Homonuclear HORROR

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... 

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)... 

It is known that, at the HORROR condition, homonuclear dipolar couplings between xHs are reintroduced, leading to partial removal of the heteronuclear coupling.28 29 It is worth noting that at 60 and 120 kHz the proton decoupling is insufficient due to the rotary resonance effect. It has been shown theoretically by Emsley and co-workers,19 that the first and second rotary resonances are partially quenched by the presence of homonuclear couplings. 

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... 

The pulse sequence for a double-quantum filtration experiment utilizing the HORROR sequence is shown in Fig. 29. The weak on-resonance RF field leads to recoupling of the homonuclear dipolar interaction under MAS if the DQ-HORROR condition Wr = 2 is fulfilled (which may be seen as equivalent to a (homonuclear) n = 1/2 7 experiment). This resonance condition leads to a purely dipolar double-quantum average Hamiltonian which has to be calculated by averaging over two rotation periods. Similarly to... 

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... 

HORROR condition fRaec = R/2 [110]. The best decoupUng was found here at v dec = 24 kHz, and it is inferred that the proximity of the HORROR condition and the reintroduction of H homonuclear couplings and spin fUp-flops help decoupUng the dipolar interaction between and H [104]. It has also been shown that a loss in decoupUng efficiency was observed for v dec = at ultrafast MAS, resulting apparently from the reintroduction of unwanted three-spin terms [47]. 

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