Echo Decay Research Articles

Relation of self-diffusion and the topological structure of flexible polymers as studied by NMR

A general approach to deriving formulas for calculating the diffusion attenuation of spin echo decay signal is developed and applied to obtaining data on random processes in linear and crosslinked polymers with different topological structures. The analysis explains the experimentally observed anomalous diffusion in polymer networks.

Electron Paramagnetic Resonance and Electron Spin Echo Studies of Co2+ Coordination by Nicotinamide Adenine Dinucleotide (NAD+) in Water Solution

Co2+ binding to the nicotinamide adenine dinucleotide (NAD+) molecule in water solution was studied by electron paramagnetic resonance (EPR) and electron spin echo at low temperatures. Cobalt is coordinated by NAD+ when the metal is in excess only, but even in such conditions, the Co/NAD+ complexes coexist with Co(H2O)6 complexes. EPR spin-Hamiltonian parameters of the Co/NAD+ complex at 6 K are g z = 2.01, g x = 2.38, g y = 3.06, A z = 94 × 10−4 cm−1, A x = 33 × 10−4 cm−1 and A y = 71 × 10−4 cm−1. They indicate the low-spin Co2+ configuration with S = 1/2. Electron spin echo envelope modulation spectroscopy with Fourier transform of the modulated spin echo decay shows a strong coordination by nitrogen atoms and excludes the coordination by phosphate and/or amide groups. Thus, Co2+ ion is coordinated in pseudo-tetrahedral geometry by four nitrogen atoms of adenine rings of two NAD+ molecules.

Primary and secondary relaxation process in plastically crystalline cyanocyclohexane studied by 2H nuclear magnetic resonance. II. Quantitative analysis

We analyze the results of our previously reported 2H nuclear magnetic resonance (NMR) experiments in the plastically crystalline (PC) phase of cyanocyclohexane (Part I of this work) to study the fast secondary relaxation (or β-process) in detail. Both, the occurrence of an additional minimum in the spin-lattice relaxation T1 and the pronounced effects arising in the solid-echo spectrum above the glass transition temperature T(g) = 134 K, allow for a direct determination of the restricting geometry of the β-process in terms of the "wobbling-in-a-cone" model. Whereas at temperatures below T(g) the reorientation is confined to rather small solid angles (below 10°), the spatial restriction decreases strongly with temperature above T(g), i.e., the distribution of cone angles shifts continuously towards higher values. The β-process in the PC phase of cyanocyclohexane proceeds via the same mechanism as found in structural glass formers. This is substantiated by demonstrating the very similar behavior (for T < T(g)) of spin-lattice relaxation, stimulated echo decays, and spectral parameters when plotted as a function of <log τ(β)> (taken from dielectric spectroscopy). We do, however, not observe a clear-cut relation between the relaxation strength of the β-process observed by NMR (calculated within the wobbling-in-a-cone model) and dielectric spectroscopy.

Probing [formula omitted] with Nuclear Magnetic Resonance echo decay and relaxation: A study on nitrile butadiene rubber

One dimensional 1H NMR measurements have been performed to probe slow molecular motions in nitrile butadiene rubber (NBR) around its calorimetric glass transition temperature Tg. The purpose is to show how software aided data analysis can extract meaningful dynamical data from these measurements. Spin–lattice relaxation time, free induction decay (FID) and magic sandwich echo (MSE) measurements have been carried out at different values of the static field, as a function of temperature. It has been evidenced how the efficiency of the MSE signal in reconstructing the original FID exhibits a sudden minimum at a given temperature, with a slight dependence from the measuring frequency. Computer simulations performed with the software SPINEVOLUTION have shown that the minimum in the efficiency reconstruction of the MSE signal corresponds to the average motional frequency taking a value around the inter-proton coupling. The FID signals have been fitted with a truncated form of a newly derived exact correlation function for the transverse magnetization of a dipolar interacting spin pair, which allows one to avoid the restriction of the stationary and Gaussian approximations. A direct estimate of the conformational dynamics on approaching the Tg is obtained, and the results are in agreement with the analysis performed via the MSE reconstruction efficiency.The occurrence of a wide distribution of correlation frequencies for the chains motion, with a Vogel–Fulcher type temperature dependence, is addressed. A route for a fruitful study of the dynamics accompanying the glass transition by a variety of NMR measurements is thus proposed.

X- and Q-Band Electron Spin Echo Study of Stochastic Molecular Librations of Spin Labels in Lipid Bilayers

Electron spin echo (ESE) of nitroxide spin labels allows detecting fast nanosecond stochastic restricted rotations (stochastic molecular librations), which is a common property of molecules in disordered media including biological systems. Under the typical experimental conditions, the anisotropic electron paramagnetic resonance (EPR) spectrum of a nitroxide is only partly excited by microwave pulses, which allows selecting an anisotropic contribution to the transverse spin relaxation by comparing echo decays at different spectral positions. On the other hand, for low-amplitude orientational motion, the excitation bandwidth is large enough to cover the range of spectral diffusion occurring during the echo formation. To verify that the two-pulse echo decay is indeed related to fast motions, the stimulated electron spin echo can be used. In addition, theory predicts an increase of the relaxation rates at higher microwave resonance frequency. To check this prediction, in the present work we performed a comparative study of ESE decays at microwave X- and Q-bands, for spin-labeled lipids in the gel phase of a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer. A good agreement found between experimental data and computer simulation provides additional justification for the model of fast stochastic molecular librations.

NMR Study of Local and Long Range Dynamics of Adsorbed Water in Zeolite NaY(Br)

AbstractThe dynamics of partially deuterated water in zeolite NaY(Br) are investigated by a combination of several NMR methods for two different loadings, 4 water molecules and 18 water molecules per supercage. Information on local dynamics is obtained by1H and2H NMR spin-lattice relaxation,2H NMR solid-echo spectra and2H NMR stimulated echo decay curves. The water long range diffusion has been studied by1H NMR static field gradient experiments. The most important findings are as follows: The mechanism of water dynamics depends strongly on the degree of loading. This is clearly reflected in both, local and long range motion. The intermediate2H NMR spectra might be explained referring to a model proposed by O'Hare et al.: It is based on two different populations of water molecules performing tetrahedral jumps respectively C2-flips of their OH bonds. Alternative models for the local water dynamics, distorted tetrahedral jumps and a distribution of correlation times, are also discussed.

Rabi oscillation and electron-spin-echo envelope modulation of the photoexcited triplet spin system in silicon

We report on a pulsed electron paramagnetic resonance (EPR) study of the photoexcited triplet state ($S=1$) of oxygen-vacancy centers in silicon. Rabi oscillations between the triplet sublevels are observed using coherent manipulation with a resonant microwave pulse. The Hahn echo and stimulated echo decay profiles are superimposed with strong modulations known as electron-spin-echo envelope modulation (ESEEM). The ESEEM spectra reveal a weak but anisotropic hyperfine coupling between the triplet electron spin and a ${}^{29}$Si nuclear spin ($I=1/2$) residing at a nearby lattice site, that cannot be resolved in conventional field-swept EPR spectra.

Using a multi-frequency acoustic backscatter system as an in situ high concentration dispersion monitor

This paper demonstrates the potential of an in situ acoustic backscatter system (ABS) to measure particle dispersion concentrations in small and large scale mixing vessels. The ABS unit employs 1, 2, 4 and 5MHz transducers that emit ultrasonic pulses and receive the resultant echo backscatter signals, with the strength of the return being related to particle concentration. In small scale studies (where the effect of depth-wise attenuation is effectively ignored), a peak is measured in the strength of the echo responses at intermediate concentrations, due to a balance of the backscatter and attenuation components on the overall signal. The average measured responses were then compared to backscatter theory, which suggested that such analysis is invalid for systems with particle levels greater than ∼2.5g/L and qualitative approaches may be necessary to correlate concentration.More detailed analysis is undertaken in a larger-scale system, where the deviation between expected depth-wise theoretical response and real experimental echo decays are quantified for individual frequencies. It is shown that theoretical estimations heavily over-predict the strength of backscatter echoes at higher particle concentrations, likely due to increasing inter-particle scattering, and such effects are most evident for the highest frequency tested. Because of these limitations, dispersion concentration is correlated using qualitative approaches. For the 4 and 5MHz responses, which had approximated linear depth-wise decays (on a dB scale), it is found that the gradient of the attenuation decay slope (in dB/m) increases linearly with respect to particle concentration, which allowed the formation of a direct correlation relationship. For the 1 and 2MHz responses, the interpolated differential is calculated for specific depth points. Again, a linear correlation is established between the gradient of attenuation and particle concentration, where importantly, it was found that this gradient is independent of dispersion depth. This result highlights the possibility of measuring concentration variation in larger scale systems, simply from the associated differential attenuation changes, and indicates the potential of acoustic techniques for the monitoring and characterisation of industrial multiphase systems.

Rheological and 1H-NMR Spin-Spin Relaxation Time for the Evaluation of the Effects of PPA Addition on Bitumen

Bitumens are currently modelled as a colloidal system and are the most used materials for road paving. Despite this large application, asphalts are still affected by some inconveniences that bring to road deterioration. This is prevalently due to temperature cycling which is related to the given local climate conditions and to the incident traffic load. In the last decades bitumen performances have been improved by means of different types of additives in order to match various expectations. This improvement is often the result of the indications given by traditional empirical standardized tests like penetration grade, temperature ductility and Fraas breaking point. The comprehension of the chemical mechanism that regulates the action of the used additives can greatly help in designing new and better performance materials. By means of a Stress Controlled Rheometer we present a laboratory evaluation of the rheological properties of a 70/100 bitumen which has been doped by different percentage of Polyphosphoric acid (PPA). In addition 1H-NMR measurements of proton transverse relaxation time (T2) have been exploited in order to corroborate the rheological data. As a novel approach to the knowledge of bitumen macro-structures, we applied an Inverse Laplace Transform (ILT) to the measured echo decay. The results show the effect of PPA addition on bitumen mechanical behaviour.

Incorporation of PbF2 into Heavy Metal Oxide Borate Glasses. Structural Studies by Solid State NMR

A series of heavy metal oxide (HMO) glasses with composition 26.66B2O3-16GeO2-4 Bi2O3-(53.33-x)PbO-xPbF2 (0 ≤ x ≤ 40) were prepared and characterized with respect to their bulk (glass transition and crystallization temperatures, densities, molar volumes) and spectroscopic properties. Homogeneous glasses are formed up to x = 30, while crystallization of β-PbF2 takes place at higher contents. Substitution of PbO by PbF2 shifts the optical band gap toward higher energies, thereby extending the UV transmission window significantly toward higher frequencies. Raman and infrared absorption spectra can be interpreted in conjunction with published reference data. Using 11B and 19F high-resolution solid state NMR as well as 11B/19F double resonance methodologies, we develop a quantitative structural description of this material. The fraction of four-coordinate boron is found to be moderately higher compared to that in glasses with the same PbO/B2O3 ratios, suggesting some participation of PbF2 in the network transformation process. This suggestion is confirmed by the 19F NMR spectra. While the majority of the fluoride ions is present as ionic fluoride, ∼20% of the fluorine inventory acts as a network modifier, resulting in the formation of four-coordinate BO3/2F– units. These units can be identified by 19F{11B} rotational echo double resonance and 11B{19F} cross-polarization magic angle spinning (CPMAS) data. These results provide the first unambiguous evidence of B–F bonding in a PbF2-modified glass system. The majority of the fluoride ions are found in a lead-dominated environment. 19F–19F homonuclear dipolar second moments measured by spin echo decay spectroscopy are quantitatively consistent with a model in which these ions are randomly distributed within the network modifier subdomain consisting of PbO, Bi2O3, and PbF2. This model, which implies both the features of atomic scale mixing with the network former borate species and some degree of fluoride ion clustering, is consistent with all of the experimental data obtained on these glasses.

EPR and potentiometric studies of copper(II) binding to nicotinamide adenine dinucleotide (NAD+) in water solution

Coordination of Cu(II) by nicotinamide adenine dinucleotide (NAD+) molecule has been studied in water solutions of various pH by potentiometry and electron paramagnetic resonance (EPR) and electron spin echo (ESE) spectroscopy. Potentiometric results indicate Cu(II) coordination by protonated NAD+ at low pH and by deprotonated NAD+ at high pH. At medium pH value (around pH=7) NAD+ is not able to coordinate Cu(II) ions effectively and mainly the Cu(H2O)6 complexes exist in the studied solution. This has been confirmed by EPR results. Electronic structure of Cu(II)–NAD complex and coordination sites is determined from EPR and ESE measurements in frozen solutions (at 77K and 6K). EPR spectra exclude coordination with nitrogen atoms. Detailed analysis of EPR parameters (g||=2.420, g⊥=2.080, A||=–131×10−4cm−1 and A⊥=8×10−4cm−1) performed in terms of molecular orbital (MO) theory shows that Cu(II)NAD complex has elongated axial octahedral symmetry with a relatively strong delocalization of unpaired electron density on in-plane and axial ligands. The distortion of octahedron is analyzed using A|| vs. g|| diagram for various CuOx complexes. Electron spin echo decay modulation excludes the coordination by oxygen atoms of phosphate groups. We postulate a coordination of Cu(II) by two hydroxyl oxygen atoms of two ribose moieties of the NAD molecules and four solvated water molecules both at low and high pH values with larger elongation of the octahedron at higher pH.

Studies on crude oil‐water biphasic mixtures by low‐field NMR

Low-field (1) H NMR was used in this work for the analysis of mixtures involving crude oils and water. CPMG experiments were performed to determine the transverse relaxation time (T2 ) distribution curves, which were computed by the inverse Laplace transform of the echo decay data. The instrument's ability of quantifying water and petroleum in biphasic mixtures following different methodologies was tested. For mixtures between deionized water and petroleum, one achieved excellent results, with root mean squared error of cross-validation (RMSECV) of 0.8% for a regression between the water content (wt %) and the relative area of the water peak in the T2 distribution curve, or a standard deviation of 0.9% for the relationship between the water content and the relative water peak area, corrected by the relative hydrogen index of the crude. In the case of biphasic mixtures of Mn(2+) -doped water and crude oils, the best result of RMSECV = 1.6% was achieved by using the raw magnetization decay data for a partial least squares regression.

Dynamic Properties of Bicellar Lipid Mixtures Observed by Rheometry and Quadrupole Echo Decay

In bicellar dispersions of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC), the transition from isotropic reorientation to partial orientational order, on warming, is known to coincide with a sharp increase in viscosity. In this work, cone-and-plate rheometry, (2)H NMR spectroscopy, and quadrupole echo decay observations have been used to obtain new insights into the dynamics of phases observed in bicellar DMPC/DHPC mixtures. Samples with 25% of the DMPC component deuterated were used to correlate rheological measurements with phase behavior observed by (2)H NMR spectroscopy. Mixtures containing only normal DMPC (DMPC/DHPC) or only chain perdeuterated DMPC (DMPC-d(54)/DHPC) were used to refine rheology and quadrupole echo decay measurements respectively. The viscosity peaked at 4-9 Pa·s, just above the isotropic-to-nematic transition, and then dropped as samples were warmed through the nematic-to-lamellar transition. Quadrupole echo decay times above the nematic-to-lamellar transition were significantly longer than typically observed in the liquid crystalline phase of saturated lipid multilamellar vesicles. This may indicate a damping of slow bilayer undulations resulting from the coupling of opposite bilayer surfaces by DHPC-lined pores.

CaBe2Ge2 type phosphides REIr2P2 (RE = La–Nd, Sm) and arsenides REIr2As2 (RE = La–Nd): synthesis, structure, and solid state NMR spectroscopy

The phosphides REIr(2)P(2) (RE = La-Nd, Sm) and arsenides REIr(2)As(2) (RE = La-Nd) were synthesized by a ceramic route via precursor compounds REIr(2) with phosphorus and arsenic, respectively. Well-shaped single crystals were obtained from lead and bismuth fluxes, respectively. The nine pnictides crystallize with the tetragonal CaBe(2)Ge(2) type structure, space group P4/nmm. The structures of CeIr(2)P(2), SmIr(2)P(2), LaIr(2)As(2) and CeIr(2)As(2) were refined from single crystal X-ray diffractometer data. The structures are composed of three-dimensional [Ir(2)P(2)] and [Ir(2)As(2)] networks in which the rare earth atoms fill cavities of coordination number 16 (8 P + 8 Ir). The phosphorus and arsenic atoms have tetrahedral and square pyramidal iridium coordination. Temperature dependent magnetic susceptibility measurements show intermediate cerium valence for CeIr(2)P(2). The rare-earth and phosphorus local environments in LaIr(2)P(2) are characterized further by (139)La and (31)P single and double resonance solid state nuclear magnetic resonance (NMR) spectroscopy. Strong (31)P Knight shifts and extremely short spin-lattice relaxation times indicate that the bonding character of the phosphide species is strongly metallic. The two crystallographically distinct phosphorus sites are well-resolved in the (31)P magic-angle spinning (MAS) spectrum and also differ significantly with respect to their effective magnetic shielding anisotropies. Unambiguous site assignments are accomplished on the basis of homonuclear (31)P-(31)P magnetic dipole-dipole interactions, which can be measured in a site-resolved fashion in this compound using static (31)P spin echo decay spectroscopy. The highly symmetric La environment in LaIr(2)P(2) is characterized by a sharp (139)La MAS-NMR spectrum, revealing rather weak nuclear electric quadrupole coupling. Furthermore, a second local environment is detected, which is characterized by stronger quadrupolar coupling and similar dipolar coupling strength with (31)P as the regular site, according to (139)La{(31)P} rotational echo double resonance (REDOR) NMR results. On the basis of these data we attribute this site to a La species next to a phosphorus vacancy. From the signal area of this resonance we deduce a composition LaIr(2)P(1.90).

A mixed basis approach in the SGP-limit

A perturbation method for computing quick estimates of the echo decay in pulsed spin echo gradient NMR diffusion experiments in the short gradient pulse limit is presented. The perturbation basis involves (relatively few) dipole distributions on the boundaries generating a small perturbation matrix in O( s 2) time, where s denotes the number of boundary elements. Several approximate eigenvalues and eigenfunctions to the diffusion operator are retrieved. The method is applied to 1D and 2D systems with Neumann boundary conditions.

Pulsed EPR analysis of tooth enamel samples exposed to UV and γ-radiations

Abstract The electron paramagnetic resonance (EPR) spectroscopy is widely applied for retrospective dosimetric purposes by means of quantitative detection of radicals in tooth enamel and bone samples. In this work we report a study by cw and pulsed EPR on two samples of human tooth enamel respectively irradiated by UV (254 nm) and γ-exposed. The continuous wave (cw) EPR spectra have shown the usual presence in both samples of two types of CO 2 − radicals, with axial and orthorombic g tensors. We have obtained the electron spin echo detected EPR (ED-EPR) spectra at 80 K of the two samples, and we have shown that they are suitable to mark the difference between the effects produced by the different irradiations. At low temperature the contribution to the ED-EPR spectrum of the mobile radical with the axial g tensor is still present in the UV irradiated sample, but not in the γ-irradiated one, where its dynamics is too slow to average the g tensor. We have moreover studied the two-pulse electron spin echo decay on varying the microwave power, a well established method for measuring the Instantaneous Diffusion. We have found that the spectral diffusion parameter is almost the same for both radiation types, whereas the Instantaneous Diffusion is significantly larger for γ-exposed samples than for UV irradiated ones. This difference is due to a higher local microscopic concentration of free radicals for samples irradiated with γ photons.

The enhanced binding energy for biexcitons in InAs quantum dots

We observed that the biexciton binding energy in InAs quantum rhombic disks (QRDs) is enhanced by twice compared with that for InAs quantum dots (QDs) so far reported around 1.24 μm nearby the telecommunication wavelength. The heterodyne-detected four-wave-mixing detected the exciton-biexciton quantum beat superposed on photon echo decay, giving the biexciton binding energy of 3.4 meV to 3 monolayer (ML) InAs QRDs and 4.1 meV to 4 ML InAs QRDs, respectively. The largest biexciton binding energy of 4.1 meV in InAs QDs is ascribed to increased electron-hole overlap in confined geometry with a minimized strain distribution.

Functional Mutations in the Force Generation Region Destabilize the Relay Helix in Myosin

We have used pulsed EPR spectroscopy (DEER) to determine the structural effects of point mutations in the force generation region of myosin (I499A, F506A, F692A, D.discoideum sequence). As it was previously reported, these myosin mutants maintain basal ATPase activity, but completely lose their motor function. It was proposed that F506A, located in the relay loop, disrupts communication between the nucleotide binding site and the force generation region in myosin, and F692A and I499A, located on the interface with the converter domain, affect coordination of the converter domain relative to the myosin head. In this study we assayed the structure of the relay helix in these mutants and in wild-type myosin. The mutations were introduced into the A639C:K498C myosin construct, both Cys were labeled with maleimide spin probes, and interprobe distance was measured. Previous studies showed that probes at these sites detect the nucleotide-induced bending of the relay helix. Observed spin echo decays were interpreted in terms of one or two Gaussian distance distributions, corresponding to one or two myosin structural states. Nucleotide analogs were used to trap myosin in several biochemical states. In the wild type, two structural states, corresponding to pre- and post-recovery stroke, are present in a single biochemical state. The mutations affect the equilibrium between these two structural states and increase the conformational disorder, indicating that these mutants destabilize the structure of the force-generation region, particularly in the post-recovery state. These results provide a structural explanation for the functional perturbation, introduced to myosin.

Fidelity decay for local perturbations: Microwave evidence for oscillating decay exponents

We study fidelity decay in classically chaotic microwave billiards for a local, pistonlike boundary perturbation. We experimentally verify a predicted nonmonotonic crossover from the Fermi golden rule to the escape-rate regime of the Loschmidt echo decay with increasing local boundary perturbation. In particular, we observe pronounced oscillations of the decay rate as a function of the piston position which quantitatively agree with corresponding theoretical results based on a refined semiclassical approach for local boundary perturbations.

NMR Studies of Half-Heusler Type Compounds YbPtSb and LuPtSb

The 121/123 Sb and 195 Pt nuclear magnetic resonance (NMR) results are presented for the half-Heusler compound YbPtSb. The characteristics of localized 4 f spins were observed in the Cure–Weiss type behavior of Knight shifts. The spin–lattice relaxation rate (1/ T 1 ) gradually increases with decreasing temperature and shows a maximum at around 20 K. An approximate analysis of the 4 f spins correlation rate 1/τ f was performed using the measured values of 1/ T 1 and bulk susceptibility. We observed a √T dependence of 1/τ f between 10 and 220 K, which is indicative of an existence of isolated local moment fluctuations. The spin–echo decay rate exhibits a distinct peak at 10 K. We suppose this peak is associated with the magnetic fluctuation originating from 4 f electrons. The crystalline defects in the half-Heusler compounds are also discussed together with the NMR results in LuPtSb.