Spin-echo experiment

This expression can be used for pulsed held gradient spin-echo experiments and also for spin-echo experiments in which the gradient is applied continuously. 

Mims W B 1972 Envelope modulation in spin-echo experiments Phys. Rev. B 5 2409-19... 

In addition to the above techniques, inverse gas chromatography, swelling experiments, tensile tests, mechanical analyses, and small-angle neutron scattering have been used to determine the cross-link density of cured networks (240—245). Si soHd-state nmr and chemical degradation methods have been used to characterize cured networks stmcturaHy (246). H- and H-nmr and spin echo experiments have been used to study the dynamics of cured sihcone networks (247—250). 

APT Attached proton test, a modification of the J-modulated spin-echo experiment to determine C//multiplicities, a less sensitive alternative to DEPT... 

The spin-echo experiment therefore leads to the refocusing not only of the individual nuclear resonances but also of the field inhomogeneity components lying in front or behind those resonances, a maximum negative amplitude being observed at time 2t after the initial 90° pulse. The frequency of rotation of each signal in the rotating frame will depend on its chemical shift and after the vector has been flipped by the 180° pulse, it... 

Figure 2.3 Spin-echo experiment. The behavior of nucleus X in an AX spin system is shown. (A) Application of the second 180° pulse to nucleus X in the AX hetero-nuclear system results in a spin-flip of the two X vectors across the x -axis. But the direction of rotation of the two X vectors does not change, and the two vectors therefore refocus along the —y axis. The spin-echo at the end of the t period along the -y axis results in a negative signal. (B) When the 180° pulse is applied to nucleus A in the AX heteronuclear system, the spin-flip of the X vectors...

Will the vectors of a doublet be refocused at time 2t in a spin-echo experiment ... 

The spin-echo is an elegant method for the measurement of transverse relaxation time Tj. In practice, this is done by repeating the spin-echo experiment many times, with different delay intervals (2t, 4t, 6t,. . . etc.). The following sequence is used ... 

No. Since the direction of precession of the vectors is reversed during the second half of the sequence, they continue to diverge from each other. The effect of a spin-echo experiment on spin-spin coupling in a first-order homonuclear spin system is shown in the following vector representation ... 

The short relaxation time of the 3D spin-echo experiment caused some saturation, otherwise the gain in SNR at the higher field would have been even larger, but at the expense of a longer experiment time. 

One of the fastest growing areas in NMR over the past decade has been the use of pulsed field gradients , or PFG-NMR, for selective ID and 2D experiments. The basic pulsed gradient spin-echo (PGSE) experiment [174] relies on the use of pulsed linear magnetic field gradients (of amplitude g, duration 8 and separation A) that are applied during a spin-echo experiment [184],... 

Modem NMR diffusion measurements are all based on the Stejskal-Tanner (S-T) pulsed field gradient spin-echo experiment [174] in which a gradient pulse is... 

Several NMR pulse sequences have been developed for measuring the diffusion coefficient. The most successful and currently used experiment is the modified spin-echo experiment (PGSE) proposed by Stejskal in 1967.62 As the echo signal is due to the transversal components of magnetic moments, the echo attenuation normalized for the attenuation due to relaxation can be written as63 ... 

Another study of the surface structure of CdSe NCs of 3.7 nm size used 31P MAS-NMR and 31P/77Se rotational-echo double-resonance (REDOR) to identify overlapping broad peaks from two surface species trioctylphosphine oxide (TOPO) at 29.3 ppm and trioctylphosphine selenide (TOPSe) at 22.2 ppm [343]. Both the isotropic chemical shift and CSA of the surface-bound TOPO were substantially different from those of the free ligand. Spin-echo experiments on 31P were stated to indicate an average P-P distance of 8-10 A at the surface, consistent with capping at alternate atomic sites (all Cd but not Se). 

ESE-ENDOR. In ESE-ENDOR, introduced by Mims1181, a rf pulse in a three-pulse electron spin echo experiment is applied during the time interval between the second and third microwave pulse. The ENDOR spectrum is obtained by monitoring the decrease... 

For the basic PFGE experiment a spin-echo experiment (either the two-pulse Hahn echo sequence, Fig. la, or the three-pulse stimulated echo sequence. Fig. lb) is combined with two magnetic field gradient pulses with duration 8 and separated by the time duration A. The gradient pulses generate a magnetic... 

MASNMR. NMR spin-echo experiments indicated that the spin-spin relaxation 29... 

Figure 1 The archetypical REDOR pulse sequence. (A) rotor-synchronised S-spin-echo experiment defining the reference echo amplitude Sq the REDOR pulse sequence in (B) with the additional rotor-synchronised /-channel 7i-pulses provides the signal intensity S.

The angles a and P define the orientation of the sample relative to the Bo-field cor denotes the rotation frequency. For the REDOR reference experiment, the rotor-synchronised spin-echo experiment for the S nuclei (cf. Figure lA), the dipolar Hamiltonian integrated over one rotor period Tr averages to zero... 

Thus, the heteronuclear dipolar information has been reintroduced into the MAS spin-echo experiment. The signal intensity follows from... 

Even if the optimisation of the use of DFS, RAPT or adiabatic inversion pulses is not straightforward for nuclei with low sensitivity, it is nonetheless worth applying one of these methods to improve sensitivity. As long as there is no influence on the CT resonance, these techniques are likely to produce an enhanced CT signal compared to standard spin-echo experiments. Therefore, for Mg (as well as for other insensitive half-integer spin quadrupolar nuclei such as S, K and Ca), it is always advisable to apply some population transfer technique before the excitation of the CT signal. 

R. Siegel, T. T. Nakashima and R. E. Wasylishen, Signal-to-noise enhancement of NMR spectra of solids using multiple-pulse spin-echo experiments. Concepts Magn. Reson., 2005, 26A, 62-77. 

Because T2 tends to be longer for molecules of lower molecular weight, spin-echo experiments tend to be very sensitive to contamination by light, diffusing impurities often found in polymer preparations. Care must be taken to remove these 32) to avoid inviting misinterpretation of experimental data. 

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