Dipolar recoupling methods

A novel tool is a symmetry-based 29Si dipolar recoupling method (SR264n) [123] for small dipolar interactions that has been initially applied in zeolite structural studies by Brouwer et al. [124], One of the advantages of the new method over INADEQUATE is that the latter misses auto-correlations of symmetry-related double-quantum coherences. The SR26411 method provides such information on auto-correlation which allows to identification of all four connectivities of a tetrahedral Si position. 

For this purpose, it has been demonstrated that the high resolution solid-state NMR approach provides one with an alternative and convenient means to distinguish a variety of crystalline polymorphs and to reveal the secondary structures of biological macromolecules, because the chemical shifts of backbone carbons are displaced (up to 8 ppm) [1, 2] depending on their respective conformations. In addition, it is emphasized that this type of empirical approach can be used as a very valuable constraint to construct the three-dimensional structure of biological molecules, such as peptides and proteins, based on a set of accurately determined interatomic distances measured by a partial dipolar recoupling method, such as REDOR (rotational echo double resonance) [3-6]. 

Key Words Correlation NMR, Quadrupolar nuclei, HMQC, Dipolar recoupling methods, Inorganic materials. Phosphates, Silicates... 

Figure 4.2 Pulse sequences of the 2D Y X cross polarisation (A) and X Y D-HMQC (B) sequences. The D-HMQC sequence is the indirect recoupling version. D-Rec is the generic term covering all the dipolar recoupling methods (R, SR4i, SFAM and REDOR) that will be detailed in this chapter.

MAS dipolar recoupling methods, (b) Determination of the cimetidine crystal structure (inset) from XRPD data using a simulated annealing (S A) algorithm, comparing the number of steps required and success rate to achieve an accurate determination when starting with an unconstrained cimetidine molecule and when incorporating the NMR distance constraints. 

Average or effective Hamiltonian theory, as introduced to NMR spectroscopy by Waugh and coworkers [55] in the late 1960s, has in all respects been the most important design tool for development of dipolar recoupling experiments (and many other important experiments). In a very simple and transparent manner, this method facilitates delineation of the impact of advanced rf irradiation schemes on the internal nuclear spin Hamiltonians. This impact is evaluated in an ordered fashion, enabling direct focus on the most important terms and, in the refinement process, the less dominant albeit still important terms in a prioritized manner. 

Floquet theory [60] represents a powerful method to analyze and describe NMR experiments including, in the present context, methods for dipolar recoupling. In particular the bimodal variants are promising in the sense that they may handle perturbations/interactions involving multiple time periodicities, as for example induced by combined sample spinning and rf irradiation in the case of dipolar... 

The future utility of exchange couplings in the NMR of semiconductors may reside less in the comparison of the limited number of experimental measurements of low or modest accuracy with more refined theoretical calculations by DFT methods, and more instead in the ability to establish correlations between chemically-shifted nuclei in MAS-NMR spectra. However, dipolar couplings can also be used in such a situation by employing the recoupling methods discussed by Nielsen in another chapter of this book. 

R. Verel. Adiabatic methods for Homonuclear Dipolar Recoupling in Magic Angle Spinning Solid-State NMR. PhD thesis, ETH Zurich, Diss. Nr. 14152, 2001. 

Fluorine-probe protocol has been applied to solid-state magic-angle spinning (SSMAS) 19F NMR analysis with the radiofrequency-driven dipolar recoupling (RFDR) method to measure the F-F distance in the microtubule-bound conformation of F2-10-Ac-docetaxel (see Figure 1.29a) [167], Moreover, five intramolecular distances of the key atoms in the microtubule-bound l9I72II/l3(Mabeled paclitaxel were determined by the rotational echo double resonance (REDOR) method (see Figure 1.29b and c) [168, 169],... 

To fulfill these needs, we recently undertook the task of developing and implementing optimal control design procedures into the SIMPSON software and described the first applications of this method to solid-state NMR spectroscopy by designing new dipolar recoupling experiments. Optimal control theory is an ideal vehicle for optimizing... 

Relevant reviews published in Annual Reports on NMR Spectroscopy have encompassed recent developments in dipolar recoupling under MAS conditions, characterization of porous media using NMR methods, NMR studies of molecular motions in solids," recent progress in solid-state NMR of low y nuclei and Co NMR. ... 

New isotropic mixing sequences suitable for scalar correlation experiments in solids have been designed using symmetry principles similar to those employed in the construction of the C7 dipolar recoupling sequence. Compared with existing methods, the new isotropic mixing sequences are... 

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