Showing papers on "Relaxation (NMR) published in 1993"

Random-field induced antiferromagnetic, ferroelectric and structural domain states

Abstract: The occurrence of metastable or stable domain states on mesoscopic length scales seems to be a widespread property of many solids undergoing phase transitions in the presence of quenched random fields A survey is given on the experimental evidence of domain states in various magnetic, ferroelectric and structural systems In particular we discuss phenomena like the excess magnetization of field-cooled diluted uniaxial antiferromagnets and its relaxation The domain state of the relaxor ferroelectric PbMg1/3Nb2/3O3 is due to the random distribution of B site cations, whereas dipolar quenched impurities give rise to mesoscopic disorder in K1−xLixTaO3 and Sr1−xCaxTiO3 Spontaneously relaxing quadrupolar domain states are observed in KTa1−xNbxO3 They are probably caused by random strain fields due to ionic size mismatch The same type of random fields determines the critical behavior of the Jahn-Teller compound DyAsxV1−xO4 but merely causes roughening of the natural ferroeleastic twin domain walls

The Spontaneous Relaxor-Ferroelectric Transition of Pb(Sc0.5ta0.5)O3

Abstract: A zero-field spontaneous relaxor-ferroelectric transition is reported in Pb(Sc0.5Ta0.5)O3 (PST). This behavior is different from that of other relaxors, where such transitions occur only under the field. A highly disordered PST that has the wide relaxation spectrum typical of relaxors is shown to transform spontaneously into a macroscopic ferroelectric state. Introduction of defects (lead vacancies) into the material impedes the transition resulting in the usual relaxor behavior. Dielectric properties of PST, with and without defects, are analyzed. For the interpretation of the observed properties, a model invoking an additional nonpolar phase is proposed. This model does not imply a freezing in the system. At the low-frequency limit, it is possible to account for the Vogel-Fulcher (VF) law for the temperature of the maximum of the dielectric constant, using only the commonly accepted assumption of an exponentially wide relaxation time spectrum that shrinks on heating. The presented approach interprets the observed proximity between the ferroelectric phase transition temperature and that of the freezing temperature obtained from the VF relation.

Light scattering and dielectric studies on glass forming liquids

Abstract: The α-relaxation behaviour of polymers and glass forming viscous liquids is well described by the Vogel-Fulcher-Tammann equation, introducing a Vogel temperature T0 at which the relaxation time τα diverges (T0 ≈ Tg - 50 K). With the recent development of the mode coupling theory a new critical temperature Tc is introduced located about 50 to 80 K above Tg. The relevance of the various temperatures is discussed on the basis of dynamic light scattering studies and dielectric relaxation data. The dynamic and static light scattering experiments revealed some unexpected features, which cannot be explained on the basis of conventional liquid state theories: (1) In static light scattering the intensity I(q→0) is no longer proportional to the isothermal compressibility. (2) This excess scattering Iexc shows a strong q-dependence (q = (4πn/λ)sin(θ/2)) corresponding to a correlation length ξ in the range of 20–200 nm. (3) The Landau-Placzek ratio IRayleigh/2IBrillouin is much too high, compared with the results of light scattering theories. (4) In photon correlation spectroscopy a new ultraslow hydrodynamic mode (Γ∼q2) is detected with relaxation rates Γ about 10-4 to 10-7 lower than those of the α-process at a given temperature. These effects are caused by long density fluctuations indicating a nonhomogeneous distribution of free volume. The redistribution of free volume in space causes the new ultraslow mode. A tentative model is proposed which describes the long range density fluctuations as a result of the coexistence of molecules with two different dynamic states, which show up in the Fabry-Perot and Raman spectroscopy.

Magnetic resonance of a single molecular spin

Abstract: THE introduction of optical detection methods for observing magnetic resonance transitions in metastable paramagnetic states1–4 has contributed enormously to our understanding of the properties of photoexcited molecules in condensed phases. In such experiments the luminescence intensity is recorded as a function of the frequency of an applied microwave field. At resonance with transitions between sublevels of a metastable paramagnetic state, the lifetime of the metastable state is altered and a consequent change in the luminescence intensity is observed. Here we report the observation of such optically detected magnetic resonance transitions for the triplet state of a single pentacene molecule embedded in a p-terphenyl host crystal. This result has been obtained by combining the conventional optical detection technique for observing magnetic resonance transitions1–4 with the new single-molecule optical detection methods developed recently5,6. This observation opens the way for magnetic resonance studies in condensed phases with single-molecule sensitivity.

Anomalies in the scaling of the dielectric alpha -relaxation.

Abstract: By measuring the complex dielectric function over 15 decades in frequency we evaluate the scaling of the α-relaxation for several glass-forming liquids including propylene carbonate. The temperature dependence of the mean relaxation time and of the relaxation strength of the relaxation function displays two dynamical regions being separated by a crossover temperature. The observed findings are essentially not in 'accordance with predictions of the mode-coupling theory and light scattering results for propylene carbonate [M. Elmroth et al., Phys. Rev. Lett. 68, 79 (1992)]

Anomalous peak effect in single crystal Bi2Sr2CaCu2O8+y studied by Hall probe magnetometry

Abstract: We have studied an anomalous peak effect in a Bi 2 Sr 2 CaCu 2 O 8+ y single crystal using a specially designed Hall probe magnetometer. In order to investigate the origin of the anomalous peak effect which manifests itself in the magnetic hysteresis loop, the relaxation (10 -2 s t 4 s), and the local field imaging experiments were performed. Decoupling of 2D vortices in neighboring CuO layers, namely, a dimensional crossover in the pinning mechanism, is proposed as a possible mechanism of the peak effect.

Photodissociation and vibrational relaxation of I2− in ethanol

Abstract: The dissociation, internal conversion, and vibrational relaxation of photoexcited I2− in ethanol have been examined using ultrafast transient‐absorption spectroscopy. I2− was photoexcited at 770 nm (1.6 eV) and probed on the subpicosecond time scale at 15 wavelengths between 580 and 950 nm, permitting a determination of the temporal evolution of the absorption spectrum. The data reveal that internal conversion and vibrational relaxation at the top of the well are extremely rapid (≤0.3 ps), with loss of the final 0.3 eV of energy (v≤20) occurring on a time scale of ∼4 ps. Simple kinetic and spectral models are able to qualitatively account for the observed behavior of the transient‐absorption signals.

Vibrational relaxation in simple fluids: Comparison of theory and simulation

Abstract: General theoretical expressions for the dephasing and energy relaxation times of a stiff oscillator in simple fluids are derived from the GLE and a critical discussion of the dynamic processes in these systems is given. In addition new methodological aspects of stochastic and full molecular dynamics simulations are discussed. The new reversible integrator based on the Trotter factorization of the classical propagator is used to directly simulate the vibrational energy and phase relaxation of a stiff classical oscillator dissolved in a Lennard‐Jones bath. We compare the ‘‘real’’ relaxation from full MD simulations with that predicted by Kubo theory and by the generalized Langevin equation (GLE) with memory friction determined from the full molecular dynamics. It is found that the GLE gives very good agreement with MD for the vibrational energy relaxation, even for nonlinear oscillators far from equilibrium. The dephasing relaxation is also well approximated by the GLE.

Gaseous 3He-3He magnetic dipolar spin relaxation.

Abstract: We derive the nuclear-spin relaxation rate of gaseous $^{3}\mathrm{He}$ due to the magnetic-dipole interaction between the $^{3}\mathrm{He}$ nuclear spins. This dipolar relaxation rate is numerically evaluated for temperatures from 0.1 K to 550 K. At room temperature, the relaxation time for a $^{3}\mathrm{He}$ density of 10 amagats is 74.4 h. We have made a series of high-density (4--12 amagat) $^{3}\mathrm{He}$ samples for which nulcear relaxation is limited by the magnetic-dipole interaction. Both our theoretical and experimental results are particularly important for the growing use of $^{3}\mathrm{He}$, polarized through spin exchange with optically pumped Rb vapor.

Protein dynamics studied by rotating frame 15N spin relaxation times

Abstract: Conformational rate processes in aqueous solutions of uniformly 15N-labeled pancreatic trypsin inhibitor (BPTI) at 36°C were investigated by measuring the rotating frame relaxation times of the backbone 15N spins as a function of the spin-lock power. Two different intramolecular exchange processes were identified. A first local rate process involved the residues Cys38 and Arg39, had a correlation time of about 1.3 ms, and was related to isomerization of the chirality of the disulfide bond Cys14-Cys38. A second, faster motional mode was superimposed on the disulfide bond isomerization and was tentatively attributed to local segmental motions in the polypeptide sequence-Cys14-Ala15-Lys16-. The correlation time for the overall rotational tumbling of the protein was found to be 2 ns, using the assumption that relaxation is dominated by dipolar coupling and chemical shift anistropy modulated by isotropic molecular reorientation.

Vibrational and rotational relaxation times of solvated molecular ions

Abstract: Infrared pump–probe and infrared polarization spectroscopy have been used to measure the vibrational relaxation times (T1) of the antisymmetric stretching mode and the reorientation times (TR) for N3−, NCS−, and NCO− in D2O and/or methanol. For N3−, experiments were also conducted in H2O and hexamethyl–phosphamide (HPMA) solutions. The rapid vibrational relaxation and slow reorientation observed demonstrate strong coupling between the ions and the solvents. Longer vibrational relaxation and shorter reorientation times measured for NCS− reveal weaker solvent interactions that may be due to the importance of the charge distribution and the form of the normal coordinate. A comparison of the T1 and TR times in different solvents permits a determination of the relative interaction strengths for the solvents investigated. The relatively weaker coupling of N3− in the aprotic solvent HMPA demonstrates the importance of hydrogen bonding in strong solvent interactions in ionic solutions. The experimental results ar...

Effects of ion binding on the backbone dynamics of calbindin D9k determined by 15N NMR relaxation

Abstract: The backbone dynamics of apo- and (Cd2+)1-calbindin D9k have been characterized by 15N nuclear magnetic resonance spectroscopy. Spin-lattice and spin-spin relaxation rate constants and steady-state [1H]-15N nuclear Overhauser effects were measured at a magnetic field strength of 11.74 T by two-dimensional, proton-detected heteronuclear NMR experiments using 15N-enriched samples. The relaxation parameters were analyzed using a model-free formalism that characterizes the dynamics of the N-H bond vectors in terms of generalized order parameters and effective correlation times. The data for the apo and (Cd2+)1 states were compared to those for the (Ca2+)2 state [Kordel, J., Skelton, N. J., Akke, M., Palmer, A. G., & Chazin, W. J. (1992) Biochemistry 31, 4856-4866] to ascertain the effects on ion ligation on the backbone dynamics of calbindin D9k. The two binding loops respond differently to ligation by metal ions: high-frequency (10(9)-10(12) s-1) fluctuations of the N-terminal ion-binding loop are not affected by ion binding, whereas residues G57, D58, G59, and E60 in the C-terminal ion-binding loop have significantly lower order parameters in the apo state than in the metal-bound states. The dynamical responses of the four helices to binding of ions are much smaller than that for the C-terminal binding loop, with the strongest effect on helix III, which is located between the linker loop and binding site II. Significant fluctuations on slower time scales also were detected in the unoccupied N-terminal ion-binding loop of the apo and (Cd2+)1 states; the apparent rates were greater for the (Cd2+)1 state. These results on the dynamical response to ion binding in calbindin D9k provide insights into the molecular details of the binding process and qualitative evidence for entropic contributions to the cooperative phenomenon of calcium binding for the pathway in which the ion binds first in the C-terminal site.

Two-dimensional inverse Laplace transform NMR: altered relaxation times allow detection of exchange correlation

Abstract: A pulse sequence for a two-dimensional inverse Laplace transform NMR experiment is proposed and demonstrated. The experiment is analogous to the two-dimensional Fourier transform protocol called EXSY, but detects exchange by monitoring alterations in the transverse relaxation time rather than the NMR frequency. The sequence may be useful for measurement of exchange and diffusion of water in vivo and for detecting slow exchange phenomena in glassy polymers. The application of the Fourier transform (FT) to nuclear magnetic resonance spectroscopy' and its extension to multiple dimensions2 has radically altered modern chemistry and medical science; NMR imaging3 and the determination of the structure of proteins in solution4 are two important advances that would be either impossible or orders-of-magnitude more difficult without the FT. The crucial feature of the transformation is the increase in availablespectroscopic resolution. If two signals havedifferent precession frequencies VI and vs as a result of some coherent interaction, the time evolution of the total transverse nuclear magnetization M(r) will be the sum of the time evolution of these two species (M1(t) + Ms(t)), and at every point in time it will be dependent on VI and us. However, the FT will still show resolved signals in frequency space, thus discriminating the contributions of the two species.

Solvent relaxation dynamics and electron transfer

Abstract: In this paper we explore the effects of medium dynamics on activationless and inverted-region electron transfer (ET), when medium-induced dynamics is slow on the time scale of the electronic processes. ET dynamics, with electron-nuclear coupling to the medium modes, was characterized in terms of incoherent population decay of vibronic states in the initial donor-acceptor manifold, which is characterized by nonadiabatic, energy ( E )-dependent, microscopic ET rates, k ( E ). These k ( E )'s are determined by average Franck-Condon densities (AFDs), which were evaluated by quantum and classical formalisms, with model calculations being performed for multimode harmonic systems with displaced potential surfaces. In spite of the intrinsic limitations of the classical AFDs, which do not account for mode specificity and nuclear tunneling effects, the classical Franck-Condon factors provide a good description of the E dependence of the microscopic ET rates. For activationless ET we show that k ( E )∝( E + n ϵ) − 1 2 , where n ϵ is the zero point energy, implying a weak energy dependence of k ( E ). Accordingly, the averaged experimental activationless ET rates exhibit a weak variation between the limits of slow medium-induced relaxation and that of fast medium-induced dynamics. Subsequently, the theory of k ( E ) was extended to include the effects of ET-induced excitations of high-frequency intramolecular vibrational modes, providing a unified description of the weak E dependence of k ( E ) in the activationless and inverted regions. We predict that for activationless and inverted-region ET the experimental ET rates are only weakly dependent on the characteristics of medium relaxation dynamics, and can be appreciably higher than the solvent-controlled values (i.e., the reciprocal values of the medium relaxation time induced by a constant charge distribution). Our analysis provides an adequate explanation for recent experimental observations of ultrafast ( k =(1 ps) −1 –(100 fs) −1 ) activationless and inverted-region ET, which apparently violate the predictions of solvent-controlled ET theory.

Fluorescence Anisotropy: Theory and Applications of Rotational Depolarization

Abstract: This review article presents the principal problems of fluorescence emission anisotropy in rigid and liquid isotropic solutions, as well as in partially ordered fluid systems (membranes), and indicates possible practical uses of these phenomena in physicochemical and biophysical investigations. It has been shown that fluorescence anisotropy provides a quantity of information on photophysical processes that occur in complex molecules. Thus, conclusions can be drawn with respect to molecular structure and properties, e.g., shapes, dimensions and conformational changes, electric dipole moments in excited states, relaxation of local temperature of excited molecules, as well as on the structure and internal dynamics of molecular systems.

Extraordinarily slow nuclear spin relaxation in frozen laser-polarized 129Xe.

Abstract: We studied the very slow nuclear spin-lattice relaxation of solid $^{129}\mathrm{Xe}$ as a function of temperature and magnetic field using laser-polarized nuclei. Relaxation times in excess of 500 h were measured. We present evidence for a new relaxation mechanism which results from a Raman spin-phonon scattering process involving the spin-rotation interaction. We also establish the existence of cross relaxation between $^{129}\mathrm{Xe}$ and the other magnetic isotope $^{131}\mathrm{Xe}$ and demonstrate that laser-polarized $^{129}\mathrm{Xe}$ can be used to cross polarize other nuclei that are present in the lattice.

Electronic dephasing studies of molecules in solution at room temperature by femtosecond degenerate four wave mixing

Abstract: The electronic dephasing of two oxazine dyes dissolved in ethylene glycol at room temperature is investigated by femtosecond degenerate four wave mixing (DFWM) experiments. Both two-pulse and three-pulse DFWM with simultaneous detection of the signals at two distinct phase-matching directions permits detailed investigation of dephasing dynamics in a room temperature liquid. At least two stochastic processes are responsible for the observed electronic dephasing: one is treated empirically as an exponential decay; the second is regarded in the full analytic form of the Kubo relaxation function. Both fast and slow (inhomogeneous) dynamics are recovered. The slow dynamics is found to bring about spectral diffusion over the inhomogeneous distribution on the time scale around a picosecond.

A unified view of relaxation in protein solutions and tissue, including hydration and magnetization transfer.

Abstract: Protein in water solution increases magnetic relaxation rates of solvent nuclei to an extent that depends on magnetic field strength and molecular weight. Koenig and Schillinger (J. Biol. Chem. 244, 3283 (1969)) showed that a small fraction of the water molecules in the first hydration shell, bound irrotationally with a residence lifetime in the range 0.1 to 10 microseconds, would account for the phenomena. No experiments, as yet, have proven the existence of such long-lived waters, nor yielded a value for their lifetime. Analogous measurements on solutions of both denatured and cross-linked protein give data different from that of native protein, but much like results for tissue. By comparing proton and deuteron relaxation rates in solutions of native and cross-linked protein, it is possible to demonstrate the existence of these relatively long-lived waters; the data indicate that 1% of a monolayer of the waters of hydration of protein have lifetimes that cluster near 1 microsecond and, it is argued, are held in place by multiple hydrogen bonds. Assigning shorter lifetimes for waters held by fewer bonds, it is possible to develop a unified view of relaxation of water nuclei in protein solutions and in tissue, and to relate it to recent crystallographic data on hydrated protein.

Proton magnetic resonance and pore size variations in reservoir sandstones

Abstract: Proton nuclear-magnetic-resonance (NMR) longitudinal relaxation time, T[sub 1], in water-saturated sandstones is related closely to pore size distributions. Most of the sandstones shown in this paper exhibit a relatively narrow T[sub 1] distribution curve, corresponding to a rather narrow distribution of the large pores visible by optical microscopy of thin sections. The surface relaxivity, [rho], which scales NMR T[sub 1] into pore size, is evaluated by comparing mean T[sub 1] for each sample with average pore diameter and volume/surface area data obtained from digitized thin sections. Permeability estimated from NMR parameters closely approximates measured permeability for these samples. These permeability estimates are closer than the variations in surface relaxivity would predict. This apparent contradiction results from parallel effects on the permeability and proton relaxation by clays in the pores.

Anomalous segment diffusion in polymers and NMR relaxation spectroscopy

Abstract: The dynamics of polymer chains (polyisoprene, polyisobutylene, poly(tetrahydrofuran), polystyrene, poly(ethylene oxide), polyethylene, and poly(dimethylsiloxane)) in melts, solutions, and network was studied by the aid of NMR relaxation spectroscopy. Proton data of the spin-lattice relaxation times in the laboratory and rotating frames, T 1 and T 1ρ , respectively, and of transverse relaxation curves are reported. Frequency, temperature, concentration, molecular weight, and cross-link density dependences have been investigated.

Magnetic‐Field‐Dependent Electronic Relaxation of Gd3+ in Aqueous Solutions of the Complexes [Gd(H2O)8]3+, [Gd(propane‐1,3‐diamine‐N,N,N′,N′‐tetraacetate)(H2O)2]−, and [Gd(N,N′‐bis[(N‐methylcarbamoyl)methyl]‐3‐azapentane‐1,5‐diamine‐3,N,N′‐triacetate)(H2O)] of interest in magnetic‐resonance imaging

Abstract: EPR Spectra have been measured for aqueous solutions of a series of Gd3+ complexes at variable temperature and a range of magnetic fields; S-band (0.14 T), X-band (0.34 T), Q-band (1.2 T), and 2-mm-band (5.0 T). The major contribution to the observed line widths is magnetic-field-dependent and is interpreted as being due to the modulation of the zero-field splitting produced by distortion of the complexes from perfect symmetry. The transverse and longitudinal relaxation matrices for an 8S ion with such an interaction have been calculated using Redfield theory with vector-coupling methods, and diagonalised numerically to obtain relaxation rates and intensities for the degenerate transitions which contribute to the multiplet. The observed line width, which is inversely proportional to the magnetic field at low temperatures, is best described by the intensity-weighted mean transverse relaxation time for the four transitions with non-zero intensity. A least-squares fit of the data yields the square of the zero-field splitting tensor, Δ2, and a correlation time, τv, with activation energy, Ev. The physical significance of these parameters and the extent of validity of the theoretical approach are considered. The parameters are used to predict the magnetic-field dependence of the longitudinal and transverse electronic relaxation times, which are discussed in the context of their relevance to 1H-NMR relaxivity.

High-field cross polarization NMR from laser-polarized xenon to a polymer surface

Abstract: We report the use of laser-polarized xenon as the source of magnetization for a high-field cross polarization experiment, obtaining surface-selective magnetization transfer. Contact between laser-polarized xenon and surface spins was achieved in high field by Hartmann-Hahn matching of the energy levels in the rotating frame with direct NMR detection of the polarized species. Proton spins are observed due to their abundance at the surface and the dominant dipolar interactions with adsorbed xenon. The sample, poly(triarylcarbinol) is a microporous, hypercross-linked, rigid-rod polymer. We have demonstrated that under favorable circumstances (high surface area, long relaxation time), laser-polarized, adsorbed xenon can be used to selectively transfer spin order to surface spins. 13 refs., 3 figs.

Proton relaxation enhancement

Abstract: Paramagnetic and superparamagnetic substances are used as contrast agents to enhance proton relaxation in magnetic resonance imaging. This review summarizes the physics of contrast agents, specifically the mechanisms by which contrast agents enhance T1 and T2 relaxation. The purpose is to provide a background for understanding the behavior of existing contrast agents in basic experimental and clinical studies. Terms such as magnetic dipole, dipole moment, magnetic susceptibility, diamagnetism, paramagnetism, superparamagnetism, and ferromagnetism are introduced. Two important interactions between the magnetic dipole moments of paramagnetic substances and the dipole moments associated with protons are described. The Solomon-Bloembergen-Morgan equations and other basic relaxation theory that has been confirmed experimentally are introduced to account for the dependence of relaxation on such parameters as the Larmor frequency, magnetic moment, accessibility of water molecules to the core of a contrast agent, and frequency of molecular motions.