Recoupling, dipolar homonuclear

The so-called HORROR experiment by Nielsen and coworkers [26] introduced continuous rf irradiation recoupling to homonuclear spin-pairs and initiated the later very widely used concept of /-encoded recoupling. Using a irreducible spherical approach as described above, the HORROR experiment (Fig. 2d) is readily described as starting out with the dipolar coupling Hamiltonian in (10) and x-phase rf irradiation in the form Hrf = ncor(Ix +SX), also here without initial constraint on n. The dipolar coupling Hamiltonian transforms into tilted frame (rotation n/2 around Iy + Sy)... 

Fig. 11.7 a Pulse sequence for rotational-resonance recoupling of homonuclear spin pairs, b The spinning frequency is matched to the isotropic chemical-shift difference, and one of the resonances is selectively inverted and the polarization exchange measured as a function of the mixing time, c The difference polarization as a function of the mixing can be evaluated to give the dipolar coupling constant. 

While the Ur — 2uj DQ-HORROR condition selectively recouples dipolar interactions, the homonuclear rotary resonance condition = leads to single-quantum and double-quantum recoupling, including recoupling of CSA. Accordingly, the homonuclear n = I condition with Wr = has also been called MSD-HORROR (mixed single- and double quantum HORROR) " and has been employed to determine the orientation of the two C chemical shielding tensors in 2,3- C2-labelled L-alanine. However, the homonuclear resonance condition simultaneously represents a heteronuclear n = 1... 

To conclude this section on recoupling of homonuclear dipolar interactions under MAS conditions, a few general remarks seem in place. The performances of some of the homonuclear dipolar recoupling sequences discussed here have... 

Heteronuclear and homonuclear dipolar recoupling under magic-angle spinning... 

We note that DRAMA has also formed the basis for refined variants such as the chemical-shift-compensated MELODRAMA [85], windowless DRAWS [29], and the DRAMA-XY4 [86] experiments, finding important applications for example for measurement of 13C-13C distances involving carbonyl spins. We should also note the existence of many other important pulsed homonuclear recoupling experiments such as the widely used Radio Frequency Driven Dipolar Recoupling (RFDR) [24] and Back-to-Back (BaBa) [87] experiments. 

As demonstrated by Griffin, Levitt, and coworkers in the late 1980s [21, 93], it is also possible to recouple homonuclear dipolar couplings through interference between isotropic chemical shifts and the rotor revolution. This phenomenon, called rotational resonance, occurs when the spinning frequency is adjusted to a submultiple of the isotropic chemical shift difference, i.e., ncor = ct> so o) °. To understand this experiment, the dipolar coupling Hamiltonian in (10) is transformed... 

Dipolar recoupling may also be accomplished using continuous rf irradiation as demonstrated in the heteronuclear and homonuclear case by the CP at MAS conditions (or for low-y heteronuclear spins called double-cross-polarization, DCP [103]) and HORROR (homonuclear rotary resonance) [26] experiments, respectively. These experiments may easily be described by transforming the description into the interaction frame of the rf irradiation using (14a) exploiting... 

Homonuclear Dipolar Recoupling and J-Coupling Based Techniques... 

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). 

Homonuclear Dipolar Recoupling Involving Quadrupolar Nuclei. 179... 

The CP can also be used for polarization transfer to MQ coherences of half-integer quadrupolar nuclei [222, 223, 228-231]. This type of transfer is mainly used in the context of MQMAS [228,229,231], although the spin-locking of MQ coherences is also featured in experiments involving homonuclear dipolar recoupling experiments [232]. 

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]. 

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