Spin-lattice Relaxation Time T1 Research Articles

Synthesis of polyesters as binders for deinkable inks. I. Structural analysis of the polyesterification betweeno-phthalic anhydride and neopentyl glycol in bulk at 200°C by high resolution13C nuclear magnetic resonance (13C-NMR)

Analysis of the polyesterification in bulk without any external catalyst at 200°C of o-phthalic anhydride with neopentyl glycol (2,2-dimethyl-1,3-propanediol) with a mole ratio ([(SINGLE BOND) COOH]/[ (SINGLE BOND) OH]) = 0.7 has been carried out by high resolution 13C nuclear magnetic resonance (13C-NMR). Polyesters can be analyzed by 13C-NMR spectra because of the fact that both o-phthalic acid (o-phthalic anhydride) and neopentyl glycol carbons are sensitive to sequence effects. Spin-lattice relaxation times T1, of quaternary, tertiary and secondary carbons in different structures are in the 0.1–6.5 s range depending on the neighboring residue effects in the polymer chain. © 1996 John Wiley & Sons, Inc.

Electron spin-lattice relaxation in layered cuprates and fullerides

A new technique of measuring extremely short electron spin-lattice relaxation times T1∼10−6–10−10 sec is developed and applied to study some oxide high-Tc materials, related compounds and fullerides RbC60.

Effect of spin correlation on133Cs nuclear magnetic relaxation in singlet ground state system CsFeCl3

To study spin correlation in singlet ground state system CsFeCl3, we have measured the temperature dependence of the nuclear spin lattice relaxation timeT 1 of133Cs in the field range up to the critical field 7.5T (H//c-axis). In low fieldsT 1 −1 exhibits remarkable temperature dependence, which is essentially expressed by an activation-type equation with the gap energy between the ground- and first-excited-state. The comparison of temperature dependence between the results above and below 5T is discussed and two model is discussed respectively.

Nuclear magnetic relaxation of1H in organic cation radical saltm-MPYNN+X− (X=ClO4, BF4)

The nuclear spin-lattice relaxation timeT 1 of1H in the organic cation radical saltm-MPYNN+X− (X=ClO4, BF4) has been measured in the temperature range 1.5∼200K and in the field range 1∼70kOe. The experimental results in high fields were well interpreted in terms of flip-flop relaxation process involving the pairs of ferromagnetic dimers, whose energy scheme consists of lower triplet- and upper singlet-states with the gap energy of 20K. An appreciable increase of the relaxation rate in low fields below about 10kOe was explained by the direct relaxation process due to the band-like spread of each Zeeman level.

NMR Study of β Relaxation in Modified and Blended Isotactic Polypropylene

Broad line NMR and spin-lattice relaxation times T1 have been used to study glass transition phenomena in isotactic polypropylene modified and blended with an ethylene aminoalkylacrylate copolymer. By means of decomposition of spectra into elementary components corresponding to the chains with different mobility a softening due to the polymer modifier was observed. The modification results in an increase of fractions of amorphous and intermediate chains, respectively, gives rise to the enhancement of the chain mobility in amorphous regions, but the higher amount of the polymer modifier, which was used in the range up to 15 wt%, the lower chain mobility in intermediate regions was found out. From the temperature dependences of the spin-lattice relaxation times T1 measured over the temperature range 290–430 K, a broadening of correlation time distribution on the side of the shorter correlation times has been found out to be and it was ascribed to overlapping of relaxation processes in amorphous regions. The sample containing the highest amount of the polymer modifier, in contrast to others, was shown to blend not only on the molecular level but also on the overmolecular.

Sensitivity Optimization in Continuous-Flow FTNMR

Equations simulating the steady-state magnetization of liquids in continuous-flow FTNMR are derived using a classical vector model, assuming plug flow. These equations are applied to calculation of (S/N)t, the relative signal/noise per unit time of any nucleus undergoing any degree of Overhauser enhancement either in the detection cell or upstream, or both, and to optimization of experimental conditions, including pulse repetition timeTrep, pulse angle β, and flow rate. Ideal parameters include a pulse angle of 90° and aTrepvalue equal to sample residence time in the NMR detection cell. Optimal flow rates are directly proportional to the premagnetization volume (the portion of sample equilibrated with the magnetic field prior to detection) and inversely proportional to spin–lattice relaxation timesT1. Optimal premagnetization times are smaller than previously assumed, varying from about 1.1 to 1.9T1values. (S/N)tfor static FTNMR is discussed in some detail, and a new graphical method is presented for its optimization. Flow advantage, the (S/N)tof optimized flow FTNMR experiments compared to that of static FTNMR in a given detection cell, is proportional to the square root of the ratio of premagnetization to detection cell volumes, and virtually independent of[formula]where[formula]is the apparent transverse-relaxation time. The theory is applied to examples from recent literature, including dynamic electron–nuclear polarization, and the literature is critically reviewed. The analysis shows that claims by previous authors of recycled flow FTNMR by itself leading to increased (S/N)tfor slowly relaxing resonances are misleading, owing to underdetermination of (S/N)tin static measurements and failure to account for greater sample sizes required in flow experiments. For monitoring and control of chemical processes, the theory presented here enables the first rational basis for the design of a flow FTNMR apparatus and for the selection of acquisition parameters.

Dipolar interactions in deuteron spin systems. I. Spin diffusion

The influence of dipolar interactions on the longitudianl relaxation of deuteron spin systems is investigated. Spin diffusion rates are evaluated, including approximate rates due to double quantum spin diffusion and three-spin flip–flop transitions. It is shown that slow molecular rotations in supercooled liquids do not affect the spin diffusion rates significantly provided that the motional correlation times are below the average spin lattice relaxation time T1 which becomes on the order of one second close to the glass transition temperature Tg. However, the broad distribution of deuteron T1 values at T<Tg results in a large effect of spin diffusion upon the long time decay of the longitudinal magnetization in T1 experiments. These effects are estimated in terms of a simple model in agreement with recent experiments. It is also shown that the initial decay determining the average rate 〈T−11〉 remains unaffected by spin diffusion. Finally, we show that small amplitude motions on a time scale of 10−6–10−3 s may cause temperature dependent spin diffusion effects.

Diffusion in Fe(II/III) radiation dosimetry gels measured by magnetic resonance imaging

We analyse the diffusion problem in the traditional Fe(II/III) agarose gel system employed in MRI studies of radiation dosimetry. The diffusion coefficient is measured using an inversion recovery null-point imaging method in a model gel/water phantom. The diffusion coefficient of Fe(III) in 1% agarose gel at pH 1.1 is D=2.7+or-0.3*10-6 cm2 s-1. The diffusion coefficient of Fe(II) is D=3.3+or-0.5*10-6 cm2 s-1. Measurement of the diffusion coefficients permits simulation of the MRI signal intensity from phantoms with model radiation dose distributions. We allow for diffusion of both Fe(II) and Fe(III) in our simulations as well as the effect of both relaxation agents on the local spin-lattice relaxation time T1. We also analyse the effects of the physical penumbra on the diffusion problem.

Quantum and classical dynamics of a methyl group with tunneling frequency 3.4 MHz studied by low field NMR

Nuclear magnetic resonance (NMR) investigations of methyl tunneling in 2,5-dimethyl-1,3,4-thiadiazole are reported. The tunneling frequency (νt = 3.39 MHz) was obtained using low field NMR spectroscopy. By means of rapid field cycling irradiation and relaxation measurements the probabilities of the absorption transitions, responsible for the spectral lines in the low field NMR spectra, can be quantified. The results obtained agree well with the calculated probabilities of rf induced transitions between the eigenstates of a methyl rotor with νt = 3.39 MHz. Measurements of the temperature dependence of νt, the spin conversion time τcon and an analysis of the proton spin lattice relaxation time T1, the latter two revealing the correlation time τc, enabled the study of the methyl group dynamics over a temperature range encompassing both the quantum mechanical and the classical regimes. The dynamical data can be explained well with a threefold hindering barrier of height V3/kB = 1175 K and are compared with existing theoretical models.

Measurements of the effect of storage at various temperatures on the T1 of ex vivo tissues

The detailed measurements of the effects of storage at various temperatures on the nuclear magnetic resonance (NMR) spin-lattice relaxation time T1 are reported. On the basis of the results, a method to monitor tissue transformation in hyperthermia of biological systems using magnetic resonance imaging (MRI) techniques is discussed. The method was found on proton relaxation time spin-lattice T1 variation measurements. Parameter F described as tissue structure transformation and/or protein denaturation rate index is discussed.

Inversion-recovery detected EPR

Inversion-recovery detected EPR is introduced. This field-swept EPR method is based on a periodically repeated inversion recovery experiment, with detection of the first harmonic of the periodically oscillating transverse magnetization. With inversion-recovery detected EPR, pure absorption spectra are obtained with signal amplitudes depending on the repetition rate of the inverting π pulses and on the strength of the probe field. The technique is particularly well suited for the measurement of broad EPR lines and for separating overlapping spectra of paramagnetic species with different spin-lattice relaxation timesT 1. The experiment is analyzed theoretically and some of the predicted features are experimentally verified. Inversion-recovery detected EPR spectra of a single crystal, a polycrystalline and a liquid solution sample are presented, and the separation of overlapping spectra is demonstrated.

Analysis of complex 1H nuclear magnetic resonance relaxation measurements in developing porous structures: A study in hydrating cement

1H nuclear magnetic resonance (NMR) is widely used for the study of water kinetics into various porous systems like sandstones, zeolites, clays, and various cementitious materials. In the case of hydrating cement pastes the evolution of the cement structure ultimately leads to a broad pore size distribution, and consequently to a multiexponential magnetization recovery represented in many cases by a Kohlraush–Williams–Watts ‘‘stretched exponential’’ curve. However, it has been proven an extremely difficult task to fully clarify and exploit the information contained in the multiexponential magnetization recovery of cementitious materials. This difficulty is due to the fact that spin-lattice relaxation times T1 exhibit broad multimodal distributions, which sometimes extend over more than two orders of magnitude. In this work, a numerical routine using a combination of Monte Carlo and Simplex iterations (MCS) is examined which appears to estimate faithfully broad and complex spin-lattice relaxation time distributions. First, the reliability of the routine has been tested on simulated data and then the algorithm has been applied to the analysis of 1H NMR time domain measurements performed on different kinds of cement paste. The development of the pore structure is directly reflected in the time evolution of the T1 distribution function.

129Xe and131Xe NMR of xenon on silica surfaces at 77 K

Little is known about129Xe NMR spectral features and spin-lattice relaxation behavior, and the dynamics of xenon atoms, for xenon adsorbed on solid surfaces at cryogenic temperatures (≤77 K), where exchange with gas-phase atoms is not a significant complication. We report129Xe NMR experiments at 9,4 T that provide such information for xenon adsorbed onto the hydroxylated surface of a number of microporous silica samples at 77 K. A convenient design for these cryogenic experiments is described. Dynamics of surface-adsorbed xenon atoms on the time scale of seconds can be observed by129Xe NMR hole-burning experiments; much slower dynamics occurring over hours and days are evidenced from changes with time of the129Xe NMR chemical shifts. The peak maxima occur in the region ca. 180–316 ppm, considerably downfield of129Xe shifts previously reported on surfaces at higher temperatures, and closer to the shift of xenon bulk solid (316.4±1 ppm). The129Xe spin-lattice relaxation timesT1 range over five orders of magnitude; possible explanations for both nonexponential relaxation behavior and extremely shortT1 values (35 ms) are discussed. Preliminary131Xe and1H NMR results are presented, as well as a method for greatly increasing the sensitivity of129Xe NMR detection at low temperatures by using closely-spaced trains of rf pulses.

Spin-lattice relaxation and magnetization transfer in intracranial tumors in vivo: effects of Gd-DTPA on relaxation parameters.

Spin-lattice relaxation time T1 and relaxation parameters in magnetization transfer (MT) imaging were measured in 11 intracranial tumors before and after injection of Gd-DTPA at 0.1 T by using the inversion recovery method and the saturation transfer technique, respectively. Preinjection T1 relaxation times of the tumors were longer than those of white matter, but after Gd-enhancement the relaxation times of most tumors were in the same range as those of white matter. Gd-DTPA shortened the apparent relaxation time in the presence of off-resonance saturation pulse (T1a) due to marked shortening of the relaxation time of mobile water (T1w). Gd-DTPA decreased the magnetization transfer contrast (MTC) but did not influence on the magnetization transfer rate (Rwm). The parameters MTC and Rwm differed clearly between Gd-enhanced tumors and normal brain, whereas the relaxation time T1a was in many Gd-enhanced tumors in the same range as in normal brain.

Estimation of water content and water mobility in the nucleus and cytoplasm of Xenopus laevis oocytes by NMR microscopy.

NMR microscopy is a noninvasive approach for studying cell structure and properties. Spatially resolved measurements of the relaxation times T1 and T2 provided information on the water proton spin density and water mobility in different parts of Xenopus laevis oocytes. The spin-lattice relaxation time T1 was determined using a saturation-recovery sequence and the common spin-echo sequence with increasing repetition times, while the transverse relaxation time T2 was measured by means of the spin-echo sequence with varying echo times. From the relaxation times, the mole fractions of possible reorientational correlation times tau c for different types of intracellular water were calculated according to a simple two-phase model. The values for T1, T2, and proton spin density (i.e., water content) are: nucleus >> animal cytoplasm > vegetal cytoplasm. Based on the estimation of tau c, nearly 90% of the nuclear water and 74.4% of the water of the animal pole was considered as free mobile water, whereas 55.5% of the water of the vegetal pole appeared as bound water.

Deuteron NMR of Methyl Groups in theTunneling Regime. A Single Crystal Study of Aspirin-CD3

We report the first single crystal deuteron NMR spectra of CD3 groups which display the socalled ±ß, ±(|α| ± ß) and ±(2|α| ± ß) lines characteristic of rotational tunneling in a sufficiently clear manner to allow a quantitative comparison with the respective theory developed in 1988 by the group of W. Müller-Warmuth. The molecular system we study is aspirin-CD3. We recorded spectra for differently oriented single crystals and measured spin-lattice relaxation times T1 in a wide temperature range. At 12.5 K we exploit the dependence of the ±(|α| ± ß) and ±(2|α| ± ß) lines on the orientation of the applied field B0 for determining the equilibrium orientation of the CD3 group in the crystal lattice. The spectra display features which allow, by comparison with simulated spectra, a measurement of the tunnel frequency vt. Its low temperature limit is (2.7 ± 0.1) MHz. It allows to infer the height V3 of the potential V(φ) in which the CD3 group moves, provided that this potential is purely threefold. We get V3 = (47.2 ± 0.5) meV. The transition from the tunneling to the classical, fast reorienting regime occurs in the 15 K ≲ T ≲ 35 K temperature range. In this range we observe a broadening, merging and eventually narrowing of the ± |α| and ±2|α| lines in very much the way predicted by Heuer. His theory, however, must be extended by taking into account all librational levels. The behaviour of the ± ß lines in the transition temperature range signalizes a reduction of the observable tunnel frequency with increasing temperature. This reduction allows an independent measurement of the potential height and represents a test of the assumption of a purely threefold potential. From the T1 -data we derive the temperature dependence of the correlation time Ƭc of the reorientational jumps. The plot of log Ƭc vs. 1 /T follows a straight line for more than five decades. From its slope we get yet another independent number for the potential height. It agrees well with the other ones, which confirms the assumption of the essentially threefold potential V(φ) in aspirin-CD3.

Non-invasive monitoring of tissue response to radiotherapy by localised proton spin-lattice relaxation time in mice

Proton spin-lattice relaxation time T1 of the localised tumour portion and its neighbouring portion of tumour-bearing mice legs is successively observed before and after radiotherapy by applying the previously proposed magnetic focusing method. Changes in T1 values are also compared with the histological radiation effects. For un-irradiated tumours, T1 increases gradually with tumour growth. T1 of the tumour portion shows significant decrease after radiotherapy. The decrease in the T1 values of the tumour clearly depends on the single radiation dose of 30, 45 and 60 Gy, respectively. A rapid decrease in the T1 values of the tumour occurs prior to the decrease in tumour volume. These decreases in the T1 values of the tumour after radiotherapy correspond positively to the increase of the histological radiation effects. After the reduction in the T1 values of the irradiated tumour portion, a successive increase in T1 values is observed and suggests local recurrence of the tumour. These results show that localised T1 relaxation time is a good reflection of the tissue response to radiotherapy.

Direct measurement of electron spin-lattice relaxation time of paramagnetic centers and non-linear modulated responses in HTSC

The report consists of two parts. 1. The method is described to measure directly the electron spin-lattice relaxation timeT 1 in high-T c superconductors (HTSC). The technique is based on registering the oscillating longitudinal spin magnetization while saturating the EPR line by an amplitude-modulated microwave field. TheT 1 values of the Cu2+ centers in YBa2Cu3O6+x (x=0.24−0.9) and Y2BaCuO5 materials are measured and found to be about 1.4·10−9 s independently ofx and temperature in the range 77–300 K. Besides much longer relaxation is displayed in degradated materials. The data obtained can be considered as an argument in favor of the “green-phase” origin of the “high-temperature” EPR spectra in the YBaCuO HTSCs. 2. Non-linear responses of the microwave (1010 Hz) and rf (105–107 Hz) absorption in HTSC materials to low-frequency magnetic modulation are studied and found to be quite different in the two frequency ranges. It is shown that at microwaves the effect is caused by non-linear interaction with shielding supercurrents whereas at lower frequencies it is due to jumps of fluxons over potential barriers. The models developed take into account the Josephson weak links and the thermo-assisted flux creep.

1H NMR Study of Water Adsorbed on Carbon Powders

1H NMR relaxation measurements are performed for H2O molecules adsorbed on the surface of carbon powders. Spin-lattice relaxation time T1 shows a minimum at T ∼ 185 K, where fast decrease in spin-spin relaxation time T2 occurs, and this corresponds to the slowing of intramolecular rotational motions. The T2 shows a maximum at a higher temperature, T ≍ 250 K. At the temperature where the T2 maximum occurs, the spin-lattice relaxation time in the rotating frame, T1ρ, also shows a maximum. The Carr-Purcell-Meiboom-Gill and stimulated echo measurements indicate the presence of nonconventional diffusion effects. These anomalies are interpreted as originating from the restricted diffusion of H2O on the surface of carbon particles with the size of restriction about 20 Å, the diffusion constant equal to (6 ± 2) × 10−12 cm2/s, and a field gradient of (4 ± 1.5) × 105 Oe/cm, measured at room temperature.

Influence of γ‐irradiation on proton spin–lattice relaxation of SBR used as antirads and its ESR investigation

AbstractProton spin–lattice relaxation time t1 was measured on SBR samples with carbon black or kaolin filler using modified linseed oil. The NMR pulse technique at 90MHz was used in the temperature range from 180 to 400 K. The temperature dependence of t1 indicates that samples filled with carbon black have similar molecular dynamics to the standard unfilled SBR samples. The activation energy for the motion of the main chain for these samples amounts to 16.4kJ/mol. Samples containing linseed oil modified with para‐toluidine showed an activation energy of about 14.6kJ/mol and were not affected by γ‐irradiation. Values of the minimum relaxation time tmin1 were increased by γ‐irradiation in comparison with a standard SBR sample. ESR measurements carried out at room temperature by means of an X‐band spectrometer indicated that unidentified radicals within the rubber were formed during its mastication with vulcanizing additives. The ESR spectra did not change during the vulcanization process. Samples filled with carbon black showed a broadening of the ESR line; this is consistent with the increase in the electrical conductivity.