Fundamentals of Recoupling Techniques Under Magic Angle Spinning

Abstract

Our discussion in Chapter 5 demonstrated the application of MAS enhances spectral resolution and sensitivity by concentrating the broad static powder pattern into sharp individual spinning side bands and the central line. In this process, the anisotropic interactions are suppressed. Meanwhile, anisotropy also encodes molecular structure and dynamics. For example, chemical shielding or dipolar tensor, their magnitudes and orientations inform us the dynamics and structural information of molecules. Therefore, NMR spectra can be modulated by selected anisotropy to retrieve dynamics and structural parameters. For example, distances between selectively isotope labeled sites can be measured accurately by recoupling of dipolar interactions, or the dynamics of amide nitrogen can be interrogated. Hence, it seems impossible to simultaneously achieve spectral resolution enhancement and retrieve maximized structural information, as they demand conflicting requirements in the same experiment: these anisotropic interactions need to be suppressed for resolution purpose, yet they need to be kept alive for probing the valuable structural and dynamic parameters. The solution is to achieve artificial control of anisotropic interactions in multidimensional NMR. By exploiting the interference effects between RF pulsing and MAS, anisotropic interactions can be selectively resurrected to modulate the spectral features along the indirect dimension, while they are dormant during direct detection dimension by MAS and decoupling. A variety of RF pulse sequences were designed for this purpose. In addition to artificially designed sequences, the recoupling effects exist intrinsically due to the interference effects associated with incommutable homogeneous interactions, which incurs undesirable line broadening despite MAS. We will start our discussion by reviewing the simple but classical recoupling of homo and heteronuclear dipolar interactions by rotary resonances1, 2. These derivations will naturally help us understand the widely the recoupling of homonuclear dipolar interactions by Dipolar-Assisted Rotational Resonance (DARR)3. Then we will review recoupling of dipolar interactions by C4, 5 and R6, 7 symmetry sequences that exploit more general symmetry principles of sequential pulse trains. Detailed analyses will be presented for C78 and SPC59 sequence. In addition to continuous irradiation, a variety of RF sequences were designed with intermittent pulses to recouple interactions. In the case of homonuclear recoupling sequences, finite pulse radio frequency driven recoupling (fpRFDR)10, RFDR, and PITHIRDS will be analyzed. Analysis of REDOR11 will be presented at the end to demonstrate the recoupling of heteronuclear dipolar interactions. It shares similar features to RFDR in the sequence layout, but was much earlier. Let's start from some classical examples in the history of ssNMR, and you will see this is a very exciting journal that you don't want to miss.