Recoupling methods

In the rotational-echo double-resonance (REDOR) [60—62] technique, the distance between two heteronuclei is determined by comparing the signal intensity in two closely related experiments. The interpretation of the experimental results assumes the existence of isolated spin pairs, and there is thus usually a requirement for selective isotopic labelling at the two sites, the distance between which is of interest. 

A number of interesting applications of the RE DOR method have been presented (see Table 1 of [9]), with a particular emphasis on samples of biological relevance. A specific example is shown in Fig. 9.14, where the distance be- 

There are small but important differences between the evolution of a given spin under a homonudear as opposed to a heteronuclear dipolar coupling [3—5] As a consequence, a different methodology is required for the determination of the intemudear distance between a homonudear dipolar-coupled pair of spins. 

Rotational resonance (RR) is an intriguing phenomenon which is observed when Vr is equal to a small integer multiple of the difference in the isotropic chemical shift frequencies of two resonances in the spectram [66, 67]. The most apparent effect of RR is that the normally narrow spectral peaks acquire sphttings and broadenings, the nature of which depend on the dipolar coupHng between the two spins. As a specific example, experimental spectra (together with best-fit simulations) corresponding to the = 1, 2, and 3 RR conditions for aU-E-[ll,20- C2]-retinal are shown in Fig. 9.15 [68]. In this case, it was possible to determine that the intemudear distance between the two labels is 0.301 + 0.008 run. 

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A novel tool is a symmetry-based 29Si dipolar recoupling method (SR264n) [123] for small dipolar interactions that has been initially applied in zeolite structural studies by Brouwer et al. [124], One of the advantages of the new method over INADEQUATE is that the latter misses auto-correlations of symmetry-related double-quantum coherences. The SR26411 method provides such information on auto-correlation which allows to identification of all four connectivities of a tetrahedral Si position. 

The future utility of exchange couplings in the NMR of semiconductors may reside less in the comparison of the limited number of experimental measurements of low or modest accuracy with more refined theoretical calculations by DFT methods, and more instead in the ability to establish correlations between chemically-shifted nuclei in MAS-NMR spectra. However, dipolar couplings can also be used in such a situation by employing the recoupling methods discussed by Nielsen in another chapter of this book. 

Rotational resonance [51-53] is mostly used as a selective recoupling method between isolated spin pairs generated chemically by selective labeling. It reintroduces the homo-... 

Sudden recoupling method Adiabatic recoupling method ZQorDQ method y-encoding References... 

J. M. Bowman and B. Gazdy, Theor. Chim. Acta, 79, 215 (1991). A Truncation/Recoupling Method for Eigenvalues and Eigenvectors Ideal for Parallel Computation. 

For the excitation of DQC under MAS, the interference with the sample rotation must be considered, as was the case for the CRAMPS experiment discussed in the previous section. In particular, if a sequence designed for a static MQ experiment is used without modification, the excitation (and reconversion) time is limited to ttr/2, since the rotor modulation causes the action of the pulse sequence in the second half of the rotor period to be the time reversal of that which occurred in the first half of the rotor period. Starting with the suggestion of Meier and Earl,89,90 who simply proposed the phase switching of the static sequences used by Baum et al.,47,48 every half rotor period to prevent the process of self-time-reversal, many different approaches have been presented that allow excitation (and reconversion) times of one or more rotor periods. Such pulse sequences that counteract the effect of MAS are referred to as recoupling methods,1112 examples that have been used in homonuclear DQ MAS NMR spectroscopy include BABA,91 C7,92 DRAMA,93 DRAWS,94 and HORROR.95 We note that Levitt and co-workers have recently introduced a very helpful classification system, based on symmetry principles, which covers such sequences.96,97... 

For an isolated spin pair and using TVtr of the BABA recoupling method for both the excitation and reconversion of DQCs, the DQ time-domain signal is given by 99... 

Gregory et al. were able to determine the relative orientation of 13C CSA tensors from an analysis of 13C DQ MAS spinning-sideband patterns obtained using the DRAWS recoupling method.117... 

For this purpose, it has been demonstrated that the high resolution solid-state NMR approach provides one with an alternative and convenient means to distinguish a variety of crystalline polymorphs and to reveal the secondary structures of biological macromolecules, because the chemical shifts of backbone carbons are displaced (up to 8 ppm) [1, 2] depending on their respective conformations. In addition, it is emphasized that this type of empirical approach can be used as a very valuable constraint to construct the three-dimensional structure of biological molecules, such as peptides and proteins, based on a set of accurately determined interatomic distances measured by a partial dipolar recoupling method, such as REDOR (rotational echo double resonance) [3-6]. 

We must first emphasize that detailed reviews of recoupling methods (that were used in the present report) are available, either for fast MAS [133,134]... 

Key Words Correlation NMR, Quadrupolar nuclei, HMQC, Dipolar recoupling methods, Inorganic materials. Phosphates, Silicates... 

Figure 4.2 Pulse sequences of the 2D Y X cross polarisation (A) and X Y D-HMQC (B) sequences. The D-HMQC sequence is the indirect recoupling version. D-Rec is the generic term covering all the dipolar recoupling methods (R, SR4i, SFAM and REDOR) that will be detailed in this chapter.

D-HMQC has also been used to investigate the structure of synthetic alumina-sihcate clays (synthetic sodium montmorillonite, S-Na MMT) [69], More precisely, the presence of impurity was highlighted in the synthetic layered material by Al Si D-HMQC performed at 17.6 MHz and 0) =5 kHz with the SR4 i recoupling method performed during 9.6 ms. In natural Na-MMT, zeolite analcime (NaAlSi20g, hH20) has been identified by XRD as the main impurity. 

In 2009, structures of hydroxyapatite (HA), a biomaterial of composition Caio(P04)g(OH)2 and oxy-hydroxyapatite (tHA) were investigated by means of H " Ca D-HMQC correlation experiments [80]. Since " Ca shows a 0.14% natural abundance, HA and tHA have been synthesised with 60% " Ca-enriched CaC03 as precursor. The direct recoupUng D-HMQC experiments were conducted at 14.1 T and 0) =9 kHz. The recoupling method was used during 1.75 ms on the H channel to generate the H " Ca ... 

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