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Techniques for the Measurement of Small Anisotropies in Magic Angle Spinning NMR Spectra
Author(s) -
Raleigh D.P.,
Kolbert A.C.,
Levitt M.H.,
Griffin R.G.
Publication year - 1988
Publication title -
israel journal of chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.908
H-Index - 54
eISSN - 1869-5868
pISSN - 0021-2148
DOI - 10.1002/ijch.198800038
Subject(s) - homonuclear molecule , chemistry , magic angle spinning , spinning , zeeman effect , magnetic dipole–dipole interaction , spins , anisotropy , spectral line , half integer , isotropy , dipole , resonance (particle physics) , residual dipolar coupling , magic angle , molecular physics , solid state nuclear magnetic resonance , spin (aerodynamics) , nuclear magnetic resonance , atomic physics , nuclear magnetic resonance spectroscopy , physics , optics , condensed matter physics , magnetic field , molecule , quantum mechanics , stereochemistry , organic chemistry , polymer chemistry , thermodynamics
Two novel methods for enhancing the effect of small anisotropies on magic angle spinning (MAS) NMR spectra are described. The first method is a simple two‐dimensional (2D) spin echo experiment, which generates rotational sidebands at a fraction of the spinning frequency in the ω 1 dimension, while maintaining the effective spinning speed in ω 2 . It is shown that the rotational sidebands in the 2D spectrum are much larger than in a normal one‐dimensional MAS spectrum obtained at the same spinning speed, and the relative intensities of the two‐dimensional sideband array can be analyzed to yield the anisotropy and asymmetry parameter of the coupling tensor. The second method is designed to measure homonuclear dipolar couplings. We show that the effect of a dipolar coupling between members of an isolated spin pair can be enhanced by adjusting the spinning speed such that n ω r = ω Δ iso , where n is an integer, ω r is the spinning speed, and ω Δ iso is the difference between isotropic shifts. When this condition, termed rotational resonance, is satisfied, a broadening or a splitting of the normally sharp resonance lines is observed. In addition, rapid exchange of Zeeman order between the dipolar coupled spins is seen. This method holds promise for the measurement of through space dipolar couplings and thus internuclear distances in polycrystalline solids.