of the sample. Relaxation time (τ) of BiFeO3 was estimated using the relaxation times of the intermediate compositions. Magnetization measurements showed field induced ferromagnetism. As the structure becomes cubic with increasing concentration of BaTiO3, paramagnetism sets in, as evidenced by the electron spin resonance spectra.

Glass transitions and dynamics in thin polymer films: Dielectric relaxation of thin films of polystyrene

Abstract: The glass transition temperature T(g) and the temperature T(alpha) corresponding to the peak in the dielectric loss due to the alpha process have been simultaneously determined as functions of film thickness d through dielectric measurements for polystyrene thin films supported on glass substrate. The dielectric loss peaks have also been investigated as functions of frequency for a given temperature. A decrease in T(g) was observed with decreasing film thickness, while T(alpha) was found to remain almost constant for d>d(c) and to decrease drastically with decreasing d for d

Electron Transfer Rates in Bridged Molecular Systems 2. A Steady-State Analysis of Coherent Tunneling and Thermal Transitions†

Abstract: The effect of dephasing and relaxation on electron transfer in bridged molecular systems is investigated using a simple molecular model. The interaction between the molecular system and the thermal environment is described on the level of the Redfield theory, modified when needed for the description of steady-state situations. Noting that transient as well as steady-state measurements are possible in such system, we discuss the relationship between the rates obtained from these different types of experiments and, in particular, the conditions under which these rates are the same. Also, a formal relation between the steady-state rate for electron transfer across a molecular bridge and the conductance of this bridge when placed between two metal contacts is established. The effect of dephasing and relaxation on the electron transfer is investigated, and new observations are made with regard to the transition from the superexchange to the thermal (hopping through bridge) regime of the transfer process. In pa...

Nanoscopic-confinement effects on local dynamics

Abstract: The segmental dynamics of 15-20 nm polymer films confined between parallel solid surfaces is investigated with dielectric spectroscopy in polymer/silicate intercalated nanocomposites The confinement effect is evident by the observation of a mode, much faster than the bulk-polymer alpha relaxation and exhibiting much weaker temperature dependence This is discussed in relation to either the interlayer spacing restricting the cooperative volume of the alpha relaxation or to the dominance of the more mobile interphase regions as predicted by simulations; the data qualitatively support the former

Femtosecond transient-absorption dynamics of colloidal gold nanorods: Shape independence of the electron-phonon relaxation time

Abstract: We studied the femtosecond dynamics of colloidal gold nanorods encapsulated in micelles after excitation with 400 nm pulses of 100 fs duration. It is found that the laser heating of the electron gas of gold nanorods with an average aspect ratio of 3.8 leads to the bleaching of both the transverse and longitudinal mode of the surface plasmon oscillation at 520 and 750 nm. The bleach recovers with the same time constant for both the transverse and longitudinal oscillation, for gold nanodots prepared by photothermal reshaping of the rods as well as for nanodots synthesized chemically by citrate reduction (and known to have twin boundaries and surface defects). Since the bleach recovery on the 3 ps time scale is assigned to electron-phonon relaxation processes, these results suggest that phonon dependent relaxation processes in gold nanoparticles are independent of the shape, size, type of the surfaces, or the mode of the surface plasmon, oscillation excited. The fact that the mean free path of the electron in metallic gold is in the nanometer length scale (\ensuremath{\sim}50 nm) raised the question of the importance of surface scattering to the electron-phonon relaxation process in gold nanoparticles. Our previous studies showed little dependence of the relaxation rate of the size of gold nanodots (from 9 to 48 nm). In the present study, the electron-phonon relaxation is measured in gold nanorods, which have different facets from those of gold nanodots.

Butterfly hysteresis loop and dissipative spin reversal in the S = 1/2, V15 molecular complex

Abstract: Time resolved magnetization measurements have been performed on a spin 1/2 molecular complex, so-called V15. Despite the absence of a barrier, magnetic hysteresis is observed over a time scale of several seconds. A detailed analysis in terms of a dissipative two-level model is given, in which fluctuations and splittings are of the same energy. Spin-phonon coupling leads to long relaxation times and to a particular "butterfly" hysteresis loop.

Water−Ethanol Mixtures at Different Compositions and Temperatures. A Dieletric Relaxation Study

Abstract: At eight temperatures T between 0 and 60 °C and at five mole fractions xe of ethanol (0 < xe ≤ 1) the complex (electric) permittivity of ethanol/water mixtures has been measured as a function of frequency ν between 1 MHz and 24 GHz. At 25 °C the ethanol permittivities are completed by literature data for the frequency range 200 MHz to 90 GHz. The spectra for ethanol and for the ethanol/water mixtures are compared to permittivity spectra for water which, at some temperatures, are available up to 900 GHz. All spectra of the ethanol/water system can be well represented by the assumption of two relaxation regions. The relaxation time τ1 of the dominating relaxation process varies between 4 ps (xe = 0, 60 °C) and 310 ps (xe = 1, 0 °C). The relaxation time τ2 of the second relaxation process is smaller. Evaluation of the extrapolated low frequency (“static”) permittivity yields a minium in the effective dipole orientation correlation of the ethanol/water system at 0.2 ≤ xe ≤ 0.4. In this composition range, othe...

Ultrafast intersubband relaxation (⩽150 fs) in AlGaN/GaN multiple quantum wells

Abstract: The ultrafast intersubband relaxation in GaN quantum wells has been verified. Al0.65Ga0.35N/GaN multiple quantum wells, with as many as 200 wells, were grown by optimizing the barrier thickness and introducing GaN intermediate layers. The intersubband absorption is sufficiently strong for the relaxation time to be measured. A pump–probe measurement is performed to investigate the relaxation. An ultrashort relaxation time of less than 150 fs is obtained at a wavelength of 4.5 μm. The transient time is shorter than that of InGaAs quantum wells by approximately an order of magnitude. This result is promising for realizing ultrafast optical switches.

Electronic Structures of Single-Walled Carbon Nanotubes Determined by NMR

Abstract: Single-walled carbon nanotubes were studied by 13C nuclear magnetic resonance (NMR). Two types of 13C nuclear spins were identified with different spin-lattice relaxation rates. The fast-relaxing component, assigned to metallic tubes, followed the relaxation behavior expected in metals, and the density-of-states at the Fermi level increased with decreasing tube diameter. The slow-relaxing component has a significantly lower density-of-states at the Fermi level. Exposure to oxygen has a substantial effect on relaxation rates of both components.

Magnetic dynamics of weakly and strongly interacting hematite nanoparticles

Abstract: The magnetic dynamics of two differently treated samples of hematite nanoparticles from the same batch with a particle size of about 20 nm have been studied by Mossbauer spectroscopy. The dynamics of the first sample, in which the particles are coated and dispersed in water, is in accordance with the Neel expression for the superparamagnetic relaxation time of noninteracting particles. From a simultaneous analysis of a series of Mossbauer spectra, measured as a function of temperature, we obtain the median energy barrier KBuVm /k 55706100 K and the preexponential factor t 051.3 20.8 11.9 310 210 s for a rotation of the sublattice magnetization directions in the rhombohedral ~111! plane. The corresponding median superparamagnetic blocking tempera- ture is about 150 K. The dynamics of the second, dry sample, in which the particles are uncoated and thus allowed to aggregate, is slowed down by interparticle interactions and a magnetically split spectrum is retained at room temperature. The temperature variation of the magnetic hyperfine field, corresponding to different quantiles in the hyperfine field distribution, can be consistently described by a mean field model for ''super- ferromagnetism'' in which the magnetic anisotropy is included. The coupling between the particles is due to exchange interactions and the interaction strength can be accounted for by just a few exchange bridges between surface atoms in neighboring crystallites.

Changing Shapes in the Nanoworld

Abstract: What are the mechanisms leading to the shape relaxation of three-dimensional crystallites? Kinetic Monte Carlo simulations of fcc clusters show that the usual theories of equilibration, via atomic surface diffusion driven by curvature, are verified only at high temperatures. Below the roughening temperature, the relaxation is much slower, kinetics being governed by the nucleation of a critical germ on a facet. We show that the energy barrier for this step linearly increases with the size of the crystallite, leading to an exponential dependence of the relaxation time.

Dielectric dispersion of the relaxor PLZT ceramics in the frequency range 20 Hz-100 THz

Abstract: The dielectric dispersion of the transparent relaxor ferroelectric ceramics PLZT 8/65/35 and 9.5/65/35 was determined in a wide frequency range including the microwave and infrared range. The number of observed polar phonons in infrared spectra gives evidence about the locally broken cubic symmetry and the presence of polar nanoclusters in the whole investigated temperature range up to 530 K. A single broad and symmetric dispersion that occurs below the polar phonon frequencies was fitted with the Cole-Cole formula and a uniform distribution of Debye relaxations. On decreasing temperature, the distribution of relaxation times becomes extremely broad which indicates increasing correlation among the clusters. The mean relaxation time diverges according to the Vogel-Fulcher law with the same freezing temperature 230±5 K for both ceramics, but different activation energies 1370 K and 1040 K for the 8/65/35 and 9.5/65/35 sample, respectively. The shortest relaxation time is about 10-12 s and remains almost temperature independent. Below room temperature, the loss spectra become essentially frequency independent and the permittivity increases linearly with decreasing logarithm of frequency. The slope of this dependence is proportional to T 4 in the investigated temperature range (above 210 K) which indicates appreciable anharmonicity of the potential for polarization fluctuations.

Glass transition and layering effects in confined water: A computer simulation study

Abstract: Single particle dynamics of water confined in a nanopore is studied through computermolecular dynamics. The pore is modeled to represent the average properties of a pore of Vycor glass. Dynamics is analyzed at different hydration levels and upon supercooling. At all hydration levels and all temperatures investigated a layering effect is observed due to the strong hydrophilicity of the substrate. The time density correlators show, already at ambient temperature, strong deviations from the Debye and the stretched exponential behavior. Both on decreasing hydration level and upon supercooling we find features that can be related to the cage effect typical of a supercooled liquid undergoing a kinetic glass transition. Nonetheless the behavior predicted by mode coupling theory can be observed only by carrying out a proper shell analysis of the density correlators. Water molecules within the first two layers from the substrate are in a glassy state already at ambient temperature (bound water). The remaining subset of molecules (free water) undergoes a kinetic glass transition; the relaxation of the density correlators agree with the main predictions of the theory. From our data we can predict the temperature of structural arrest of free water.

Evolution of vibrational excitations in glassy systems

Abstract: The equations of the mode-coupling theory (MCT) for ideal liquid-glass transitions are used for a discussion of the evolution of the density-fluctuation spectra of glass-forming systems for frequencies within the dynamical window between the band of high-frequency motion and the band of low-frequency-structural-relaxation processes. It is shown that the strong interaction between density fluctuations with microscopic wavelength and the arrested glass structure causes an anomalous-oscillation peak, which exhibits the properties of the so-called boson peak. It produces an elastic modulus which governs the hybridization of density fluctuations of mesoscopic wavelength with the boson-peak oscillations. This leads to the existence of high-frequency sound with properties as found by x-ray-scattering spectroscopy of glasses and glassy liquids. The results of the theory are demonstrated for a model of the hard-sphere system. It is also derived that certain schematic MCT models, whose spectra for the stiff-glass states can be expressed by elementary formulas, provide reasonable approximations for the solutions of the general MCT equations.

Impact of Transverse Relaxation Optimized Spectroscopy (TROSY) on NMR as a technique in structural biology

Abstract: 1. Transverse relaxation and the molecular size limit in liquid state NMR 1612. TROSY: how does it work? 1632.1 Transverse relaxation in coupled spin systems 1632.2 The TROSY effect, relaxation due to remote protons and 2H isotope labeling 1653. Direct heteronuclear chemical shift correlations 1683.1 Single-Quantum [15N,1H]-TROSY 1683.2 Zero-Quantum [15N,1H]-TROSY 1713.3 Single-Quantum TROSY with aromatic 13C–1H moieties 1764. Resonance assignment and NOE spectroscopy of large biomolecules 1804.1 TROSY-based triple resonance experiments for 13C, 15N and 1HN backbone resonance assignment in uniformly 2H, 13C, 15N labeled proteins 1804.2 TROSY-type NOE spectroscopy 1865. Scalar coupling across hydrogen bonds observed by TROSY 1876. The use of TROSY for measurements of residual dipolar coupling constants 1907. Conclusions 1918. Acknowledgements 1919. References 191The application of nuclear magnetic resonance (NMR) spectroscopy for structure determination of proteins and nucleic acids (Wuthrich, 1986) with molecular mass exceeding 30 kDa is largely constrained by two factors, fast transverse relaxation of spins of interest and complexity of NMR spectra, both of which increase with increasing molecular size (Wagner, 1993b; Clore & Gronenborn, 1997, 1998b; Kay & Gardner, 1997). The good news is that neither of these factors represent a fundamental limit for the application of NMR techniques to protein structure determination in solution (Clore & Gronenborn, 1998a; Wuthrich, 1998; Wider & Wuthrich, 1999). In fact, in the past few years the size limitations imposed by these factors have been pushed up to 50–70 kDa by the use of 13C, 15N and 2H isotope labeling combined with selective reprotonation of individual chemical groups in conjunction with the use of triple-resonance experiments (Bax, 1994; Gardner et al. 1997; Gardner & Kay, 1998) and heteronuclear-resolved NMR (Fesik & Zuiderweg, 1988; Marion et al. 1989a; Otting & Wuthrich, 1990). Among the largest biomolecules whose 3D structure was solved by NMR are the 44 kDa trimeric ectodomain of simian immunodeficiency virus (SIV) gp41 (Caffrey et al. 1998) and 40–60 kDa particles of the elongation initiation factor 4E solubilized in CHAPS micelles (Matsuo et al. 1997; McGuire et al. 1998).

Dielectric properties of low molecular weight poly(ethylene glycol)s

Abstract: This paper reports the measured values of dielectric permittivity e′ and dielectric loss e″ of ethylene glycol, diethylene glycol and poly(ethylene glycol)s of average molecular weight 200, 300, 400 and 600 g mol−1 in the pure liquid state. The measurements have been carried out in the frequency range 200 MHz to 20 GHz at four different temperatures of 25, 35, 45 and 55 °C. The complex plane plots (e″ versus e′) of these molecules are Cole–Cole arcs. The static dielectric constant e0, high-frequency limiting dielectric constant e∞, average relaxation time τ0 and distribution parameter α have been determined from these plots. The value of the Kirkwood correlation factor g and the dielectric rate free energy of activation ΔF have also been evaluated. The dependence of relaxation time on molecular size and viscosity has been discussed. A comparison has also been made with the dielectric behaviour of these molecules in dilute solutions of non-polar solvents, which were carried out earlier in this laboratory. The influences of intermolecular hydrogen bonding and molecular chain coiling on the dielectric relaxation of these molecules have been recognized. © 2000 Society of Chemical Industry

Hydrophobic hydration and molecular association in methanol-water mixtures studied by microwave dielectric analysis

Abstract: Dielectric relaxation measurements on the methanol–water mixtures for the entire concentration range were carried out using time domain reflectometry in the frequency range from 500 MHz to 25 GHz at 20, 25, and 30 °C. The excess partial molar activation free energy, enthalpy, and entropy for methanol, ΔGMAE, ΔHMAE, and ΔSMAE, and those for water, ΔGWE, ΔHWE, and ΔSWE, were calculated from accurately measured concentration and temperature dependence of the dielectric relaxation time of the mixtures. The behavior of the excess partial molar quantities in the regions below and above X (molar fraction of methanol) ∼0.3 are quite different from each other. In a water-rich region, ΔHMAE and ΔSMAE exhibit two maxima at X∼0.045 and X∼0.12, which is clearly attributed to structural enhancement of the hydrogen bond network of water, the so-called hydrophobic hydration. Appearance of two maxima in ΔHMAE and ΔSMAE implies that water molecules surround methanol molecules in qualitatively different manners around the t...

Size dependence of the properties of hematite nanoparticles

Abstract: Samples of nanoparticles of hematite (α-Fe2O3) with average size between 6 and 27 nm have been studied by use of Mossbauer spectroscopy. The superparamagnetic relaxation was analysed on the basis of the Neel-Brown expression for the relaxation time, τ = τ0 exp[KV/kT]. It was found that the value of τ0 increases with increasing particle volume, V, whereas the magnetic anisotropy energy constant, K, decreases. The electric quadrupole interaction, the isomer shift and the magnetic hyperfine field extrapolated to 0 K were found to be essentially independent of particle size.

Progressive suppression of spin relaxation in two-dimensional channels of finite width

Abstract: We have investigated spatiotemporal kinetics of electron spin polarization in a semiconductor narrow two-dimensional (2D) strip and explored the ability to manipulate spin relaxation. Information about the conduction electron spin and mechanisms of spin rotation is incorporated into a Monte Carlo transport simulation program. A model problem, involving linear-in-$k$ splitting of the conduction band responsible for the D'yakonov-Perel' mechanism of spin relaxation in the zinc-blende semiconductors and heterostructures, is solved numerically to yield the decay of spin polarization of an electron ensemble in the 2D channel of finite width. For very wide channels, a conventional 2D value of spin relaxation is obtained. With decreasing channel width, the relaxation time increases rapidly by orders of magnitude. Surprisingly, the crossover point between 2D and quasi-1D behavior is found to be at tens of electron mean-free paths. Thus, classically wide channels can effectively suppress electron spin relaxation.

Ultrafast Raman-induced Kerr-effect of water: Single molecule versus collective motions

Abstract: The ultrafast optical Kerr-response of water and heavy water has been measured at 1 bar in the temperature range between 273 and 373 K. The nuclear Kerr response of the liquid exhibits a pronounced double exponential decay on longer time scales after dephasing of impulsively perturbed acoustic modes is completed. The time constant, τ2, characterizing the slowly decaying exponential component of the Kerr-response function is in quantitative agreement with rotational diffusion time constants of the water molecules obtained form nuclear magnetic resonance (NMR) spin-lattice relaxation rates. A detailed comparison with THz time domain spectroscopy demonstrates that the reorientational dynamics responsible for the long time tail of the Kerr response are due to single molecule as opposed to collective effects. Furthermore, a good agreement between the single molecule rotational diffusion and the Stokes–Einstein–Debye equation is found in the temperature range of thermodynamic stability of the liquid. The time constant, τ1, characterizing the fast exponential component of the Kerr-response of water is found to be in qualitative agreement with central Lorentzian linewidths obtained from frequency-domain, depolarized Raman scattering experiments. The temperature dependence of τ2 does not follow an Arrhenius-type behavior, which was previously taken as evidence for thermally activated crossing of a librational barrier with concomitant hydrogen-bond breakage. Instead, the temperature dependence of the fast relaxation time constant can be represented adequately by the Speedy–Angell relation which has been shown to accurately describe a number of transport parameters and thermodynamic properties of water.

Light ion irradiation of Co/Pt systems: Structural origin of the decrease in magnetic anisotropy

Abstract: We study the structural properties of Pt/Co/Pt systems submitted to ${\mathrm{He}}^{+}$ ion irradiation, in order to understand why the magnetic anisotropy can be decreased in a controlled way. It is shown by grazing x-ray reflectometry that the irradiation-induced Pt and Co atom displacements can be largely accounted for by a simple ballistic recoil mechanism model. Our results indicate that even in these nm-thick films, irradiation may affect the upper and lower interfaces differently. Specifically, the upper Co interface undergoes short-range mixing, resulting in roughness, whereas the lower Co interface mostly evolves by longer-range mixing, leading to alloy formation. Irradiation also releases strain in these Co-Pt systems, but has no chemical ordering effect. Together with slow asymmetric interface roughening, the cobalt tensile strain relaxation at low fluences accounts for the magnetic anisotropy decrease. The type of analysis we propose could be useful to understand why other magnetic properties, such as interlayer exchange coupling, can be controlled by light ion irradiation.

NC100150 Injection, a preparation of optimized iron oxide nanoparticles for positive-contrast MR angiography.

Abstract: A preparation of monocrystalline iron oxide nanoparticles with an oxidized starch coating, currently in clinical trials (NC100150 Injection; CLARISCAN), was characterized by magnetization measurements, relaxometry, and photon correlation spectroscopy. By combining the results with a measure of iron content, one can obtain the size and magnetic attributes of the iron cores, including the relevant correlation times for outer sphere relaxation (tau(SO) and tau(D)), and information about the interaction of the organic coating with both core and solvent. The results are 6.43 nm for the iron oxide core diameter, a magnetic moment of 4.38x10(-17) erg/G, and a water-penetrable coating region of oxidized oligomeric starch fragments and entrained water molecules. The latter extends the hydrodynamic diameter to 11.9 nm and lowers the average diffusivity of solvent about 64% (which increases tau(D) accordingly). The nanoparticles show little size-polydispersity, evidenced by the lowest value of r(2)/r(1) at 20 MHz reported to date, an asset for magnetic resonance angiography.