Recoupling techniques

As previously hinted, structure assessment through the interatomic distances determination can be done by recoupling techniques. Till few... 

The homonuclear dipolar recoupling technique of radio frequency-driven recoupling (RFDR) involves a hard n pulse per rotor period and its recoupling mechanism is based on the modulation of chemical shift difference [35-37]. When the delta-pulse approximation is relaxed so that the pulse width of the 7t pulse is about one-third of the rotor period, the so-called finite-pulse RFDR (fpRFDR) could selectively reintroduce the homonuclear dipole-dipole interaction under fast MAS conditions [38], Because the recoupling mechanism of fpRFDR does not require the presence of chemical shift difference, it can be applied to study samples with a singly labeled site. For the study of amyloid fibrils, the technique of fpRFDR is usually applied in a constant-time framework (see below). 

The idea of frequency selectivity is certainly not restricted to J-coupling mediated polarization transfer. Furthermore, frequency selective polarization transfer can also be realized by DQ recoupling techniques. For the technique of shift-evolution-assisted selective homonuclear recoupling (SEASHORE), which employs POST-C7 to construct an effective DQ coupling [82, 83], frequency selective polarization can be achieved when the transmitter offset is set to the... 

Any heteronuclear dipolar recoupling techniques developed for systems containing multiple 3H spins must have the property to suppress actively the dipolar... 

The techniques discussed in this chapter can be divided into two general classes homonuclear and heteronuclear polarization-transfer techniques. In each of the two classes, further distinctions can be made and are used to structure the following discussion of the different techniques. We do not want to discuss the details of the experiments, which can be found in the original literature or in several reviews about dipolar recoupling techniques [48-50]. 

In a similar way one can compute matrix elements of any interbond interaction. The use of recoupling techniques (Racah algebra) allows one to reduce calculations of properties of molecules with n bonds to those of molecules with 2 bonds. 

In the same way as discussed in the preceding sections, one can include Majorana operators, M13 and M23. Since these are in the wrong coupling scheme, one must use the recoupling techniques of Section 2. The matrix elements of Mn and M23 are given by... 

The second and third term in Eq. (5.26) are not diagonal. However, their matrix elements can be constructed using the recoupling technique of Section 5.2. They are once more given by... 

Although the assumption that all apatite platelets are of equal thickness may not be well justified, it provides a useful approximation to the real situation. In fact, such assumption is inevitable for an ensemble technique like NMR. It is anticipated that HARDSHIP may be well suited to determine the relative thickness of apatite crystallites in different bone or dentin samples. We note in passing that C-REDOR, which is a hetero-nuclear recoupling technique with active suppression of homonuclear dipolar interaction,42 43 can be used to probe for the size of nanoparticles embedded in polymer matrix,44 provided that the nanoparticles do not... 

To determine orientation of an adsorbed molecule relative to the mineral surface, one would need to employ dipolar recoupling techniques to extract the distance constraints between the selected spin species of the molecule and of the surface. For polypeptide-HAp systems, 13C 31P REDOR has been carried out for uniformly 13C labeled molecules, where numerical simulations show that the effect of 13C dipole-dipole interaction is relatively minor.125 For a study of bone sample, o-phospho-L-serine was taken as the model compound for the setup of the 13C 31P REDOR experiments, where the data can be well analyzed by a l3C-3lP spin-pair model with the intemuclear distance equal to 2.7 A.126 Concerning the effect of 31P homonuclear dipolar interaction on the spin dynamics, Drobny and co-workers have carried out a detailed REDOR NMR study of polycrystalline diammonium hydrogen phosphate ((NH4)2F1P04).127,128 The results show that the 15N 31P REDOR data can... 

Section 6 reviews developments in cross-polarization and spin diffusion/ exchange. This section is closely related to Section 4 because of the interplay between these spin dynamics processes and the recoupling schemes at highspeed spinning, which now is almost invariably used for high resolution. Actually, many recoupling techniques were motivated by effecting crosspolarization and spin diffusion under fast MAS. 

A number of recoupling techniques are utilized for specific purposes, but they all rely on synchronizing measurements, and, in most instances, pulse sequences, with the rotation period. For homonuclear recoupling (e.g., 13C-13C or 31P-31P), rotational resonance (R2) is based on magnetization exchange when the spinning rate wr is adjusted so that a multiple of it equals the chemical shift difference between the two dipolar coupled nuclei, and DRAMA (dipolar recovery at the... 

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