When a powder having linewidth A arising from inhomogeneous second-rank tensor interactions is rotated about the magic axis at trot < A, the free induction decay is a spin echo train, the Fourier transform of whose envelope is the isotropic spectrum.
In obtaining isotropic chemical shift spectra of 13C in powders by combining
[ 1 ] strong proton decoupling [2] with magic-angle sample spinning [3] it might be thought that one would require spinning frequencies vr % A, A being the width of the powder pattern arising from anisotropic chemical shifts. While this is indeed the criterion for complete collapse to the isotropic resonance, Lippmaa et al. [4] and subsequently Stejskal et al. [S] have strikingly demonstrated that much slower spinning suffices to break up the powder pattern into an array of spinning sidebands of width 6 & A; the criterion for this behavior is vr > 6.
We have studied what amounts to the same phenomenon from a different point of view which is generally illuminating and leads to a greatly simplified method for obtaining the isotropic spectrum. We believe that this method is applicable to a considerably wider class of powder spectra, including in particular 2H resonances, from which isotropic proton chemical shifts can be extracted as precisely as and more simply than by competitive methods [6,7], though perhaps at soln= penalty in sensitivity.