Spin-lattice Relaxation Time In Rotating Frame Research Articles

Phase behaviour of blend systems containing nitrile copolymers examined by solid-state nuclear magnetic resonance and differential scanning calorimetry

Solid-state cross-polarization magic angle spinning (CP-MAS) nuclear magnetic resonance (n.m.r.) and differential scanning calorimetry (d.s.c.) methods have been applied to examine the phase behaviour of binary blends of acrylonitrile/methyl acrylate/butadiene terpolymer (B210) with poly(ethylene- co-maleic anhydride) (PEMA) and poly(oxycarbonyloxy-1,4-phenyleneisopropylylidene-1,4-phenylene) (PC). Spinlattice relaxation times of the protons in the rotating frame for blends and pure components have been measured by monitoring the CP-MAS generated carbon signal intensities as a function of variable proton spin-lock time. Since single component n.m.r. relaxation behaviour was observed over the entire range of compositions, phase homogeneity was demonstrated for the B210/PEMA system. These monophase blends exhibit single-component rotating frame spin-lattice relaxation times, intermediate in value as compared to the pure components. The d.s.c. examinations also showed that B210/PEMA blends have a single composition-dependent glass transition temperature ( T g). By contrast, the solid state CP-MAS n.m.r. study of B210/PC blends revealed that they display multicomponent relaxation behaviour indicating the presence of a mixed phase along with pure phases of each individual polymer. Additionally, based on the d.s.c. measurements, the T g behaviour of B210/PC blends appears to have a three-component character consistent with a multiphase system.

Effects of paramagnetic lanthanides on the study of carbonaceous deposits on zeolite catalysts by carbon-13 solid-state nuclear magnetic resonanace spectroscopy

Relaxation and spin-counting measurements have been used to probe the extent to which {sup 13}C NMR spectroscopy with cross polarization and magic-angle spinning is a useful technique for characterizing carbonaceous (coke) deposits in zeolite catalysts containing rare-earth cations. Such ions were anticipated to complicate this analysis as a result of paramagnetic shifts and/or relaxation effects. Contrary to expectations, no paramagnetic shifts were observed for carbonaceous deposits in the rare-earth-exchanged Y-type zeolites examined. Even more surprising was that the mixture of rare earths used in industrial catalysts has negligible effects upon the relaxation processes central to cross polarization. Small concentrations of lanthanides with long electron spin-lattice relaxation times (in particular gadolinium), however, have a deleterious effect on characterizing carbonaceous deposits by decreasing the proton rotating-frame spin-lattice relaxation time.

Proton T1 r and diffusion in (Ni 0.5Zr 0.5) 0.95P 0.05H 0.54 and (Ni 0.5Zr 0.5) 0.93P 0.07H 0.58 amorphous alloys

The proton rotating-frame spin lattice relaxation time T 1 r has been measured in (Ni 0.5Zr 0.5) 0.95P 0.05H 0.54 and (Ni 0.5Zr 0.5)Po 0.07H 0.58 amorphous alloys in the temperature range 140–370 K. A single-exponential relaxation decay was obtained from 200 to 370 K only. The experimental data above 200 K were interpreted on the basis of a model involving diffusion of the hydrogen atoms. It is demonstrated that, by using a gaussian distribution of diffusion activation energies about a temperature independent mean energy E ̄ a = 0.32 eV atom −1 and a full width at half maximum ΔE = 0.07 eV, the temperature dependence of T 1r can be well fitted in the 200–370 K range. These values are compared with relevant hydrogen diffusion data from the literature on other amorphous alloys

Ionic reorientations and phase transition in solid IF6AsF6

19F static and rotating frame spin–lattice relaxation times (T1 and T1ρ) as well as broadline NMR studies have been carried out on solid IF6AsF6 between 213 and 393 K. From the T1– and T1ρ–temperature behaviour, we inferred a phase transition at 350(± 2) K and a supercooled high temperature phase. In both phases the second moments are consistent with a model where both IF6+ and AsF6− reorient about random axes. The T1 and T1ρ data in the low temperature phase can be interpreted in terms of a single frequency factor τ0 = 3.15(± 0.2) × 10−16 s and activation energy Ea = 10.4(± 0.4) kcal/mol for both cation and anion reorientation. At the temperature of the phase transition the ionic-reorientational frequencies become faster by ~ 2 orders of magnitude. In the high temperature phase T1 and T1ρ are essentially independent of temperature which may be associated with a spin–rotation relaxation mechanism.

Variation in measured transverse relaxation in tissue resulting from spin locking with the CPMG sequence

It is shown both theoretically and experimentally that the Carr-Purcell-Meiboom-Gill (CPMG) sequence is equivalent to a spin-locking sequence in the limit of short interpulse spacing ( t cp). The increase apparent in the CPMG relaxation time observed in biological tissue as t cp decreases is explained in terms of spin locking. At an effective locking field frequency f 1 = 1 2 t cp the transverse relaxation time ( T 2 † ) measured with the CPMG sequence is indistinguishable from the rotating-frame spin-lattice relaxation time ( T 1 ϱ ). The origin of the variation in measured relaxation times in tissue is discussed in terms of low-frequency dipolar interactions of water.

Oxygen-17 cross-polarization nmr spectroscopy of inorganic solids

We have obtained 17O nuclear magnetic resonance spectra of a variety of 17O-labeled solids (Mg(OH) 2, Ca(OH) 2, boehmite (AlO(OH)), talc (Mg 3Si 4O 10(OH) 2), (C 6H 5) 3SiOH, and amorphous SiO 2) using high-field static and “magic-angle” sample spinning techniques, together with 1H cross polarization and dipolar decoupling. Our results show that large cross-polarization enhancements can be obtained and that reliable second-order quadrupolar powder lineshapes can be observed under cross-polarization conditions. We have also investigated the dynamics of cross polarization for several samples, including measurements of cross-relaxation rates and 1H and 17O rotating-frame spin-lattice relaxation times. We show that rapid 17O rotating-frame spin-lattice relaxation reduces the cross-polarization enhancement in some cases and that differences in cross-relaxation rates can be used to “edit” spectra by selectively enhancing protonated oxygen resonances (in general, hydroxide versus oxide ions, in inorganic solids). When applied to high surface area metal oxides such as amorphous silica, this selectivity enables the observation of resonances from surface hydroxyl groups that are difficult to detect by conventional 17O NMR. Overall, the cross-polarization approach appears to have considerable utility for aiding in the interpretation of 17O NMR spectra of complex inorganic solids.

T1 ρ study of pitch softening

Proton rotating frame spin-lattice relaxation time ( T 1 ρ ) measurements have been made in 10 samples of pitch from room temperature to 673 K to obtain information about the mechanisms of pitch softening. The T 1 ρ minimum of pitch was found to occur at approximately the same temperature as the softening point. This result suggests that the softening phenomenon of pitch can be detected at the molecular level by T 1 ρ measurements. Laboratory frame spin-lattice relaxation time ( T 1) measurements were also made on pitches to obtain information about the molecular motion of pitch in the high frequency ranges.

CP/MAS Carbon-13 NMR Study of Spin Relaxation Phenomena of Cellulose Containing Crystalline and Noncrystalline Components

Abstract Cross-polarization, 13C rotating frame spin-lattice relaxation and C laboratory frame spin-lattice relaxation processes have been studied for different cellulose samples by CP/MAS 13C NMR spectroscopy. It was found that the CP process can be described by a simple thermodynamic model and relative intensities of the respective resonance lines are consistent with the atomic ratios for the spectra obtained at a contact time of about 1 ms. The observed rotating frame spin-lattice relaxation times TC 10 were dominantly dependent on the time constant TD CH by which 13C nuclei were coupled to the 1H dipolar spin system. It was, therefore, impossible to obtain information about molecular

Inter and Intramolecular Contributions to the Proton Dipolar and Rotating Frame Spin-Lattice Relaxation Times in the Nematic Phase of the Liquid Crystal 5CB-d 11

Abstract An isotopic dilution study of the proton dipolar and rotating frame spin-lattice relaxation times (T 1 D and T 1 p) in the nematic liquid crystal 4-cyano-4′-n-pentyl-d 11-biphenyl (5CB-d 11) at 13.8 MHz over the nematic phase temperature range was used to separate the intermolecular and intramolecular contributions to the ring proton relaxation rates. This is the first such study to be reported. Combined with our earlier proton study of inter/intra Zeeman spin-lattice relaxation (T 1) in 5CB-d 11, these data permit a critical study of the soft-mode dynamics in the system. A qualitative explanation of the T 1 D> results is presented, but a complete explanation of the T 1 p results is not possible with existing theory.

Nuclear magnetic resonance study of atomic motions in vanadium

Atomic motions in solid vanadium have been investigated by means of NMR. For that purpose, the Zeeman spin-lattice relaxation time T 1, the rotating-frame spin-lattice relaxation time T 1 ρ , and the Knight shift K of 51V in polycrystalline vanadium have been measured as a function of temperature in the range between 300 and 2000 K. At all temperatures, the relaxation time T 1 is mainly determined by electronic contributions leading to a temperature-dependent expression for the Korringa relation T 1, T · K 2. In contrast to the T 1,-data, two pronounced peaks in the relaxation rate T −1 1 ρ at about 750 and 1600 K, respectively, are observed. The peak at 750 K arises from fluctuations in the nuclear quadrupolar interaction between vanadium and interstitially migrating oxygen whereas the maximum at 1600 K is caused by fluctuations in the nuclear dipole interaction due to atomic self-diffusion. From the low-temperature data, the diffusivity of oxygen is determined to be D 1, = 3.2·10 −6exp(−0.73 eV/ kT)cm 2/s. By comparing the self-diffusion contribution to the relaxation rate with the tracer measurements of Pelleg, the correlation factor ∝ was found to be ∝= 0.77 ±0.11 consistent with a monovacancy diffusion mechanism. The uncorrelated self-diffusion coefficient was obtained as D SD = 0.14 exp(−3.09 eV/ kT)cm 2/s.

An NMR study of 13C-enriched galactose attached to the single carbohydrate chain of hen ovalbumin. Motions of the carbohydrates of glycoproteins.

In order to facilitate the study of motions of the carbohydrate moiety of glycoproteins, a method of enzymatically attaching uniformly 13C-enriched galactopyranose ([U-13C]galactose) onto the nonreducing end of the carbohydrate chain has been developed. Specifically, [U-13C]glucose (20% 13C) was converted through a sequence of four enzymatic steps to [U-13C]UDP-galactose which was then attached, via a beta-1,4 glycosidic bond, to a nonreducing terminal N-acetylglucosamine of hen ovalbumin using the enzyme bovine galactosyltransferase. The overall reaction was followed using 31P and 13C NMR. 13C NMR spectra of the modified ovalbumin show six well resolved resonances arising from the six carbons of the attached [U-13C]galactose. Values of the spin-lattice relaxation time, the line width, the 13C (1H) nuclear Overhauser effect, and the intensity ratio, a measure of the off-resonance rotating frame spin-lattice relaxation time, were obtained for the individual carbons of the [U13-C]galactose as well as other carbon resonances of the protein. Relaxation data for the protein alpha-carbon and aromatic methine carbon resonances could be fit to theoretical values by assuming a correlation time of 25 ns for overall isotropic motion of the glycoprotein. Relaxation data for the anomeric carbon of the attached [U-13C]galactose could only be fit by assuming an additional correlation time between 40 and 80 ps for free internal motion, subject to the constraint that its carbon-proton bond vector makes an angle of about 30 degrees with an effective axis of rotation.

Strong Collision Induced Spin-Lattice Relaxation for Tunneling NH4-ion Systems in the Rotating Frame

Abstract In order to calculate the rotating frame spin-lattice relaxation time T1ρ for tunneling NH4-ion systems at low temperature, a model of relaxation, which is available when γHL τr⪢1, is proposed, where HL is the local dipolar field and τr the correlation time. The present model is useful especially in two cases: (1) when the NH4-ion is relatively magnetically isolated and the assumption of a common spin temperature cannot be used. (2) when the level crossing of the NH4-ion occurs in the rotating frame. In case (1), non-single exponential relaxation is predicted. In case (2), T1ρ is shown to be proportional to τr, while the prediction of the weak collision theory is T1ρ ∞cc τr-1. The present model is compared with the strong collision theory of Slichter and Ailion.

Nuclear magnetic relaxation study of solid sulfolan

Sulfolan has been studied by pulsed NMR measurements of laboratory frame spin–lattice relaxation T1 and rotating frame spin–lattice relaxation times T1ρ in the temperature range from 186 to 339 K. The previously reported phase transition at 288 K has been observed, but the data also show the presence of two additional, as yet unreported, phase transitions. The two crystalline phases are characterized by twofold rotations of the sulfolan molecules while the two plastic phases are both characterized by isotropic rotation and translational diffusion. It is suggested that the lower temperature plastic phase involves monovacancy diffusion while the higher temperature one involves relaxed vacancy diffusion. The relaxation times at most temperatures in the solid phases are dependent on the previous thermal history of the sample, with the transitions being sluggish and displaying hysteresis effects.

The Motionally Narrowed Nmr Line Shape of Ammonium Selenate,

ABSTRACTThe proton free induction decay f(t) of ammonium selenate is measured from temperatures of no apparent effects of lattice motion (−185°C) upon f(t), to substantial motional effects (−145°C) upon f(t). Our measured results are consistent with a classical hindered rotational motion of the ammonium group. The deviation of f(t) from the rigid-lattice fid fo(t) can be expanded in a Taylor series asFrom the Δ1f(t) data, a3 and a4 are derived, and they are well fitted by the simple relationswhere ν is the jump frequency of the rotational motion, T1r is the rotating-frame spin-lattice relaxation time, and ω1 the strength of the rotating magnetic field in frequency units. α, C3 and C4 are calculable constants of order unity.

Methyl group rotation and bonding in solid hexamethyldialuminum, Al2(CH3)6

Continuous wave proton second moment and line shape studies of the solid state trimethylaluminum dimer, Al2(CH3)6, have shown that all the methyl groups in this compound rotate about their triad axes at 77° K. An activation energy of 840±40 cal/mole was found for this methyl group rotational process from the temperature dependent static and rotating frame spin–lattice relaxation times (T1 and T1ρ, respectively) studies. These observations tend to support the two-electron three-center Al–C–Al bond model for the bridging methyl group carbons.

AnomalousH1spin-lattice relaxation in antiferroelectric ammonium dihydrogen phosphate (ADP)

$^{1}\mathrm{H}$ NMR static, and rotating-frame spin-lattice relaxation times and dipolar relaxation times were measured between 135 and 270 K for a single crystal of ammonium dihydrogen phosphate. Random fluctuation in nuclear dipolar interactions by means of ammonium-ion reorientation was found to be the dominant relaxation process. A critical fluctuation contribution to relaxation near the paraelectric-antiferroelectric phase transition, which was postulated in a previous study, was found to be unnecessary to account for the relaxation-time results.

Rotating frame spin-lattice relaxation in the presence of an off-resonance radio frequency field. Investigation of intermediate molecular motions

An NMR relaxation technique is described that should prove valuable in studies of intermediate molecular motions, i.e., these motions having correlation times greater than the inverse of the Larmor frequency. The technique is applicable to multiline spectra, the experiment being carried out on a commmercial Fourier transform NMR spectrometer. The experiment consists of measuring resonance peak intensities in the absence and presence of a radio frequency field applied far off-resonance as a function of the off-resonance frequency. Although it is not necessary to determine relaxation times in order to examine molecular motions, a value for the nuclear spin-lattice relaxation time in the rotating frame with off-resonance rf irradiation, T 1 Q off , may be determined if a value for T 1, the nuclear Zeeman spin-lattice relaxation time, is known. This technique is thus designed to supplement or supercede spin-spin relaxation time ( T 2) and normal on-resonance rotating frame spin-lattice relaxation time ( T 1 Q ) measurements. The utility of the off-resonance T 1 Q technique for investigating molecular motions is illustrated with a viscous liquid, glycerol, at low temperatures.

Nuclear magnetic resonance in lithium metal: Thermal mixing

We present a calculation of the thermal mixing rate ${\ensuremath{\tau}}_{m}^{\ensuremath{-}1}$ between nuclear Zeeman and dipolar reservoirs in the presence of large rf fields. We applied this calculation to slow-motion diffusion in lithium metal. In particular, we measured the rotating frame spin-lattice relaxation time ${T}_{1\ensuremath{\rho}}$ as a function of ${H}_{1}$ and observed the transition between the regions of validity of low-field and high-field theories. Furthermore, we demonstrated that this transition occurs when ${T}_{1\ensuremath{\rho}}\ensuremath{\sim}{\ensuremath{\tau}}_{m}$.

An investigation of “bound” water in frozen erythrocytes by proton magnetic resonance spin-lattice, spin-spin, and rotating frame spin-lattice relaxation time measurements

The water proton spin-lattice relaxation time data for frozen normal and sickle erythrocytes at temperatures between −20 and −80°C can be adequately described by a log-normal distribution of correlation times for water proton motion with a single peak centered at approximately 10 −9 sec at −35°C. However, the dependence of the rotating frame spin-lattice relaxation time ( T 1 ϱ ) on the strength of the radio-frequency field suggests that water proton motions with correlation times greater than 10 −7 sec are present in this system. The magnitude of the spin-spin relaxation time for these frozen erythrocytes (0.1 msec at −55°C) is consistent with the T 1 ϱ data, suggesting that a bimodal distribution of correlation times may be a better model for all of the low-temperature relaxation time data. We find that such a model with peaks centered at about 10 −9 sec at −35°C and about 10 −6 sec at −55°C fits all of the relaxation time data obtained at low temperatures. This bimodal distribution of correlation times for the water motion in frozen erythrocytes is discussed in terms of a model in which the peak centered at 10 −9 sec may represent a “loose” hydration shell and that centered at 10 −6 sec may represent a “tight” hydration shell, and/or water interactions with protein sidechains.

On the anisotropy of T1ρand diffusion parameters in solids

The 19F rotating-frame spin-lattice relaxation time T1 rho has been measured in a single crystal of pure BaF2 from 540 to 1000K. The process responsible for relaxation is F- self-diffusion. A large anisotropy was found that influences the apparent slopes of T1 rho at temperatures above the minimum.