Heteronuclear Dipolar Recoupling Techniques

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]. 

The technique applied most frequently for heteronuclear dipolar recoupling is cross polarization. Heteronuclear dipolar interactions can facilitate polarization transfer between spins in the doubly rotating frame if two rf spinlock fields with appropriate field strengths are applied simultaneously on both channels. The ratio between the rf field amplitudes and on the proton and the heteronuclear rf channel under MAS at a spinning frequency Vrai has to match the Hartmann-Hahn condition ... 

An in-depth account of the effects of homonuclear couphngs and multiple heteronuclear couplings has been given for a recently published technique for H- C dipolar correlation in solids under very fast MAS, where the heteronuclear dipolar coupling is recoupled by means of REDOR n-pulse trains." The method bears similarities to well-known solution NMR techniques, which form the framework of a heteronuclear MQ experiment. The so-called... 

A new heteronuclear dipolar interaction recoupling scheme suitable for the characterization of heteronuclear dipolar couplings for multiple-spin systems under very fast MAS condition has been proposed The new technique has been shown to be superior to the rotational echo double resonance method and other recoupling schemes as far as the interference of homonuclear dipolar interaction is concerned. The potential of the technique is illustrated for fluorapatite sample at a spinning frequency of 25 kHz. 

A novel MAS NMR approach has been presented for the determination of motionally averaged heteronuclear dipolar couplings in unoriented materials. The technique is based on the proton-detected local field protocol and achieves dipolar recoupling by R-type RF irradiation. The experiment, which is called R-PDLF spectroscopy, is demonstrated on solid and liquid-crystalline systems. For mobile systems, it is shown that the R-PDLF scheme provides better dipolar resolution as compared to techniques combining conventional separated local field spectroscopy with R-type recoupling. 

A new pulse sequence has been described for the recoupling of heteronuclear dipolar interactions under MAS. " The method is similar to the PISEMA experiment, but employs a well-defined amplitude modulation of one of the two RF fields. The technique was used for measurements of dipolar couplings in... 

R-SLF and R-PDLF techniques. Levitt and co-workers have presented symmetry theorems that are useful in the design of rf schemes for selective recoupling and decoupling in MAS solid-state NMR [36]. For example, it was found that certain R-type sequences [60] lead to reactivation of heteronuclear dipolar couplings... 

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