Spin-lattice Relaxation Time T1 Research Articles

Anomalies of Spin–Lattice Relaxation and NMR Spectra and Phase Transition in Fluorinated Graphite Intercalation Compounds

AbstractThe anomalies of NMR spectra and spin–lattice relaxation time T1 in a fluorinated graphite layer intercalated with acetone (stage 1) suggest a phase transition and the existence of an “intermediate” phase in the range of fast molecular diffusion. In the high temperature phase T1 and the activation energy Ea do not depend on the molecular content x. Meanwhile, the concentration dependence of Ea and T1 in the “intermediate” phase suggests the essential role of intermolecular interactions leading to cooperative effects. An increase of Ea with decreasing x is interpreted as a consequence of the existence of a molecular sublattice which is incommensurate with the fluorographite lattice. The nature of the phase transition is discussed.

The electronic quantum size effect observed by195Pt NMR in the metal cluster compound Pt309Phen 36 * O30

195Pt NMR on the organic ligand stabilized metal cluster compound Pt309Phen 36 * O30 reveals two separate peaks in the lineshape. Ligand-bonded platinum atoms at the surface of the core are thought to be responsible for the peak that does not show any Knight shift. The corresponding spin-lattice relaxation timeT1 is of the order of seconds. The second peak is Knight shifted and is attributed to the other Pt atoms, for which metallic behavior is inferred from the temperature dependence ofT1. The Korringa relation holds down to 65 K. Below 65 K the relaxation of the magnetization becomes increasingly non-exponential with decreasing temperature. The relaxation process can be successfully modelled under the assumption of a Poisson distribution of the energy levels around the Fermi energy (the electronic quantum size effect).

Spin-orbit coupling, spin-lattice relaxation, and spin-memory studies ofF-center-molecular-ion pairs in alkali halides

The spin-orbit parameter Δ, spin-lattice relaxation timeT1, and spin-mixing parameter ɛ ofFH(OH−) andFH(CN−) centers in several alkali halides have been studied with magnetic circular dichroism at ∼2 K. A close comparison of the experimental results before and after optically induced association of theF center with the molecular ion has been made. In crystals of NaCl structure the negative spin-orbit parameter Δ changes little betweenF andFH centers in the same host. For CsCl and CsBr two values of Δ have been derived forFH(CN−) centers with axis parallel or perpendicular to the magnetic field. In all studied systems, the spin-lattice relaxation timeT1 is already shorter before aggregation compared toF formation. The spin-mixing parameter ɛ decreases slightly forFH(OH−) compared toF centers, while it increases drastically forFH(CN−) defects and reaches its maximum possible value ɛ=0.5 in cesium halides. First attempts to interpret these magneto-optical results will be presented.

Ionic dynamics in the rotator phase of n-alkylammonium chlorides (C6–C10), studied by1H nuclear magnetic resonance, electrical conductivity and thermal measurements

1 H NMR, electrical conductivity and thermal measurements have been performed on the rotator phase of n-alkylammonium chlorides CnH2n+1 NH3Cl (n= 6–10). Rapid two-dimensional self-diffusion and uniaxial rotation of the long rod-like cations were observed in all salts studied by measuring 1H NMR spin–lattice relaxation times T1 and T1ρ, indicating the highly disordered structure of the rotator phase. Marked ionic self-diffusion was detected in this phase by measuring a.c. electrical conductivity which increased to ca. 10–2 S m–1 near the respective melting temperatures. Analysing the NMR relaxation and conductivity data, it was found that the anions also diffuse in this phase to a similar extent to the cations. From the characteristic dynamic behaviour in this phase, together with the small melting entropies (<20 J K–1 mol–1) reported, the rotator phase can be considered to be a low-dimensional plastic crystal or an intermediate state between the plastic crystal and smectic liquid crystal.

Nuclear spin polarization in InSb detected by spin-flip Raman gain spectroscopy

The conduction electron spin resonance (CESR) in n-type InSb is shifted by the spin polarization of the nuclear and electron spins (Overhauser shift). The authors measured this shift by optically detecting the magnetic CESR using coherent spin-flip Raman scattering of two CO lasers in the infrared region. This method is very useful for studying the hyperfine interaction in narrow-gap semiconductors with high effective g-factors. By using a combined CESR/NMR technique, the effective g-factor in InSb was determined at different magnetic fields with the high precision available from NMR experiments. The Overhauser shift is directly proportional to the probability mod Psi (0) mod 2 of finding an electron at the nuclear site of the 115In or the 121Sb isotope, respectively. In thermal equilibrium, when the spin polarization of the nuclei is known, measurements of the Overhauser shift directly yield information on mod Psi (0) mod 2 of the conduction electrons at the respective nuclear site. Observing the shift while either building up dynamic nuclear polarization or studying the relaxation of the nuclei from high spin polarization to thermal equilibrium yields information on the nuclear spin-lattice relaxation time T1 of each of the isotopes.

High-field spin-lattice relaxation of methyl groups: relation to neutron scattering

The modern theory of high-field spin-lattice relaxation is applied to methyl group rotation. It is shown that the same time correlation functions are observed in nuclear magnetic resonance (NMR) relaxation experiments and in inelastic neutron scattering (INS) experiments. A linear relationship is derived between the spectral function S( omega ) observed in INS and the spin-lattice relaxation rate T1-1( omega ) as a function of the Larmor frequency. The similarities between the two methods are pointed out. For CD3, NMR experiments on systems with high tunneling frequencies yield the width of the quasi-elastic Ea to or from Eb line of INS. For CH3, the effect of intermolecular dipolar interactions on the spin-lattice relaxation time T1 is calculated using a series expansion in r/Ri, where r is the radius of the methyl group and Ri is the distance of the considered proton i from the centre of the CH3 group. It is shown that for systems with high tunneling frequencies T1 is mainly determined by intermolecular dipolar interactions, whereas these have a negligible effect on spin conversion. T1 experiments on diluted CH3-containing systems yield the width of the inelastic A to or from E line, also in the so-called 'quasi-classical' temperature regime.

7Li‐NMR Study of Diffusion‐Induced Spin‐Lattice Relaxation in Glassy and Crystalline LiAlSi2O6

AbstractIn order to study the effect of structural disorder on the diffusion of mobile ions, both the glass Li2O · Al2O3 · 4SiO2 and its crystalline counterpart (β‐spodumene) were investigated by 7Li‐NMR. The spinlattice relaxation (SLR) time T1 of 7Li was measured as a function of temperature T from 150 K to about 960K in the glass (glass transition temperature) and 1300 K in the polycrystalline sample. The frequency ν was varied in the range 10 MHz to 100 MHz. Both in the glass and the crystal a diffusion‐induced peak in T1−1(T) was observed. Comparison of the positions of the maxima and the slopes of the low‐T sides of log T1−1 vs 1/T curves shows that the Li+ jump rate in the glass is higher and the activation energy is lower than in the crystal. In both cases the ν and T dependences of the diffusional SLR rate strongly deviate from the BPP model. In particular, the ν dependence of T1−1 on the low‐T sides is sublinear or, at most, linear. The activation energies obtained from the temperature regime above the maximum and those below at various frequencies are briefly compared with corresponding values from a.c. impedance measurements on the same samples.

Pulsed NMRON measurements on insulators

Selective single and double quantum excitation pulsed NMRON has been utilized to obtain rotation patterns, free induction decays, and spin echoes in antiferromagnetic54Mn−MnCl2·4H2O and the quasi-2 dimensional ferromagnet54Mn−Mn(COOCH3)2·4H2O. The pulsed technique is well suited to observing fast spin-lattice relaxation and T1 values down to 100 ms have been measured in these systems. These short times may make magnetic insulators viable hosts for on-line experiments. The dependence on field and temperature of the54Mn spin-lattice relaxation time T1 has been investigated and a T1 minimum at high fields observed in both systems. A spin-spin relaxation time T2≈50 μs is measured in54Mn−MnCl2·4H2O. The observation of NMRON in the paramagnetic phase of MnCl2·4H2O allows the resonant frequencies in the antiferromagnetic and paramagnetic phases to be compared, yielding a value for the zero point spin deviation in the former phase.

Interpretation of 25Mg spin relaxation in Mg-DNA solutions: temperature variation and chemical exchange effects.

The temperature dependencies of line shapes and spin-lattice relaxation times T1 have been measured for 25Mg in dilute solutions of Na-DNA/NaCl containing varying amounts of added magnesium(II) ions. The 25Mg spectrum is clearly non-Lorentzian, due to the presence of motions modulating the quadrupolar interaction that are slow compared to the inverse of the Larmor frequency. The weakly temperature-dependent line shapes and relaxation rates appear to be influenced by the relatively slow exchange of the Mg2+ ions between the DNA surface and the aqueous bulk phase. The observed temperature dependencies depend on the ratio of total magnesium to DNA phosphate, Mg/P. The line shape as well as the temperature dependence of the line width at half height can be qualitatively reproduced with a two-site discrete exchange model for the quadrupolar relaxation of a spin 5/2 nucleus in isotropic solution. The calculations give a value of the lifetime for magnesium bound to DNA of 4 ms at room temperature. Previously reported temperature-dependent 43Ca relaxation measurements in DNA solution can be reproduced under the assumption of a mean lifetime of bound calcium that is not larger than 2 ms but not smaller than 50 microseconds at room temperature. The temperature variation of T1 for 25Mg has been calculated, giving some qualitative agreement with the data. The correlation time for bound 25Mg has been found to be about 40 ns at room temperature.

Inertial and attractive potential effects on rotation of solitary water molecules in apolar and polar solvents

Deuteron nuclear magnetic resonance spin–lattice relaxation times T1 have been measured for solitary water molecules (D2O) at very low concentrations (8–70 mM) in a series of solvents over a wide range of temperatures; the solvents studied are carbon tetrachloride (10 to 40 °C), benzene (10 to 60 °C), chloroform (−40 to 50 °C), acetonitrile (−40 to 50 °C), and acetone (−40 to 50 °C). The orientational correlation times τ2R for D2O with extremely small moments of inertia (I) in CCl4, C6H6, CHCl3, CH3CN, and (CH3)2CO at 30 °C are, respectively, 96, 224, 230, 625, and 826 fs, which are all by far smaller than the bulk value (2210 fs). The τ2R value is proportional to the proton chemical shift of water in each solvent which is taken as a measure of the strength of solute–solvent interactions in the short range. On the other hand, τ2R is almost inversely proportional to solvent viscosity in disagreement with the simple hydrodynamic friction model. The correlation time (τ*) scaled by the free rotator correlation time √I/kBT is 1.20, 1.01, and 0.89 in CCl4, respectively, at 10, 30, and 40 °C, suggesting a strong inertial effect on the water rotation in the weakly coupled solvent cage. In other solvents, the quantity τ* increases with an increase in the water proton chemical shift, and the larger the τ* value the stronger the temperature dependence. The usefulness of the solvent cage model is illustrated and limitations of primitive models so far often used are discussed.

Two-dimensional nuclear magnetic resonance study of phason and amplitudon relaxation mechanisms in structurally incommensurate Rb2ZnCl4

In structurally incommensurate systems phason and amplitudon thermal fluctuations of the frozen-in modulation wave provide for two different nuclear magnetic resonance (NMR) spin-lattice relaxation mechanisms. These two relaxation mechanisms influence different parts of the inhomogeneously broadened NMR absorption spectrum differently. They can be resolved in principle by a conventional one-dimensional measurement of the spin-lattice relaxation time T1 over the NMR lineshape. Here the authors show that a two-dimensional NMR spin-lattice relaxation technique provides a very convenient way of determining the variation of T1 over the lineshape by the use of normalized contour plots. The results for the central transition of 87Rb in Rb2ZnCl4 show that the measured phason-induced spin-lattice relaxation time is temperature-independent. The much less efficient amplitudon relaxation mechanism is bypassed in part by the cross-relaxation process of the amplitudon-relaxed parts to the phason-relaxed parts of the NMR lineshape. The theoretical prediction for the total variation of T1 over the lineshape for the cross-relaxation process agrees with the experimental value of three.

Molecular Motions in Solid [Sb(CH3)4]PF6. A Combined 11H,9F Nuclear Magnetic Resonance and Quasielastic Neutron Scattering Study

Abstract The molecular dynamics of tetramethylstibonium hexafluorophosphate, [Sb(CH3)4]PF6, is investigated over a broad temperature range. NMR spin lattice relaxation times T1 and the NMR second moments of the 1H and 19F nuclei were determined in the range 8.6 ≦ T/K ≦ 332.3 for polycrystalline [Sb(CH3)4]PF6. The complex cation undergoes isotropic tumbling for T &gt; 260 K and thermally activated methyl group rotation in the temperature range T &lt; 196 K. The activation energies for the transition from methyl group rotation to cation reorientation, as derived from NMR wideline (18.1 kJ/mol) and relaxation (22.7 kJ/mol) measurements, match. At very low temperatures pseudo classical line narrowing is observed, indicating tunneling motions of the methyl groups. The existence of two crystallographically inequivalent methyl groups is found by X-ray structure analysis at room temperature. The space group is P63mc, Z = 2; a = 738.6 pm, c = 1089.3 pm. It is confirmed by two steps in the temperature dependence of the signal intensity of the quasielastic line in neutron fixed window measurements in the temperature range 2 &lt; T/K &lt;148. The low temperature spin lattice relaxation times can be explained qualitatively by contributions of two crystallographically inequivalent methyl groups. Apparent activation energies for the two crystallographically different methyl groups are estimated. The complex anion undergoes isotropic tumbling in the temperature range 95 &lt; T/K &lt; 330. Above 330 K additionally translational motion is activated. Below 95 K the rotational motion of PF- 6 is freezing in via an uniaxial state in range 40 &lt; T/K &lt;80. Activation energies for both isotropical tumbling (10.5 kJ/mol) and uniaxial rotation (5.8 kJ/mol) have been derived from 19F NMR spin lattice relaxation

1H relaxation and diffusion of water and their dependence on suspended γ-Al2O3 particles

The 1H spin-lattice relaxation time of water molecules is measured for the aqueous colloidal suspensions of γ-alumina (Al2O3) particles. We observe that the spin-lattice relaxation time T1(c) decreases gradually as the concentration c of the alumina particles is increased. We also observe a parallel decrease in the diffusion constant D(c). We attribute the simultaneous decrease of T1(c) and D(c) as c gets larger to the increase in the effective viscosity of the suspension, as explained in the light of the Bloembergen–Purcell–Pound [Phys. Rev. 73, 679 (1948)] theory of spin relaxation in liquids.

(TiO4)- in alpha -quartz studied by low-temperature electron paramagnetic resonance

The centre (TiO4)-, formed by X-irradiation at 77 K in naturally occurring crystalline rose quartz, has been studied by single-crystal electron paramagnetic resonance (EPR) spectroscopy. Here the central 3d1Ti3+ ion exists within a slightly distorted oxygen tetrahedron. The hyperfine matrix of the low-abundance isotope 49Ti was measured at 8 K and the g matrix was measured at several temperatures from 30 K down to 4 K The analysis of be spin-population distribution within the centre showed that the unpaired spin is almost entirely in the Ti 3d orbital mod 3z2-r2). Estimates of the spin-lattice relaxation time T1 and of the EPR linewidth in be temperature range 4-30 K were also obtained. T1 decreases exponentially with increasing temperature, presumably owing to phonon coupling to the low-lying excited state mod xy) which is 8.1 meV (65 cm-1) above the ground state, via an Orbach process. At temperatures above approximately 15 K the abnormally large EPR linewidth is predominantly caused by lifetime broadening through T1. The near degeneracy of mod 3z2-r2) and mod xy) indicate that (TiO4)- has almost tetrahedral symmetry.

13C and1H NMR study of molecular motion in the deoxycholic acid-ferrocene solid inclusion compound

Using solid-state13C NMR spectroscopy and also measurements of proton spin-lattice relaxation times T1 and second moments M2, we have investigated molecular motion in solid inclusion compounds of deoxycholic acid with ferrocene in the interval 100–400 K. We have identified various dynamic processes occurring with participation of molecules of the matrix and the included molecules. We have shown that in addition to rapid reorientations of the cyclopentadienyl rings, the ferrocene molecules undergo 180° jumps about the C2 axis. We have determined the parameters for all the molecular motions. We discuss the conformational state of the included molecules.

35Cl NQR and Crystal Structure Studies of Salts of Chlorodifluoro- and Dichloroacetic Acid

Abstract The 35Cl NQR spectra of several chlorodifluoroacetates were studied as a function of temperature, including the acid ClF2CCOOH. The cations were: Ammonium, guanidinium, paramethylanilinium. Also some acid salts M⊕ClF2CCOO⊖ • n - ClF2CCOOH ( n &gt; l ) were studied by 35Cl NQR. The bleaching temperatures of the NQR signals were determined. In the para-methylanilinium salt and in the guanidinium salt a phase transition has been observed. The crystal structure of guanidinium chlorodifluoroacetate has been determined at room temperature (a = 1089 pm, 6 = 845 pm, c = 832 pm, space group Pnma, Z = 4). For comparison, guanidinium dichloroacetate was studied by 35Cl NQR and by X-ray diffraction, too: P21/c, Z = 4 , a = 804pm, b = 1202 pm, c = 1080 pm, ß = 131.58°. For guanidinium chlorodifluoroacetate and chlorodifluoroacetic acid, the 35Cl spin lattice relaxation time T1 and the line width have been followed up as a function of temperature. Therefrom, the activation energies of the reorientation motion of the group -CF2C1 have been determined to be 14 kJ • mol-1 (from T1) and 12.5 kJ • mol- 1 (from Δv) for the pure acid and 9.2 kJ • mol-1 and 8.8 kJ • mol-1 , respectively, for the guanidinium salt.

Nucleus-Carrier Interactions in the Group V Semimetals: Dependence on Temperature and Pressure

Abstract A review is given of pulsed NQR spin-lattice relaxation measurements on the rhombohedral semimetals, arsenic (As) and antimony (Sb). The nuclear spin-lattice relaxation time T1 has been measured in the temperature range 100 mK-600 K. The data follow the Korringa relation at low temperatures, but deviations from this relation are found as the temperature is increased. The results are quantitatively explained by considering interactions of the nuclei with the carriers at the Fermi surface. Above the Debye temperature (θD) the data may be understood by including a two phonon process in the analysis. The relaxation data for As and Sb are found to scale with θD. T1 has been measured for As at 295 K in the pressure range 0 - 10 kbar. The results are discussed in terms of changes in the Fermi surface parameters.

Confined geometry effects on reorientational dynamics of molecular liquids in porous silica glasses

This work investigates the relative role of the pure geometrical confinement and the strength of the surface effect on the dynamics of liquids in porous silica glasses prepared by the sol-gel process. The deuteron NMR spin-lattice relaxation times T1 of several molecular liquids in porous silica glasses are reported as function of pore size in the range from 18 to 143 Å over the temperature range from 260 to 310 K. Molecular liquids studied include strongly interacting polar liquids such as pyridine-d5, aniline-d5, and nitrobenzene-d5, whereas the saturated cyclic hydrocarbon liquids of cyclohexane-d12 and cis-decalin-d18 represent the weakly interacting liquids. In a first approximation, toluene-d1 and dioxane-d18 are chosen as examples of liquids with intermediate interactions with the silica surface. The experimental relaxation data are analyzed by using the two-state, fast-exchange model which is found to be valid for the strongly interacting liquids and liquids with intermediate interactions. In terms of this model, the viscosity of the surface layer for pyridine-d5 is about 30 times higher than that for bulk liquid pyridine. The importance of the two-dimensional approach to describe motional dynamics of liquids confined to pores smaller than 30 Å is illustrated in the case of weakly interacting liquid of cyclohexane-d12. Additional information on the relative role of surface interactions and the pure topological effects on the dynamics of liquids in confined geometries was obtained by using surface-modified glasses in which the surface hydroxyl groups were replaced by OSi(CH3)2OC2H5 groups. Indeed, the effects of surface modifications on the 2H T−11 are most pronounced for strongly interacting liquids whereas they are absent for cyclohexane. In agreement with the concept of two-dimensional behavior of liquids in small pores, one finds that the low-frequency relaxation times, namely, the spin–spin relaxation time T2, and the spin-lattice relaxation time in the rotating coordinate frame, T1ρ, remain unchanged by surface modification. In fact, this is a consequence of logarithmic enhancement of the spectral density at low frequencies so that the effect of pure geometrical confinement on the T−12 and T−11ρ relaxation rates is much larger than any relaxation rate changes arising from surface modification. Several selected NMR T1 experiments on pyridine-d5 confined to anopore and zeolites are also presented.

Hippocampal abnormalities in depression

A quantitative magnetic resonance imaging study of regional brain T1 spin-lattice relaxation times in 29 normal volunteers and in 20 patients with major depression revealed significantly shortened T1 relaxation times for the hippocampus in depressed patients. These differences were particularly prominent in elderly depressed patients. T1 relaxation times are reflective of the content and macromolecular environment of tissue water protons; shorter hippocampal T1 values may reflect differences in the content or organizational properties of hippocampal water protons. These findings are consistent with several lines of evidence that have implicated a role for the hippocampus in the regulation of mood and in the pathophysiology of the stress response, and they suggest that major depression may be associated with biophysical tissue changes in the aging hippocampus.

Aging of the human corpus callosum: magnetic resonance imaging in normal volunteers

A brain magnetic resonance imaging (MRI) study of the corpus callosum in 36 normal volunteers, ages 26-79 years, revealed that age was positively correlated with the T1 spin-lattice relaxation time of the corpus callosum and was negatively correlated with the corpus callosum cross-sectional area. T1 relaxation times are sensitive to the content and macromolecular environment of tissue water, and variations in callosal T1 may reflect differences in the content or relaxation properties of callosal water. Such changes also potentially could occur secondary to alterations in the composition of callosal myelin. This study confirms and extends prior morphological studies that have reported age-related dimensional changes in the corpus callosum and, in addition, provides evidence of age-related biophysical tissue differences in the human corpus callosum.