Echo double pulse

The REDOR experiment has formed the basis for a large number of ideal pulse type recoupling experiments, and later finite pulse variants, for heteronuclear dipolar recoupling. These include experiments such as frequency selective REDOR (FS-REDOR) [80], TEDOR (Transferred Echo DOuble Resonance) [25], and 3D variants of TEDOR [81, 82], which have found important applications, e.g., for measurement of intemuclear 13C-15N distances in biological solids. We should also mention that rotor-encoded variants of TEDOR, such as REPT, HDOR [83], and REREDOR [84], have been proposed for 1H13C dipolar recoupling under high-speed MAS conditions. 

Rotational-echo double resonance (REDOR), originally introduced by Gullion and Schaefer [102], is a method to recouple heteronuclear spin pairs. The sequence relies on a train of rotor-synchronized n pulses applied to the I spin to interrupt the spatial averaging of the heteronuclear dipolar coupling under MAS to give a nonvanishing dipolar Hamiltonian over a full rotor cycle (Fig. 11.8). Typically, REDOR data are collected by col-... 

OPPOSE double pulsed field gradient spin echo... 

Since its introduction, several other coherence transfer pathways have been employed. The described FSLG-HETCOR and recently proposed MAS-J-HMQC pulse sequence uses heteronuclear dipolar couplings, while the REPT-HMQC employs the rotational-echo double resonance (REDOR) recoupling pulse sequence. 

The Slichter group (see Section IV.F) uses the following terminology the spin echo double resonance (SEDOR) fraction (Sf) for a pulse... 

The double pulsed field gradient spin echo (DPFGSE) pulse sequence has been used to improve the measurement of proton-proton nuclear Overhauser effect (NOE) [28]. The DPFGSE NOE does not rely on difference spectroscopy and very small NOEs can be measured. This technique has been used to determine the structure of organosilicon compounds [28]. 

Figure 6-41 The double pulse field gradient spin echo (DPFGSE) NOE experiment with llp-hydroxyproges-terone (a) the unirradiated spectrum (b)-(g) spectra with irradiation at selected frequencies. (Reproduced with permission from K. Stott,...

Figure 7-5 The double pulsed field gradient spin echo-NOE pulse sequence. The relative strengths of gradients Gi, G2, and Gm are 16,7, and 13 G cm, respectively. The mixing time is 650 ms.

One of the important applications of butylcalix[4]arenes arises from their ability to trap alkali metal ions. In particular, Cs+-calixarene complexes have received much attention because of the need to remove the Cs radionucleotide from nuclear wastes. Benevelli et al. have used one-pulse solid state NMR experiments to directly observe Li, Na and Cs ions in the host cavity [52]. More advanced experiments, which allow the investigation of metal lattice interactions were also reported. Rotational-echo double resonance (REDOR) NMR is a useful tool for obtaining structural details of butylcalix[4]arene [53]. Gullion and coworkers used REDOR to determine the position of the... 

Static spin echo decay spectroscopy also forms the basis for the measurement of magnetic dipole-dipole interactions between two unlike nuclei I and S. While this interaction is refocused by the Hahn spin echo, it can be recoupled by applying a 7i-pulse to the S-spins during the dipolar evolution period [12]. This manipulation inverts the sign of the heterodipolar Hamiltonian, and thereby interferes with the ability of the Hahn spin echo technique to refocus this interaction. The corresponding pulse sequence, termed SEDOR spin echo double resonance) shown in Fig. 4, compares the I-spin echo intensity as a function of dipolar evolution time (a) in the absence and (b) in the presence of the ti(S) pulses. Experiment (a) produces a decay F(2ti)/Fo, which is dominated by homonuclear dipole-dipole interactions, while experiment (b) results in an accelerated decay, reflecting the contribution from the heteronuclear I-S dipole-dipole interaction, which is now re-introduced into the spin Hamiltonian. For multi-spin systems, a Gaussian decay is expected ... 

Fig. 4 Pulse sequence of spin echo double resonance (SEDOR) spectroscopy...

The rotational echo double resonance (REDOR) technique is essentially the MAS version of SEDOR, affording a site-resolved measurement of heteronu-clear dipolar coupling information on rotating samples. Figure 8 shows two commonly used pulse sequences [21-24]. 

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