Spin-locking Sequence Research Articles

The use of multi-pulse nuclear quadrupole resonance techniques for the detection of explosives containing RDX

The multi-pulse method of detecting nuclear quadrupole resonance (NQR) in explosives containing polycrystalline RDX has been investigated. The following sequences were used: a spin-locking type sequence (SLS), a phase-alternated pulse sequence (PAPS) and a sequence without a preparatory pulse. The SLS, PAPS and strong off-resonance comb methods with the same parameters are of great interest for practical applications. The influence of the frequency offset on NQR signals in multi-pulse experiments was studied.

Detection of Double-Quantum Coherences in a Spin- [formula omitted]System Excited by Spin-Lock Sequences

The density matrix of a spin- 5 2 system excited by a spin-lock sequence, { Y} {− X}-[acquisition( y)], is calculated from the equilibrium state to the acquisition period using the program Mathematica. The interaction involved throughout the computation is the first-order quadrupolar interaction. Consequently, the results are valid for any ratio of the quadrupolar coupling ω Q to the amplitude ω RF of the RF pulse. The behavior of the central-line intensities versus the second-pulse length is discussed for several sequences with various phase cyclings. It is shown that double-quantum coherences developed by the first pulse can be detected selectively. Some predictions are illustrated with the 27Al nuclei in a single crystal of corundum α-Al 2O 3.

Cross polarization under fast magic angle sample spinning using amplitude-modulated spin-lock sequences

Amplitude-modulated cross-polarization (AMCP) schemes that can greatly improve the transfer efficiency under fast magic angle spinning are discussed. A novel class of pulse sequences (S-AMCP) that employ an amplitude-modulated spin-lock field on the S-spin channel only and cw irradiation on the I-spin channel is introduced. It leads to efficient cross polarization at the standard Hartmann-Hahn condition while maintaining good spin-lock properties. S-AMCP does, however, not decrease the sensitivity to exact Hartmann-Hahn matching that can become critical under fast rotation. A double amplitude modulation scheme (D-AMCP) that uses amplitude modulation on both radiofrequency channels and significantly broadens the Hartmann-Hahn matching condition is presented. It can, however, be applied only to spin systems with relatively low I-spin homonuclear dipolar interactions due to the absence of an effective spin-locking field. S-AMCP pulse sequences, in contrast, are generally applicable and yield cross-polarization efficiencies that are always better than or equal to the standard continuous-wave cross-polarization experiment.

Double-quantum coherence detection in a spin-3/2 system excited by two consecutive pulses

The density matrix describing the evolution of a spin-3/2 system excited by a spin-lock sequence is calculated from the thermal equilibrium to the end of the second radio-frequency pulse by taking into account only the first-order quadrupolar interaction. It is shown that certain combinations of spin-lock and rotary-echo sequences with appropriate phase cycling detect the double-quantum coherence and the magnetization independently. Moreover, applying the progressive saturation method would alloy the study of spin-lattice relaxation. Some predictions are illustrated with 7Li nuclei in a single crystal of LiTaO3.

Dynamics of phosphate head groups in biomembranes. Comprehensive analysis using phosphorus-31 nuclear magnetic resonance lineshape and relaxation time measurements

Phospholipid head group dynamics have been studied by pulsed phosphorus-31 nuclear magnetic resonance (31P-NMR) of unoriented and macroscopically aligned dimyristoylphosphatidylcholine model membranes in the temperature range, 203-343 K. Lineshapes and echo intensities have been recorded as a function of interpulse delay times, temperature and macroscopic orientation of the bilayer normal with respect to the magnetic field. The dipolar proton-phosphorus (1H-31P) contribution to the transverse relaxation time, T2E, and to lineshapes was eliminated by means of a proton spin-lock sequence. In case of longitudinal spin relaxation, T1Z, the amount of dipolar coupling was evaluated by measuring the maximum nuclear Overhauser enhancement. Hence, the results could be analyzed by considering chemical shift anisotropy as the only relaxation mechanism. The presence of various minima both in T1Z and T2E temperature plots as well as the angular dependence of these relaxation times allowed description of the dynamics of the phosphate head group in the 31P-NMR time window, by three different motional classes, i.e., intramolecular, intermolecular and collective motions. The intramolecular motions consist of two hindered rotations and one free rotation around the bonds linking the phosphate head group to the glycerol backbone. These motions are the fastest in the hierarchy of time with correlation times varying from less than 10(-12) to 10(-6) s in the temperature range investigated. The intermolecular motions are assigned to phospholipid long axis rotation and fluctuation. They have correlation times ranging from 10(-11) s at high temperatures to 10(-3) s at low temperatures. The slowest motion affecting the 31P-NMR observables is assigned to viscoelastic modes, i.e., so called order director fluctuations and is only detected at high temperatures, above the main transition in pulse frequency dependent T2ECP experiments. Comprehensive analysis of the phosphate head group dynamics is achieved by a dynamic NMR model based on the stochastic Liouville equation. In addition to correlation times, this analysis provides activation energies and order parameters for the various motions, and a value for the bilayer elastic constant.

Spin-lock techniques and CPMG imaging sequences: A critical appraisal of T 1p contrast at 0.15 T

The addition of a spin-lock preparatory sequence to a Carr-Purcell-Meiboom-Gill (CPMG) imaging sequence provides a method which allows an accurate and simple comparison of T 1p and T 2 contrast. Sagittal and axial brain images, produced with the application of a three pulse preparatory spin-lock sequence prior to a sixteen-echo CPMG imaging sequence, are compared with images acquired without the spin-lock sequence. The CPMG sequence uses non-selective refocusing pulses. Therefore, observed echo signals accurately reflect T 2 relaxation. This allows a convenient method for assessing the degree to which T 1p and T 2 contrast differ. The spin-lock CPMG (SL-CPMG) images were acquired with a spin-locking field amplitude of 0.4 G and resemble heavily T 2-weighted images at 0.15 T. Quantitative analyses of signal intensities from edema and normal brain tissue confirm the qualitative observations. This in vivo method should prove useful for determining when the additional RF power deposition associated with spin-locking techniques will provide an alternate form of tissue contrast than that available from additional echo collection.

Multiple-pulse investigations in pure NQR spectroscopy: influence of spin-locking and phase alternated pulse sequence (PAPS) on polycrystalline samples containing 35Cl(( I= [formula omitted]) nuclei

The NQR responses if 35Cl ( I= 3 2 ) in KClO 3 and NaClO 3 powder samples to two general categories of multiple-pulse sequences have been experimentally investigated. The sequences are: (i) spin-locking sequence (SLS), i.e. 90 x −[τ-θ ( X+φ) −τ] N and (ii) phase-alternated multiple-pulse sequence (PAPS), i.e. 90 X−[τ-θ − X −2τ-θ X −τ] N . θ φ refers to an rf pulse with pulse angle θ and phase φ. SLS yields spin-locking for arbitrary θ and φ values as longas τ⩽ T 2 and the multiple-echo decay time constant ( T 2ϵ) for this case has been found to be much larger than the spin-spin relaxation time ( T 2) indicating that the resonance lines are narrowed. The observation that the spin-locking is insensitive to φ in powder specimen is a new result and cannot be explained on the basis of existing theories of multiple-pulse spin-locking in NQR spectroscopy. Line narrowing has also been observed with PAPS for arbitrary θ values and for τ values ⩽ T 2. T 2ϵ has been observed to be more sensitive to θ in this case than in the case of the spin-locking sequence.

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.

Relayed-NOE experiments in the rotating frame for sequence analysis of peptides

Proton spectra of peptides consist of a set of spin systems for the individual amino acid residues, which are usually identified via their characteristic J-coupling patterns (I) in 2D correlation spectra. However, the examination of a peptide with unknown amino acid sequence or the presence of several amino acids with identical spin systems requires an assignment of each spin system to its position in the peptide chain. The sequence analysis of a peptide makes use of correlations across the peptide bonds between a-protons and the amide protons of the following amino acid, either via heteronuclear long-range couplings to carbonyl carbons (favorably by COLOC (2) or its inverse variant (3)) or directly via NOE effects (I) (NOESY (4)). However, especially in linear peptides the H, resonances are often not sufficiently resolved for an unambiguous assignment. This serious problem can be circumvented by the use of the Relayed-NOESY experiment (5), advantageously with double-quantum filter (6), where the magnetization transfer via NOE between NH’+’ and HL is followed by a transfer through J coupling from HL to NH’ within the same amino acid residue. This results in a cross peak between NH’ and NH’+’ which appears only on one side of the diagonal due to the asymmetry of the mixing sequence (7) (cf. Fig. 1). Unfortunately, many oligopeptides with molecular weights of about 1000 show only very weak NOE effects in high-field spectrometers due to their unfavorable correlation times. Instead of conventional NOE spectra, ROESY (8, 9) experiments can be used to measure NOE effects in the rotating frame (ROE effects), which are almost independent of the correlation time. We describe here the recently proposed (IO) possibility of using spin-lock sequences in Relayed-NOESY experiments for both the NOE and the Jtransfer part of the mixing time. Figure 2 shows the pulse sequences which are discussed in this paper. The DQF-Relayed-NOESY experiment exists in two versions, which differ in the order of NOE transfer and J trandkr. Since the desired cross peaks always show antiphaae s&uctaue in one dime i$ seems to be favoraMe to have antiphase in w2 because of’tlte bd%cs resduhta in that dimension. This can be achieved by placing ~JVrans~&z&eNOEtrans&ass&owninF~ Z?a.Tbecorrespondingsequence wi~theJ~r~~eNOE~(givingrise~bo~inwl)is~f~ omitted in Fig. 2.

Spin-polarization torsional spectroscopy in beta -phase palladium hydride.

A spin-polarization torsional spectroscopy experiment is reported for three samples of \ensuremath{\beta}-phase palladium hydride (${\mathrm{PdH}}_{\mathrm{x}}$) with hydrogen concentrations x=0.80, 0.75, and 0.71. The measurements were carried out at a proton magnetic-resonance frequency of 30 MHz and a temperature of 40\ifmmode\pm\else\textpm\fi{}0.5 K. Frequency spectra were obtained by Fourier transformation of the transient signals detected in the time domain as a function of the duration of the locking pulse after the spin-locking sequence. These spectra provide numerical values of the two lowest tunneling splittings. Both tunneling frequencies decrease in an approximately linear fashion with increasing hydrogen concentration as expected if the changes are related to the reduction in the number of vacancies to which a proton can tunnel. These data should provide a useful test for future theoretical models proposed to describe phonon-assisted tunneling of hydrogen in metals at low temperatures.

Application of the SPOTS technique to the measurement of proton tunnelling splittings in beta -phase palladium hydride

A proton magnetic resonance experiment known as spin polarisation torsional spectroscopy is reported for a sample of beta -phase palladium hydride with a hydrogen concentration of 0.71+or-0.015. The experiment was carried out at a resonance frequency of 30 MHz and at a temperature of 40+or-0.5K. The transient signal detected in the time domain after the spin-locking sequence and the resulting frequency spectrum obtained by Fourier transformation give definitive proof of the occurrence of low-frequency tunnelling involving groups of two or more protons in this metal hydride sample.

The orientation dependence of dislocation slip in NaCl single crystals

Abstract Results are presented of investigations into the role of dislocation motion in deforming NaCl single crystals, using the technique of pulsed N.M.R. In particular, the spin-locking pulse sequence is used to measure T 1p, which is the spin-lattice relaxation time in the presence of a resonant r.f. magnetic field. T 1p, depends strongly on the mean free path L which is covered by the dislocations during successive jumps between obstacles in their glide plane. The mean free path is determined in crystals compressed in different directions, in which cases either the {110} or the {100} planes are primarily activated. It is shown that both the mean free path, measured as a function of strain rate and of the applied stress, and the work hardening differ remarkably for the two different primary slip planes. The magnitude of the mean free path is compared with the mean distance between dislocations in the crystal and the activation length of dislocations at obstacles, as obtained from mechanical stress relaxation experiments.

Spin-echo methods for resolution control of lanthanide-shifted n.m.r. spectra

At high fields, lanthanide-shifted n.m.r. spectra are severely broadened by chemical exchange; the Carr–Purcell–Meiboom–Gill spin-lock sequence can be used to remove broadened peaks selectively and controllably without distorting the remaining signals.