REDOR pulse sequence

Figure 2. Pulse sequence for 13C-serve REDOR NMR. This sequence differs from the original REDOR pulse sequence (ref. 18) in that n pulses alternate between 13C and 15N r.f channels. On alternate scans of the REDOR experiment, the 15N iz pulses are either applied or omitted. This figure illustrates that the REDOR pulse sequence with four rotor periods of 13C-15N dipolar-coupling evolution (Nc = 4) NC can be increased (in increments of two) by adding rotor periods and pairs of 13C and 15N n pulses between the end of the cross-polarization preparation and the start of data acquisition.

Fig. 11.8 (a) REDOR pulse sequence for the determination of dipolar couplings between 13C and 15N. Initially 13C polarization is generated by cross polarization from the protons. During the following evolution period n pulses are used to prevent the averaging of the heteronuclear dipolar... 

In our context, especially C-Li distances are of interest. A first successful Li- C REDOR study was undertaken in order to establish the structure of the previously discussed TMEDA complex of fluorenyllithium °, prepared from Li-enriched w-butyllithium and fluorene with at natural abundance. The REDOR pulse sequence used is depicted in Figure 20. The number of rotor cycles is increased in a symmetric fashion about the central jr-pulse in order to increase the dephasing time. 

FIGURE 20. The first two experiments in the REDOR pulse sequence, where the dephasing delay is increased by adding more rotor periods and Li jr-pulses symmetrically around the jr-pulse. Reproduced with permission of Blackwell Pubhshing from Reference 221... 

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.

Thus, from a parabolic fit to the REDOR evolution data, the second moment can be evaluated. As mentioned in Section 1, this analysis has to be restricted to the initial part of the evolution curves AS/Sq <0.3, as exemplified in Figure 2. However, the first order approximation entails a systematic imderestimation of M2, as shovm by Bertmer and Eckert. Numerous variations of the original REDOR pulse sequence have been established to adapt the technique to specific needs. To accoimt for pulse imperfections and other experimental errors, Chan and Eckert introduced compensated REDOR. In this approach, an /-channel 7r-pulse in the centre of the pulse sequence cancels the reintroduction of the 7-S dipolar couplings hence the echo amplitudes are solely attenuated by the... 

Figure 3.2.20 Scheme of REDOR pulse sequence. Dephasing curve 13C magnetization in 13C- and 15N-labeled glycine as a function of the length of the dephasing time T. A S/S0 (the REDOR difference) is the normalized difference between the 13C signal intensities with and without the 15N pulses. Reproduced with permission from Ref. [51]. 

These dipolar-transition frequencies are time-dependent and repeat the cycle in the spinning. In the REDOR pulse sequence, a 7r-pulse is applied in the centre of the rotor period. In this case, the averaged angular velocity over one rotor cycle for each resonance is given by... 

The procedure that is useful for intermolecular distance determination in the solid materials is termed REDOR (rotational echo double resonance) experiment [25,26]. It allows recovering dipolar interaction under MAS. REDOR is a spin-echo double-resonance experiment. A typical REDOR pulse sequence is shown in Fig. 5B. The rotor-synchronized echo sequence is applied to the spin I system and the echoes are detected after the time 2t equal the even number n of rotation periods t, . For recoupling the dipolar interaction between the spins I and S, r-pulses are applied to the spin S system at every half rotation period Tj. The dipolar coupling is determined by measuring the REDOR fraction AS/S, which describes the decrease of the echo amplitude 5 as a function of the number of rotation periods n [25] ... 

Figure Bl.12.15. Some double-resonance pulse sequences for providing distance infomiation in solids (a) SEDOR, (b) REDOR, (c) TEDOR and (d) TRAPDOR. In all sequences the narrow pulses are 90° and the wide pulses 180°. For sequences that employ MAS the luimber of rotor cycles N is shown along the bottom.

Figure 7. REDOR 13C NMR spectra of poly (aery lie acid) (PA) imbibed with [3-13C]Ala/[15N]Ala (50 1 1 by weight). The bottom curve represents the echo spectrum of full sample (S0) The top curve is the REDOR difference. (AS). Spectra were collected using the pulse sequence of Figure 2 with VR = 3 kHz NC = 30.

Fig. 5 Radio frequency pulse sequences for measurements of Sj and Si in DSQ-REDOR experiments. The MAS period rR is 100 ps. XY represents a train of 15N n pulses with XY-16 phase patterns [98]. TPPM represents two-pulse phase modulation [99]. In these experiments, M = Nt 4, N2+ N3 = 48, and N2 is incremented from 0 to 48 to produce effective dephasing times from 0 to 9.6 ms. Signals arising from intraresidue 15N-13C DSQ coherence (Si) are selected by standard phase cycling. Signal decay due to the pulse imperfection of 15N pulses is estimated by S2. Decay due to the intermolecular 15N-I3C dipole-dipole couplings is calculated as Si(N2)/S2(N2). The phase cycling scheme can be found in the original figure and caption. (Figure and caption adapted from [45])...

Fig. 11.15 Diagram showing the relative proximity of the two ligands Glp and S3P, which form a stable ternary complex with the enzyme EPSP synthase. The distance constraints were obtained from both homonuclear and heteronuclear dipolar couplings obtained using the REDOR and DRAMA pulse sequence, together with a model showing a...

Figure 3 Pulse sequences for the different CT-REDOR versions for two rotor cycles (A) MAS spin-echo reference experiment (B) CT-VPP-REDOR with the pulse position fpp of the /-spin dephasing pulses stepped from 0 to Tp and (C) CT-VPD-REDOR with the pulse width fpp of the /-spin dephasing pulses stepped from 0 to In.

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. 

Fig. 21 a-c a Pulse sequence and coherence transfer pathway of the STMAS REDOR... 

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]. 

Figure 9a summarizes the effect of the nuclear electric quadrupolar coupling constant on the REDOR curve for the S=l/2 [1=3/2 case, as calculated by the SIMPSON code [31] for the pulse sequence of Fig. 8a. Note that interpretable REDOR results are obtained only in the limit Cq—>0. For higher Cq-values, the simulations show greatly diminished REDOR effects, which turn out to be highly sensitive to experimental parameters and dependent on the specifics of the spin system considered. It is disappointing in particular, that in the limit of large Cq-values the REDOR effect is found to be much weaker than expected from the selective excitation limit. Furthermore, the curves are not universal...

Finally, a few words remain to be said about case (d) (S>l/2,I>l/2). At the present stage of knowledge, this case remains seemingly intractable for quantitative applications outside the non-selective excitation limit Vj/Vq l. The chief problem in applying pulse sequence at Fig. 8b) arises from the nuclear electric quadrupolar interactions affecting the S spins, which interfere with the ability of the 71-pulse trains to refocus the transverse magnetization. In contrast, pulse sequence at Fig. 8a) is able to produce experimental REDOR curves, which are, however, difficult to interpret quantitatively. As a best-effort solution, one may resort to sample-to-sample comparisons on a relative basis and the use of empirical calibration procedures with closely related model compounds [32]. Alternatively, the SEDOR approach, which does not suffer from these restrictions, would be preferable. 

Fig. 18 B Na REDOR results for sodium borate glasses recorded with the pulse sequence of Fig. 8a. The 180° solid pulse length on Na was 8 ps and the rotor speed was 12 kHz. (a, top) site resolved REDOR data on trigonal and tetrahedral boron species, (b, bottom) site-...

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