Nuclear spin-lattice relaxation in the triply rotating frame and ultraslow molecular motions in solids

A method for measuring nuclear magnetic spin-lattice relaxation in solids in the effective field H_e3 acting in the triply rotating frame (TRF) is described. The method advances the previously described techniques whereby nuclear magnetic resonance and relaxation in the rotating (RF) and doubly rotating frames (DRF) are measured directly. In the present work, the RF and DRF are employed for suppressing the secular part of nuclear dipole-dipole (DD) interactions in the first two orders. As a result, the higher-order DD interactions (four- and five-particle ones) were separated, and their contribution to the nuclear spin-lattice relaxation in the TRF was studied experimentally. The experiments were carried out on protons in polycrystalline benzene. With the introduced technique, an overall spin-lattice relaxation decay in the TRF was recorded continuously during a single radio-frequency pulse with a length not exceeding 1 s. The contribution of multiproton nonsecular DD interactions to the proton spin-lattice relaxation in the TRF was observed selectively as a pronounced local minimum in the temperature dependence of the relaxation timeT _1ϱϱϱ. This contribution corresponds to ultraslow motion of benzene molecules with a rate about γH_e3 ≈ 2π · (10¹-10³) s^-1 and is determined quantitatively by specific correlation functions corresponding to the multiparticle nonsecular DD interactions of protons. The prospects of using this method for studying ultraslow atomic and molecular dynamics in solids are discussed.     

Effect of solvents on carbon-13 spin-lattice relaxation in 1-phenyladamantane

Rotational diffusion of 1-phenyladamantane was studied by means of carbon-13 spin-lattice relaxation. Seven different solvents were used. Rotational correlation times showed a linear dependence on solvent viscosity, except for chloroform and 1,1,2,2-tetrachloroethane which retarded motion of the phenyl moiety through specific interactions.     

EPR and spin-lattice relaxation of rare-earth ions in LiLuF4 monocrystals

The EPR and spin-lattice relaxation are studied of impurity rare-earth ions in LiLuF₄ crystals at liquid-helium temperatures. It is detected that paramagnetic relaxation of Er³⁺ ions is retarded by the effect of the phonon throat. The effect of resonance attenuation of the phonon throat is clarified in two-phonon resonance relaxation processes of Er³⁺ ions. The Debye temperature of the crystal is determined from an analysis of experimental results.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 24–27, February, 1988.     

Exchange-mediated spin-lattice relaxation of Fe3+ ions in borate glasses

Spin-lattice relaxation times (T1) of two borate glasses doped with different concentrations of Fe2O3 were measured using the Electron Spin-Echo (ESE) technique at X-band (9.630 GHz) in the temperature range 2-6K. In comparison with a previous investigation of Fe3+-doped silicate glasses, the relaxation rates were comparable and differed by no more than a factor of two. The data presented here extend those previously reported for borate glasses in the 10-250K range but measured using the amplitude-modulation technique. The T1 values were found to depend on temperature (T) as T(n) with n approximately 1 for the 1% and 0.1% Fe2O3-doped glass samples. These results are consistent with spin-lattice relaxation as effected by exchange interaction of a Fe3+ spin exchange-coupled to another Fe3+ spin in an amorphous material.     

Characterization of molecular motion in the solid state by carbon-13 spin-lattice relaxation times

Relaxation calculations for rapidly spinning samples show that spin-lattice relaxation time (T(1Z)) anisotropy varies with the angle between the rotor spinning axis and the external field. When the rate of molecular motion is in the extreme narrowing limit, the measurement of T(1Z) anisotropies for two different values of the spinning angle allows the determination of two linear combinations of the three static spectral densities, J(0)(0), J(1)(0), and J(2)(0). These functions are sensitive to molecular geometry and the rate and trajectory of motion. The utility of these linear combinations in the investigation of molecular dynamics in solids has been demonstrated with natural abundance (13)C NMR experiments on ferrocene. In an isolated (13)C-(1,2)H group, the dipole-dipole interaction has the same orientational dependence as the quadrupole interaction. Thus, the spectral densities that are responsible for dipolar relaxation of (13)C are the same as those responsible for deuteron quadrupolar relaxation. For ferrocene-d(10), deuteron T(1Z) and T(1Q) anisotropies and the relaxation time of the (13)C magic angle spinning peak provide sufficient information to determine the orientation dependence of all three individual spectral densities.     

Study of vibrational relaxation and molecular reorientation in liquids: a Raman spectroscopic study

The vibrational relaxation and molecular reorientational processes in liquids have been studied in some carbonyl‐containing molecules using the acetonitrile (ACN) solvent. The vibrational and reorientational processes have been studied in correspondence with correlation times. The screening effect due to dielectric has been studied and the Onsager reaction field model has been tested. The study shows that repulsive types of intermolecular forces play an important role in complex systems. Copyright © 2010 John Wiley & Sons, Ltd.     

Some peculiarities of spin-lattice relaxation of impurity rare-earth ions in crystals caused by the structure defects

The spin-lattice relaxation times for Nd³⁺ ions in yttrium-aluminum garnets (YAG) and for Yb³⁺ ions in CaF₂ in the low-temperature range have been measured. For the first system the temperature dependence of the relaxation rate is determined to a great extent by the method of sample preparation. For samples grow by the method of the horizontally oriented crystallization the dependence is described asT ₁ ^?1 =AT ⁿ ,n ? 4.7, which is an evidence of an influence of local structure disordering on the relaxation. The temperature dependence of the relaxation rate in CaF₂:Yb is also “anomalous”:T ₁ ^?1 =AT ^3.3. The results are compared with the previous data on the relaxation in similar systems, and with other cases of observation of “anomalous” temperature dependences. Different manifestations of the local crystal defects in spin-lattice relaxation are discussed.     

NMR relaxation study of molecular motions in liquid chlorobenzotrifluorides

The temperature dependences of the ¹H and ¹⁹F nuclear spin-lattice relaxation times T₁ in liquid o-, m-, and p-chlorobenzotrifluorides have been measured. The analysis of the temperature dependences of the ¹H spin-lattice relaxation times leads to the conclusion that the overall molecular reorientational motion in o-, m-, and p-benzotrifluorides is nearly the same. Data for ¹H and ¹⁹F spin-lattice relaxation times of o-chlorobenzotrifluoride jointly lead to the determination of the individual contributions to relaxation rate in the entire temperature range studied. A knowledge of these contributions for o-chlorobenzotrifluoride, together with the assumption of equal correlation times for overall molecular reorientation in o- and p-chlorobenzotrifluorides, leads to the determination of the spin-internal-rotation interaction contribution to relaxation for p-chlorobenzotrifluoride in the same range of temperature.     

On the fluorescence of methyl salicylate in hydrogen bonding solvents

Two fluorescence bands have been observed in methanolic solutions of methyl salicylate whose relative intensity depends on excitation wavelength. This effect which is not observed in methylcyclohexane is explained by the existence of a small fraction of ester molecules, each with a broken hydrogen bond.     

Approximation of weak stability constants of paramagnetic complexes by spin-lattice relaxation measurements of water protons

The literature was briefly reviewed concernling the determination of stability constants of metal ions with various complexones using water proton relaxation methods. Experimentally, weak stability constants (K _stab) of ethylenediaminetetraacetic acid (EDTA) and citric acid complexes with several transition and lanthanide ions were obtained at constant pHvia measurements of spin-lattice relaxation times (T ₁) of water protons. The results were in general agreement with those determined earlier by various methods. In cases where more than one kind of complex was formed, the experimentalK _stab reflected an average value of all species. Although this method is limited to measurements of weak complexes, it provides experimentally a simple approach with satisfactory results where neither great accuracy nor knowledge of equilibria and other parameters are required. It is believed that this method could be more widely applied for screening new contrast agents for MRI, and for exploration by MRI of weak low molecular weight complexes derived from biological sources.     

Systematic elimination of RF pulse defect errors in Fourier transform spin-lattice relaxation measurements

The elimination of rf pulse defect errors in Fourier transform spin-lattice relaxation measurements is discussed. The standard 180°−τ−90°−t pulse sequence is used. However, the phase of the 90° pulse is shifted by 180° on every repetition of the sequence, while the phase of the 180° pulse is unaltered. Implementation of this phase shifted pulse sequence is discussed for a JEOL PFT-100 single phase detector spectrometer.     

Investigation of the diffusion of hydrogen in palladium by means of spin-lattice relaxation in the rotating frame

The diffusion of hydrogen in palladium $\text{(}\text{H}\text{Pd}\text{= 0.73)}$ has been investigated from 170 to 300K by measurements of the proton spin-lattice relaxation time in the rotating frame, T_1?. In contrast to previous T₁ measurements, a single activation energy of 0.225 eV is obtained, in agreement with the high-temperature T₁ data and with internal friction experiments at about 120K.     

Spin-spin and spin-lattice relaxation of nitrogen in electron irradiated and annealed synthetic diamond

Transient nutation ESR spectroscopy has been used to study the broadening of isolated lines in the triplet of P1 nitrogen centers in disperse synthetic diamond bombarded by electrons and annealed at 670–1070 K. On the basis of measurements of the spin-spin relaxation time, it was established that at nitrogen concentrations up to 1.2×10¹⁹ cm⁻³ the homogeneous line width is caused by dipole interaction between the nitrogen atoms and exchange interactions make no contribution. Fiz. Tverd. Tela (St. Petersburg) 40, 1235–1237 (July 1998)     

Nuclear relaxation,molecular motions and interactions. Part II. 14N nuclear quadrupole relaxation from the1H line-shape analysis,and viscosity measurements of pyrrole in “non-active” solvents

The ¹⁴N relaxation time of pyrrole in cyclohexane, carbon tetrachloride and pentafluororyridine solution has been determined at a number of temperatures through a line-shape analysis of the imido proton pmr signal of pyrrole (scalar relaxation of the second kind). The viscosity of pyrrole in the same solvents has also been measured. The results are discussed in terms of solvent effects on the association equilibria of pyrrole.     

Electron spin-lattice relaxation of Yb3+ and Gd3+ ions in glasses

The electron spin-lattice relaxation rate (T ₁ ^?1) was measured in two glass samples: (i) a phosphate glass doped with 1 wt% Yb₂O₃ and (ii) a Li₂Si₄O₉ glass sample doped with 0.2 wt% Gd₂O₃. In the Yb³⁺-doped glass sample,T ₁ was measured by an electron-spin-echo technique from 4.2 to 6 K, by the modulation method from 10 to 26 K and by the EPR linewidth from 30 to 100 K. It was found thatT ₁ ^?1 αT ⁿ withn=9 in the range 4.2–6 K.n decreased gradually as the temperature was increased and tended towards 2 above 40 K. Over the entire temperature range 4.2–100 K,T ₁ ^?1 was fitted toAT+BT ⁹ J ₈ (Θ _D/T) (whereA andB are two temperature-independent constants,J ₈ is the well-known Van Vleck integral andΘ _D is the Debye temperature). The value ofΘ _D (=46.3±0.9 K) so determined is in good agreement with that of Stevens and Stapleton from theirT ₁ measurements in the range 1.5 to 7 K. In the Gd³⁺-doped glass, it was observed thatT ₁ ^?1 αT over the range 50–150 K. The data for Ye³⁺-doped glass sample were accounted for by assuming that the phonon modulation of the ligand field is the dominant mechanism, associated with a low Debye temperature in accordance with the published data obtained by using other techniques to study lattice dynamics. On the other hand, the data on the Gd³⁺-doped glass sample were explained to be predominantly due to a mechanism involving Two-Level-Systems (TLS)     

NMR spin-lattice relaxation of the129Xe nucleus of xenon gas dissolved in various isotropic liquids

The spin-lattice relaxation time of the¹²⁹Xe nucleus of natural xenon gas dissolved in various isotropic liquids, acetonitrile, benzene, carbon tetrachloride and cyclohexane, was studied as a function of temperature at the magnetic fields of 9.4 and 4.7 T. The utilization of hydrogenated and deuterated benzene and cyclohexane reveals that the intermolecular¹²⁹Xe-¹H dipole-dipole interaction constitutes an important relaxation mechanism in hydrogenated solvents. According to this interpretation the interaction is rather strongly temperature-dependent, and increases with increasing temperature. An important observation of an experimental nature is also noted, namely convective flow present in non-spinning sample tubes at elevated temperatures disturbs inversion-recovery measurements and leads to erroneous and unreliable relaxation time values.     

Membrane fluidity profiles as deduced by saturation-recovery EPR measurements of spin-lattice relaxation times of spin labels

There are no easily obtainable EPR spectral parameters for lipid spin labels that describe profiles of membrane fluidity. The order parameter, which is most often used as a measure of membrane fluidity, describes the amplitude of wobbling motion of alkyl chains relative to the membrane normal and does not contain explicitly time or velocity. Thus, this parameter can be considered as nondynamic. The spin-lattice relaxation rate () obtained from saturation-recovery EPR measurements of lipid spin labels in deoxygenated samples depends primarily on the rotational correlation time of the nitroxide moiety within the lipid bilayer. Thus, can be used as a convenient quantitative measure of membrane fluidity that reflects local membrane dynamics. profiles obtained for 1-palmitoyl-2-(n-doxylstearoyl)phosphatidylcholine (n-PC) spin labels in dimyristoylphosphatidylcholine (DMPC) membranes with and without 50 mol% cholesterol are presented in parallel with profiles of the rotational diffusion coefficient, R_⊥, obtained from simulation of EPR spectra using Freed’s model. These profiles are compared with profiles of the order parameter obtained directly from EPR spectra and with profiles of the order parameter obtained from simulation of EPR spectra. It is shown that and R_⊥ profiles reveal changes in membrane fluidity that depend on the motional properties of the lipid alkyl chain. We find that cholesterol has a rigidifying effect only to the depth occupied by the rigid steroid ring structure and a fluidizing effect at deeper locations. These effects cannot be differentiated by profiles of the order parameter. All profiles in this study were obtained at X-band (9.5 GHz).