REDOR double-resonance

Figure Bl.12.15. Some double-resonance pulse sequences for providing distance infomiation in solids (a) SEDOR, (b) REDOR, (c) TEDOR and (d) TRAPDOR. In all sequences the narrow pulses are 90° and the wide pulses 180°. For sequences that employ MAS the luimber of rotor cycles N is shown along the bottom.

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. 

The REDOR experiment has formed the basis for a large number of ideal pulse type recoupling experiments, and later finite pulse variants, for heteronuclear dipolar recoupling. These include experiments such as frequency selective REDOR (FS-REDOR) [80], TEDOR (Transferred Echo DOuble Resonance) [25], and 3D variants of TEDOR [81, 82], which have found important applications, e.g., for measurement of intemuclear 13C-15N distances in biological solids. We should also mention that rotor-encoded variants of TEDOR, such as REPT, HDOR [83], and REREDOR [84], have been proposed for 1H13C dipolar recoupling under high-speed MAS conditions. 

Rotational echo double resonance (REDOR) [86] is one of the most important techniques developed for solid-state NMR spectroscopy. The theory of REDOR is... 

Another study of the surface structure of CdSe NCs of 3.7 nm size used 31P MAS-NMR and 31P/77Se rotational-echo double-resonance (REDOR) to identify overlapping broad peaks from two surface species trioctylphosphine oxide (TOPO) at 29.3 ppm and trioctylphosphine selenide (TOPSe) at 22.2 ppm [343]. Both the isotropic chemical shift and CSA of the surface-bound TOPO were substantially different from those of the free ligand. Spin-echo experiments on 31P were stated to indicate an average P-P distance of 8-10 A at the surface, consistent with capping at alternate atomic sites (all Cd but not Se). 

Poly(benzyl ether) dendrimers synthesized by Frechet el al. have been studied with many techniques in order to reveal their conformational properties. Size exclusion measurements performed by Mourey et al. [154], rotational-echo double resonance (REDOR) NMR studies by Wooley et al. [155] and spin lattice relaxation measurements by Gorman et al. [156] reveal that back-folding takes place and the end-groups can be found throughout the molecule. The observed trends are in qualitative agreement with the model of Lescanec and Muthukumar [54],... 

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-... 

J. C.C. Chan, C-REDOR Rotational double resonance under very fast magic angle spinning, Chem. Phys. Lett. 335 (2001) 289-297. 

NMR-spectroscopic investigations of dendrimers can provide comparable information. Thus Wooley et al. used REDOR (Rational Echo Double-Resonance) solid state NMR experiments for the study of the density distribution and back-folding of terminal groups in fifth-generation poly(benzyl ether) dendrimers... 

REDOR rotational echo double resonance (NMR spectroscopy)... 

REDOR Rotational Echo Double Resonance tr-NOE, transferred Nuclear Overhauser Enhancement... 

It was shown that REDOR or its variants can be used when one coupling partner is a quadrupolar spin.113,114 However, there are some limitations, such as the fact that only a fraction of the spin-1, nuclei are involved in the dephasing.115 Accordingly, more specific dephasing methods have been proposed for this type of system. One is TRAPDOR (transfer of population in double resonance), which uses continuous irradiation.116 The other is REAPDOR (rotational-echo, adiabatic passage, double resonance),117,118 which is a more straightforward extension of REDOR. 

The application of 31P-19F rotational echo double resonance (REDOR) measurements in structural studies of nucleic acids has been reviewed by Iuga et al.13 In recent years, this methodology has been further developed.85... 

The symmetric geometry of the TMEDA complex of 68 was also confirmed by using the Rotational-Echo Double Resonance (REDOR) method, and the position of the lithium cation was determined with an error of about 0.2 A127. However, the accuracy of the REDOR method could not be confirmed, because this complex has not been investigated by X-ray crystallography. 

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