Spin-lattice Relaxation Time T1 Research Articles

Real-time pulsed nuclear magnetic resonance measurement system for the study of nonequilibrium phenomena in polymers

A real-time pulsed nuclear magnetic resonance measurement system controlled by a microcomputer has been constructed. The real-time measurements of the spin-spin relaxation time T2, the spin-lattice relaxation time in the rotating frame T1ρ, and the spin-lattice relaxation time T1 become possible by the use of this system. It is applicable to the studies of nonequilibrium phenomena in polymers such as crystallization, phase separation, gelation, polymerization, etc. It is also applicable to wider fields such as biological reaction and chemical reaction. The application of this system to polymer crystallization is presented as an example.

Order‐Disorder in Solid 1,2,3‐Trichlorobenzene. A Single Crystal 35CI NQR Study

AbstractBy single crystal Zeeman split NQR spectroscopy the nuclear quadrupole coupling tensors, NQCT, of 35CI in magnitude, orientation, and symmetry were studied in the range 140 ≦ T/K ≦ 250. Neither the asymmetry parameters nor the orientations of the NQCTs depend significantly on temperature. The disappearance of the 35CI NQR signal well below the melting point was investigated by studying the line width Δv and the spin lattice relaxation time T1 as a function of temperature. From Δv = f(T) an activation energy of 31 kj mol−1 and 36 kJ mol−1, respectively, was found for the two crystallographically inequivalent molecules in the unit cell. The onset of static order‐disorder or the creation of lattice vacancies may be responsible for the line broadening and the change in T1.

Nuclear spin relaxation studies of the spin-rotation interaction of 13C in CO in various buffer gases

Nuclear spin-lattice relaxation times T1 have been measured for 13C in 13C16O in pure CO gas and in CO in Ar, Kr, Xe, N2, O2, CO2, HCl, CH4, and SF6 gases as a function of temperature. The relaxation is completely dominated by the spin-rotation mechanism so that empirical values of the cross sections for rotational angular momentum transfer σJ are obtained as a function of temperature.

Proton Spin Lattice Relaxation in Vinylidene Fluoride/Trifluoroethylene Copolymer I. Vinylidene Fluoride 72 mol% Copolymer

The proton spin lattice relaxation times T1 of drawn films of a VDF/TrFE copolymer (VDF 72 mol%) were measured for various values of γ, which was the angle between the magnetic field and the draw direction of the film, and for different Larmor frequencies ω, over a temperature range from −23 to 130°C. Three kinds of T1 relaxation processes were observed below the Curie temperature (Tc), around Tc, and at a certain temperature in the paraelectric phase. The curve of ln T1 against reciprocal temperature 1/T was linear for the relaxation process in the paraelectric phase. The T1 became larger as the frequency ω and the angle γ increased.The formulation of T1 in the paraelectric phase for the oriented copolymer was derived, using the non-exponential correlation function which describes a one dimensional diffusion motion of conformational defects along the chain. The existence of the 1-D diffusion motion of the conformation defect was confirmed by a reasonable agreement between the theoretical and experimental results.

Thermotropic ionic liquid crystals. V. Deuterium NMR study of sodium n-alkanoates

Deuterium NMR spectra and spin-lattice relaxation times T1 were obtained as functions of temperature for α-deuterated sodium n-alkanoates (nc=4–7) in the smectic A and ‘‘isotropic’’ liquid phases. The temperature dependence of the orientational order parameter is found to disagree with the predictions of McMillan’s and Woo’s theories for the smectic A phase. Reorientational models are presented to explain the experimental order parameters in terms of the structure of the anhydrous solid or the lyotropic gel phase of these compounds. The same models are used in the analysis of reorientational contributions to the 2D T1 in the mesophase; order director fluctuations are also estimated to contribute significantly to the relaxation times. The 2D T1 temperature dependence in the mesophase resembles that of the 23Na T1 measured previously, implying shared relaxation mechanisms for the two nuclei. Estimated correlation times for 2D relaxation in the isotropic phase provide evidence for the persistence of local structure characterized by motion on a time scale of tens of picoseconds.

Dynamics of helical worm-like chains. VIII. Higher-order subspace approximations to dielectric and magnetic relaxation and fluorescence depolarization for flexible chains

Following the general scheme developed in the preceding paper (paper VII), dielectric and magnetic relaxation and fluorescence depolarization for flexible chain polymers in dilute solution are reinvestigated on the basis of the discrete helical worm-like chain in the higher-order subspace approximation. A comparison of theory with experiment is made with respect to the dielectric correlation time τD, the spin-lattice relaxation time T1, the spin–spin relaxation time T2, the nuclear Overhauser enhancement (NOE), the fluorescence emission anisotropy r(t), the average fluorescence anisotropy r̄, and the fluorescence correlation time τF. It is found that there is agreement between the diameters of the chains determined from these dynamic properties and those from chemical structures, better than in the previous crude subspace approximation, indicating that the theory is remarkably improved in the present approximation. The magnetic correlation time τM is in general not an observable, and therefore an empirical equation to be used for its determination from the observed T1 is constructed. It is then found that there is good correlation between the dynamic chain stiffness τX/τ0X and the static chain stiffness λ−1, where τ0X is the correlation time of the isolated subbody (monomer unit) with X=D, M, and F; τX/τ0X is a monotonically increasing function of λ−1 nearly independent of X as far as perpendicular dipoles are concerned. An explanation of this result is given. However, the dependence of τX on temperature cannot be explained very satisfactorily.

1H NMR Studies on the Reorientational Motions of Cations in Four Solid Phases of Methylammonium Iodide and the Self-Diffusion of Ions in Its Highest-Temperature Solid Phase

Abstract Four solid phases of CH3NH3I named as α′, β′, δ, and ε were studied by the temperature dependence of 1H spin-lattice relaxation time T1 and 1H spin–spin relaxation time T2 (for ε) at several NMR frequencies, and of 1H NMR second moment. Phase transitions were studied by means of differential thermal analysis and 1H T1. The X-ray powder patterns of the highest-temperature ε phase taken at ca. 440 K revealed that the crystal forms a CsCl-type cubic lattice. The measurements of differential scanning calorimetry gave unusually small entropy of melting indicating that the cations of the ε phase are in a state having the most part of their motional freedom. From 1H NMR T1 experiments, it was found that the transformation of the room-temperature α′ phase to the low-temperature stable β′ phase needed longer time than three months at Dry Ice temperature for samples carefully purified and dried for a long time under vacuum whereas it was completed within two weeks for roughly dried samples. The NMR results indicated that the cations are bound tightly in the β′ phase whereas they are fairly free to reorient in the low-temperature substable δ phase. In the α′ phase, the cations perform C3 reorientations very rapidly about their C–N bond axis. The cations of the ε phase undergo overall rotation as well as self-diffusion which was confirmed by electrical conductivity measurements. The mechanism of the self-diffusion of the cations is discussed.

Nuclear-magnetic-resonance determinations of fluorine content and relaxation times in bone powder

The detection of trace amounts of fluoride ions from ground rat-bone samples using a 27-MHz solid-state single-coil pulsed nuclear-magnetic-resonance (NMR) spectrometer is described. The fluorine content in the bone samples as determined from the amplitudes of the free-induction-decay (FID) signals is in good agreement with analyses based on standard chemical and neutron-activation techniques. The value of the spin-lattice relaxation time T1 is 1.46 ± 0.07 s in dry rat-bone powder and is slightly longer in wet bone powder depending on the water content. The contribution of cross-relaxation processes to these results is discussed. The value of the spin–spin relaxation time T2 (as determined from the point where the 19F FID signal has decayed to 1/e of its maximum amplitude) is approximately 75 μs. The Hahn echo sequence [(π/2)0-τ-(π)0] applied to 7 g of bone powder with a fluorine concentration of 3 mg F/g bone in a relatively homogeneous region of the external magnetic field yields echoes that persist for times 2τ up to ~0.5 ms.

A Phase Transition and Molecular Dynamics in Crystalline C6H5CHCI2 by 35Cl NQR Investigations

The NQR frequency ν and the spin-lattice relaxation time T1 of 35Cl nuclei in crystalline (ω,ω)-dichlorotoluene, C6H5CHCl2, have been measured at temperatures above 77 K. The existence of a second-order phase transition was discovered at Tc = 94 K. At this point the low-temperature NQR spectrum consisting of five lines passes into the high-temperature doublet. The behaviour of ν and T1 as functions of temperature near the critical point Tc is investigated and corresponding parameters are obtained. The NQR signals fade out about 185 K due to the reorientational motion with the potential barrier of 33.9 kJ/mole.

Unsaturated polyester resin: kinetics of the cross-linking reaction by pulsed NMR

Pulsed NMR at 20 MHz has been used to monitor the time course of the intermediate-final stage of the radical crosslinking reaction (cure of unsaturated polyester-styrene resins.) The measurement of Spin lattice relaxation time T1 allowed us to follow the hardening process having as a parameter the loss of general mobility of the system at the molecular level.

1H nuclear magnetic resonance studies on structural phase transitions and molecular dynamics of five-membered rings in solid ferrocene, azaferrocene and ruthenocene

The temperature variations of the 1H spin–lattice relaxation time T1 and the second moment M2 of 1H n.m.r. absorptions have been studied for solid ferrocene, azaferrocene and ruthenocene. Differential thermal analysis and 1H T1 measurements have revealed that azaferrocene has a new low-temperature stable phase below 281 K, while its monoclinic phase, stable at room temperature, is easily supercooled to extremely low temperatures to form a metastable phase, and ruthenocene shows a second-order phase transition at 392 K. The 1H n.m.r. results obtained indicate that the five-membered rings of the respective phases of these metallocenes perform in-plane C5 or pseudo-C5 reorientations with different activation energies, which were estimated from the 1H T1 data. For the low-temperature stable phases of ferrocene and azaferrocene, large activation energies for the C5(including pseudo-C5) reorientation of the rings were obtained, indicating that they form closely packed crystals. The metastable phase of azaferrocene indicates the presence of a dipole-glass-like state at lower temperatures for the electric dipoles of C4H4N rings randomly frozen. The solid-state properties of each phase and the mechanism by which phase transitions occur in the three metallocenes are discussed with reference to the dynamics of five-membered rings. A new explanation of the transition entropies reported for solid ferrocene is proposed based on the present n.m.r. results.

13C NMR Spin-lattice relaxation time of poly[vinyl-(1-adamantyl)carboxylate] in solution

1. The selective spin-lattice relaxation times T1 for the carbon nuclei of poly[vinyl-(1-adamantyl)carboxylate] were measured in tetrachloroethylene and the correlation time for internal rotation of the adamantyl group was evaluated. 2. The nature of the intramolecular interactions in this compound was examined.

Tunneling Spectroscopy by Nuclear Magnetic Resonance: Analysis of Rotational Tunneling in Solid Pentamethylbenzene

The frequency dependence of the spin-lattice relaxation time T1 was measured at three temperatures near one of the Zeeman-tunneling level matching resonances for pentamethylbenzene. These measurements are correlated with 71 temperature dependence data from the literature. It is shown that the frequency dependence of the Zeeman-torsion coupling time cannot be explained in terms of the semiclassical perturbation theory using time correlation functions. A three bath polarization transfer model is also employed and the applicability of both models discussed. Zeeman-torsion coupling is further investigated using a saturation sequence and the results are compared with the predictions of the three bath polarization transfer model.

Optical pumping study of GaAs before and after copper diffusion

We have investigated the optical-pumping properties of n-type GaAs and the modification of these results produced by compensation of the crystal by diffusion of copper acceptors. The rate of circular polarization and its decrease as a function of a transverse magnetic field have been measured for the various luminescence lines. We obtain the values of lifetime τ and spin-lattice relaxation time T1 of conduction electrons. Whereas T1 does not change upon copper introduction, τ is found to be decreased by two orders of magnitude because copper acts as a killer center. The compensation of the crystal can be characterized in a straightforward manner from the study of the luminescence polarization as a function of energy in the vicinity of the band gap. Finally, the inhomogeneities of the crystal and the local strains can be investigated respectively from the measure of the nonlorentzian tail of the depolarization curve in a transverse magnetic field and of the polarization of acceptor-related luminescence. All these results show that optical-pumping techniques can allow simple nondestructive characterization of semiconductors.

Motion of CF3 Groups and Phase Transition in [Cu4(CF3COO)4]·2C6H6 as Studied by 1H and 19F NMR Techniques

Abstract The temperature variation of 1H and 19F nuclear spin-lattice relaxation times T1 in [Cu4(CF3COO)4]·2C6H6 was observed at different resonance frequencies. From the NMR experiments, it was found that this complex undergoes a first order phase transition at 130 K, which was confirmed by DTA measurements. In the room-temperature phase, 1H gave normal exponential magnetization recovery curves and logT1 linearly decreased with increasing 103T−1 whereas 19F gave nonexponential magnetization recovery curves, from which long- and short-T1 components could be derived. This indicates that the C3 reorientation of CF3 groups about their respective symmetry axis is responsible for 19F T1 and for 1H T1 as well through 1H-19F cross relaxation. This was confirmed from the coincidence of the logT1 vs. 103T−1 curve of the long component and that of 1H T1. A very small activation energy of 5.5 kJ mol−1 was obtained for the CF3 reorientation. For the low-temperature phase, both 1H and 19F yielded nonexponential magnetization decay curves.

14N1H and 2H1H cross-relaxation in hydrated proteins

The frequency dependence of the proton spin-lattice relaxation time T1 of solid hydrated bovine serum albumin and alpha-chymotrypsin has been measured over 4.5 decades in the range 10(4) to 3 X 10(8) Hz mainly by the aid of the field-cycling technique. The comparison between H2O- and D2O-hydrated samples permitted the distinction of exchangeable and unexchangeable protons. In all cases the 14N1H cross-relaxation dips due mainly to the amide groups have been observed. In addition, in the case of the deuterium exchanged proteins a 2H1H quadrupole dip appears. The amide groups act as relaxation sinks due to the coupling of the amide proton to 14N and adjacent protons. Outside of the dip regions the proton-proton coupling dominates. The fluctuations of the 14N1H and 1H1H interactions are of a different type. The unexchangeable protons show a T1 dispersion outside of the quadrupole dip regions given by the exceptional power law T1 approximately v0.75 +/- 0.05. It is shown that apart from structural information of the 14N spectra, 14N1H cross-relaxation spectroscopy permits the determination of correlation times in the range 10(-7) s less than tau less than 10(-4)S.

Temperature behaviour of35Cl NQR frequencies and spin-lattice relaxation in solid p-bromobenzene sulphony chloride

A semi-rigid molecule model is used to explain both the temperature dependence of the 35Cl nuclear quadrupole resonance (NQR) frequency nu Q(T) and the spin-lattice relaxation time T1(T) in p-bromobenzene sulphonyl chloride. This paper reports measurements of T1(T) over the temperature range 80 to 285K which reflect the appearance of thermally activated semi-external modes (SEM) of the SO2Cl group, giving a strong spin-lattice relaxation for temperatures greater than 160K. The measured temperature dependence of nu Q(T) is explained by assuming the existence of two thermally activated SEM.

Calculation of zero-crossing and spin-lattice relaxation time pictures in inversion recovery NMR imaging.

A method is described which permits accurate pixel-by-pixel estimation of zero-crossing time t10 and spin-lattice relaxation time T1 images from a set of inversion recovery images with different inversion times. The method is effective when at least three inversion recovery images of a particular object sike are available, and does not require starting values or additional knowledge with respect to any of the NMR parameters involved. In the proposed method T1 is calculated from the zero-crossing time t10 of the inversion recovery curve for each point in the image, in a way analogous to the null-point determination of T1 in a conventional NMR experiment.

Nuclear spin relaxation in Li2BeF4

Nuclear spin lattice relaxation time T1 of 19F and 7Li in the solid and liquid phases of lithium tetrafluoroberyllate (Li2BeF4) were measured in the temperature range between 250 and 630 °C by means of pulse NMR technique. Nuclear magnetic resonance absorption spectra were also measured by use of a spectrometer. T1 of 19F and 7Li in the solid phase decreases exponentially as the temperature is increased. The temperature dependence of T1 of 7Li can be attributed to the diffusion of Li nuclei with an activation energy of 1.03 eV while T1 of 19F can be explained by the dipole interaction modulated by the diffusion of F ions with an activation energy of 1.23 eV. In the liquid phase T1 of 19F goes through a minimum at about 550 °C and is expressed as T1−1∝τ/(1+ω2τ2) in terms of a correlation time, τ. We obtained τ=5×10−16 exp (1.08/kT) s. It is suggested that the relaxation mechanism of 19F in the liquid phase is due to the dissociation of F ions from BeF4 complex ions and the long distance diffusion of the F ions in the liquid.

Evidence of hydrogen anti-trapping behaviour by manganese impurities in titanium dihydride from measurements of the proton spin relaxation time

The behaviour of manganese in titanium hydride has been studied by NMR to determine the effects which this alloying addition has on hydrogen distribution and diffusion. Measurements of the temperature dependence of the proton spin-spin and spin-lattice relaxation times T1 and T2 were made on two series of TiHx samples, one at fixed hydrogen concentration with varying Mn content and the other at fixed Mn content with varying hydrogen concentration. The measurements were compared with those made on pure TiHx to determine the effects of the Mn. The relaxation rate R1p due to the proton-Mn-moment interaction increases sharply with increasing hydrogen concentration, showing that Mn, which is not a hydride-forming metal, is avoided by the hydrogen. Increasing the hydrogen concentration forces it closer to the Mn and increases the relaxation rate. Thus, the Mn ions act as 'anti-trapping' centres. At intermediate temperatures the slopes of the R1p curves yield an activation energy Ea=0.11 eV/atom, independent of the hydrogen concentration. This value is much less than that appropriate to regions far from Mn ions where Ea=0.50 eV/atom in both the pure and Mn-alloy samples, showing that hydrogen hopping motion in the vicinity of the Mn anti-traps is much faster than in the bulk. These measurements demonstrate clearly the anti-trapping character of Mn as an alloy addition in a hydride-forming host metal and the strong modification of hydrogen hopping motion that it causes.