Rotational echo double resonance heteronuclear dipolar coupling

Rotational-echo double-resonance (REDOR)(75,79) is a new solid-state NMR technique which is sensitive to through-space carbon-nitrogen interactions between selectively 13C and 15N-enriched sites separated by up to 5A (20-22). The parameter directly measured in a REDOR experiment is the heteronuclear dipolar coupling constant DCN, which is in itself proportional to the inverse third power of the intemuclear distance, rCN. It is this dependence on (icn)3 which accounts both for REDOR s ability to accurately measure short distances and its insensitivity to longer-range interactions. As a technique which can probe, in detail, intermolecular interactions over a distance range of 5A, REDOR is well suited to studying the distribution of small selectively-labeled molecules in polymer delivery systems. 

Rotational-echo double resonance (REDOR), originally introduced by Gullion and Schaefer [102], is a method to recouple heteronuclear spin pairs. The sequence relies on a train of rotor-synchronized n pulses applied to the I spin to interrupt the spatial averaging of the heteronuclear dipolar coupling under MAS to give a nonvanishing dipolar Hamiltonian over a full rotor cycle (Fig. 11.8). Typically, REDOR data are collected by col-... 

Since its introduction, several other coherence transfer pathways have been employed. The described FSLG-HETCOR and recently proposed MAS-J-HMQC pulse sequence uses heteronuclear dipolar couplings, while the REPT-HMQC employs the rotational-echo double resonance (REDOR) recoupling pulse sequence. 

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. 

Rotational-echo double resonance (REDOR) experiments follow another strategy to achieve recoupling of weak heteronuclear dipolar coupling interactions under MAS conditions, quite different from the approach. REDOR experiments prevent the refocusing of weak heteronuclear dipolar couphng interactions under MAS by applying rotation-synchronized trains of 7T pulses. Figure 9 depicts some (of many possible) versions of REDOR sequences. The theory of REDOR has been treated extensively in the literature. 

Having demonstrated the achievement of high-resolution sohd state NMR capability, the authors describe experiments that combine the high-resolution aspect of MAS NMR with methods that retain the structure and/or dynamic information inherent in the anisotropic interactions. Rotational-echo double resonance (REDOR) allows the determination of D between isolated heteronuclear spin pairs. D is related simply and without approximation to intemuclear separation. Hence, REDOR makes possible the unambiguous direct determination of intemuclear distance between the labeled spin pair, independent of pair orientation, i. e., in amorphous and /or microaystaUine solids, and extends our abihty to quantitatively explore complex materials. It is also possible to extract intemuclear distance from homonuclear dipolar coupled spin pairs, and these experiments are also reviewed. 

It has been shown by Trebosc et that the FS REDOR (Frequency Selective Rotational Echo Double Resonance) experiment can be used for accurate through-space measurements in spin pairs that involve the quadrupolar nuclei. The experiment reveals both heteronuclear dipolar and scalar couplings, which can be re-introduced selectively site after site. As an example, couplings between the aluminium and phosphorus nuclei in the VPI5 zeohte have been measured. They agree very well with those reported in the literature, which validates the authors approach. 

Heteronuclear dipolar-coupling-based double-resonance NMR techniques such as rotational-echo double-resonance [98], rotational-echo adiabatic passage double-resonance (REAPDOR) [99] and transfer of populations in double-resonance [100] are useful tools for the characterization of intermediate-range ordering phenomena involving two unhke spins in a wide variety of materials. These techniques utilize the distance-dependent heteronuclear dipole—dipole interaction to obtain information on intemuclear distance between the two spins involved under the MAS conditions. 

Only the longitudinal spin term is present in Eq. (14) and aU heteronuclear dipole-dipole coupling interactions commute in a multispin system making the analysis much more straightforward. One of the best known pulse sequences in solid-state NMR spectroscopy, the rotational-echo double-resonance NMR (REDOR) experiment [44], employs appropriately placed pulses to avoid averaging of the heteronuclear dipolar coupling interactions by MAS. REDOR has been used in numerous cases to extract precise dipolar couplings and the inventor. Prof Schaefer, was commemorated recently for his contributions to solid-state NMR spectroscopy [45]. 

The development of the double resonance experiments described in Section 3.8.2 was targeted at spin- /i systems. TRAPDOR and Rotational-Echo Adiabatic Passage Double Resonance (REAPDOR) experiments are designed specifically for quadrupolar nuclei and do not work on systems containing only spin- /2 nuclei. These MAS experiments still rely on the modulation of the heteronuclear dipolar coupling (as in the REDOR experiment) to prevent an echo from refocusing, but the modulation is no longer by 180° pulses. 

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