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Chemical shift anisotropy produces inhomogeneous broadening of NMR lines in glassy and polycrystalline solids. Holes can be burned into such lines, tagging molecules which are at certain orientations. Subsequent molecular reorientations result in a spectral diffusion which is not related to spin-spin interactions. By measuring the broadening and recovery of the hole as a function of time, detailed knowledge of the reorientations is obtained (e.g., mean jump rate and angular jump size). The reorientation rate in supercooled glycerol was followed with this technique from ∼101 to 10−2 s−1. This rate agrees well with higher frequency dielectric results and the location of the glass transition temperature. It was determined that the mean angular jump size is greater than 45°. The hole recovery curves were not exponential, but fit the Williams–Watts function exp−(t/τ)β with β = 0.5. The spin-spin contribution to spectral diffusion is very slow, making this technique widely applicable for the study of slow motions, particularly in organic solids and polymers. The hole-burning experiment is compared with related NMR techniques.
Kuhns et al. (Sun,) studied this question.