Magic Angle Spinning and Truncated Field Concept in NMR

J Jenczyk

doi:10.1155/2019/5895206

Abstract

In order to thoroughly comprehend and adequtely interpret NMR data, it is necessary to perceive the complex structure of spin Hamiltonian. Although NMR principles have been extensively discussed in a number of distinguished introductory publications, it still remains difficult to find illustrative graphical models revealing the tensorial nature of spin interaction. Exposure of the structure standing behind mathematical formulas can clarify intangible concepts and provide a coherent image of basic phenomena. This approach is essential when it comes to hard to manage, time-dependent processes such as Magic Angle Spinning (MAS), where the anisotropic character of the spin system interactions couple with experimentally introduced time evolution processes. The presented work concerns fundamental aspects of solid state NMR namely: the uniqueness of the tetrahedral angle and evolution of both dipolar D and chemical shield σ coupling tensors under MAS conditions.

Highlights

Developed conceptually in the 1950s [1, 2] Magic Angle Spinning (MAS) has become a routine technique in solid state NMR spectroscopy [3]. is unique concept, together with the exceptional progress made in the eld of NMR probe engineering, has made possible the acquisition of high resolution spectra for rigid phase samples
MAS eliminates the broadening of NMR signals by orientation-dependent spin interactions, yielding signi cant improvements in resolution and signal-to-noise ratio. e excellent resolution observed in the case of liquid samples stems from the fact that during NMR signal acquisition each molecule undergoes extremely fast and random tumbling
On the contrary, powdered samples, consisting of a large ensemble of randomly oriented tiny crystals, reveal broad NMR spectra representing superposition of lines at all possible frequencies weighed by their probability. Both the abovementioned results are due to the anisotropic nature of spin interactions. e work presented below (i) introduces the concept of truncated eld which represents the e ective local magnetic eld generated by dipolar nucleus ( = 1/2) under the in uence of Zeeman eld, (ii) illustrates the tensorial character of dipolar coupling and chemical shi anisotropy (CSA), and (iii) explains the in uence of MAS on spin systems

Summary

Introduction

Developed conceptually in the 1950s [1, 2] Magic Angle Spinning (MAS) has become a routine technique in solid state NMR spectroscopy [3]. is unique concept, together with the exceptional progress made in the eld of NMR probe engineering, has made possible the acquisition of high resolution spectra for rigid phase samples. E excellent resolution observed in the case of liquid samples stems from the fact that during NMR signal acquisition each molecule undergoes extremely fast and random tumbling Both dipolar and quadrupolar interactions are e ectively averaged out and only an isotropic chemical shielding is manifested in the spectrum, together with a subtle J coupling e ect. On the contrary, powdered samples, consisting of a large ensemble of randomly oriented tiny crystals, reveal broad NMR spectra representing superposition of lines at all possible frequencies (orientations) weighed by their probability. Both the abovementioned results are due to the anisotropic nature of spin interactions.

Discussion

10: Schematically presented evolution of a spin pair under

Conclusions