Rotational-echo double-resonance dipolar interactions

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. 

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. 

From the success of the cross-polarization experiments, one may predict success for a variety of related experiments that depend on the dipolar interaction (such as those of Veeman [951, where spin echoes are observed under magic-angle spinning and double resonance conditions). Further experiments that allow determination of internuclear distances should also be possible, including rotational-echo double-resonance (REDOR) NMR [96], and transferred-echo double-resonance (TEDOR) NMR [97], the latter being related to INEPT experiments in liquids. 

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. 

Several methods have been developed that yield structural information based on the dependence of the dipolar interaction on the internuclear separation. These include rotational echo double resonance (REDOR), ... 

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 procedure that is useful for intermolecular distance determination in the solid materials is termed REDOR (rotational echo double resonance) experiment [25,26]. It allows recovering dipolar interaction under MAS. REDOR is a spin-echo double-resonance experiment. A typical REDOR pulse sequence is shown in Fig. 5B. The rotor-synchronized echo sequence is applied to the spin I system and the echoes are detected after the time 2t equal the even number n of rotation periods t, . For recoupling the dipolar interaction between the spins I and S, r-pulses are applied to the spin S system at every half rotation period Tj. The dipolar coupling is determined by measuring the REDOR fraction AS/S, which describes the decrease of the echo amplitude 5 as a function of the number of rotation periods n [25] ... 

T. Gulhon, Measurement of dipolar interactions between spin-12 and quadrupolar nuclei by rotational-echo, adiabatic-passage, double-resonance NMR, Chem. Phys. Lett. 246 (1995) 325-330. 

Ganapathy et al. (2004) studied molecular sieve ETAS-10 and showed that the silicon sites tetrahedrally connected to aluminum in framework positions of a molecular sieve can be identified by a selective reintroduction of the heteronuclear Al- Si dipolar interaction through Rotational Echo Adiabatic Passage DOuble Resonance (REAPDOR) NMR. They used an effective dipolar dephasing of the Si-O-Al, over Si-O-Si, environments to identily silicon sites in the immediate vicinity of aluminum. The direct NMR estimation of Al-Si distance gave =0.323 nm. REAPDOR... 

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