Techniques for the Measurement of Small Anisotropies in Magic Angle Spinning NMR Spectra |
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| Authors: | D.P. Raleigh A.C. Kolbert M.H. Levitt R.G. Griffin |
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| Affiliation: | 1. Francis Bitter National Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A. Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A.;2. Francis Bitter National Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A. |
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| Abstract: | 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 ω1 dimension, while maintaining the effective spinning speed in ω2. 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 nωr = ωΔiso, where n is an integer, ωr is the spinning speed, and ωΔiso 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. |
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