Rotary resonance

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... 

Recently a new type of proton assisted recoupling experiments has been developed for coherence transfer where rf irradiation is taking place on all involved rf channels. This embraces the homonuclear proton assisted recoupling (PAR) [45, 140, 141] and the later resonant second-order transfer (RESORT) [142] experiments, as well as the heteronuclear proton assisted insensitive nuclei (PAIN) cross polarization [44] experiments. In PAR and PAIN, spin-lock CW irradiation is applied on both passive ( H) and active spins (13C, 15N) without matching rotary resonance conditions. In RESORT a phase alternation irradiation scheme for the passive spins is used. 

Finally, we should also mention that assisted spin recoupling is by no means restricted to 1H, 13C, and 15N spin species but may also be used to facilitate H and 2H to low- spin transfers. This may be exploited in extensively deuterated samples using rf fields on JH and 2H which are matched to a rotary resonance condition to facilitate spin-diffusion between 13C spins in a longitudinal mixing process. This was recently demonstrated by van Rossum and coworkers in terms of the doublenucleus enhanced recoupling (DONER) experiment [143]. 

Rotary resonance recoupling (R3) was used to measure chemical shift tensors,153 and the relative orientation of the shift and the dipolar tensors.154 It can also be used to conduct a 3D chemical shift correlation experiment to investigate molecular rearrangement.155 Another interesting application of the rotary resonance is to obtain an NMR spectrum with peaks at the principal values of the chemical shift tensor.156 The principle is that the spins in principal-value orientations experience an effective field of zero magnitude at the rotary resonance, hence their magnetization vectors do not oscillate while spins at other orientations may be dephased. 

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. 

Thus, the occurrence of broad C resonances under H high-power decoupling and MAS conditions should neither lead to the automatic conclusion of lack of crystallinity of the sample, nor be considered a nuisance in contrast line broadening mechanisms under MAS are a valuable source of information for the extraction of kinetic parameters and activation energies of thermally activated processes. It should also be mentioned that further line broadening effects may arise from interference between ooi and the MAS frequency, leading to so-called rotary-resonance recoupling of dipolar... 

N. C. Nielsen, H. Bildspe, H. J. Jakobsen and M. H. Levitt, Double-quantum homonuclear rotary resonance efficient dipolar recovery in in magic-angle spinning nuclear magnetic resonance. J. Cherrt Phys., 1994. 101, 1805-1812. 

T. Vosegaard, P. Florian, D. Massiot and P. J. Grandinetti, Multiple quantum magic-angle spinning using rotary resonance excitation. J. Chem. Phys., 2001, 4618-4624. 

The utility of simple rotary resonance experiments for the determination of the magnitude and orientation of chemical shift tensors relative to one or more internuclear axes from MAS NMR experiments has been... 

Rotary resonance recoupling of heteronuclear C- H dipolar interactions in NMR spectra (recorded under conditions of H decoupling at frequency vi and MAS at frequency Vr) has been studied for three examples of molecular solids (adamantane, ferrocene and hexamethylbenzene) in which substantial molecular motion is known to occur. It has been shown that when rotary resonance conditions are satisfied (i.e. l Vr = , for n = 1 or 2), the recoupling can lead to motionally averaged Pake-like powder patterns from which information on C- H intemuclear distances and/or molecular motion can be derived. 

Fig. 2. Pulse sequence of the rotary resonance recoupling experiment (R ) where the amplitude of u>2 matches an integer multiple of Wr. Here we have assumed a spin system with additional presence of H spins in the sample, hence the pulse sequence includes (ramped) cross polarization H— S and H decoupling during acquisition. This assumption will be made throughout as this represents the most typical circumstance. However, replacing CP by direct single-pulse Si excitation and omitting the H decoupling, the R pulse sequence and all sequences discussed in the following are also applicable to (Si, S2) spin systems in the absence of H spins.

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... 

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