Rotational echo double resonance sequence

An alternative approach is based on a coherent suppression of the signals representing protonated species. By incorporating a A1 H REDOR (rotational echo double resonance) sequence [91] into the evolution of... 

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. 

The rotational echo double resonance (REDOR) technique is essentially the MAS version of SEDOR, affording a site-resolved measurement of heteronu-clear dipolar coupling information on rotating samples. Figure 8 shows two commonly used pulse sequences [21-24]. 

As with the 2DTOSS studies, Munson and co-workers showed that solid-state NMR can provide important information on the structures of polymorphs and solvates whose crystal structure is not known. As we seek more detailed structural information of drugs in order to understand more about solid-state behavior, these 2D techniques will no doubt become more and more important. Additional 2D techniques have been utilized to study DNA complexes (104) and perform distance measurements (105), specifically using the rotational-echo double-resonance (REDOR) sequence (106,107). 

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. 

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] ... 

In this context, we must at least briefly mention the REAPDOR experiment [94] (rotational-echo adiabatic-passage double resonance), a technique that is very similar to the BJ3DOR pulse sequence, although it is more efficient, as it is designed especially for quadrupolar nuclei utilizing an adiabatic pulse of length for the polarization transfer (Fig. 25). In this... 

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