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Abstract:
The effect of molecular reorientation on the NMR powder spectra of nuclei with spin I=, governed by magnetic tensors, has been investigated. Numerical calculations were performed for the case of axially symmetric shielding tensors. Examples discussed explicitly are the nuclei at the corners of regular octahedron, tetrahedron, and trigonal bipyramid. In all cases it was assumed that the molecules can jump between their equilibrium positions, thereby interchanging the nuclei. The calculation described is completely general and can be used to obtain the lineshapes for molecules of lower symmetry as well. The spectra display characteristic features easily recognizable experiments, especially for values of the jump frequency τ −1, such that Δωτ = 1 (Δω = ω‖−gw⊥). But even for much smaller jump frequencies, where Δωτ ≈ 30, the deviations from the powder pattern without motion are significant. Comparison with lineshape calculations for liquids shows that the spectra for the same values of the jump frequency differ markedly for reorientational models applicable to liquids or solids. Experimental examples are presented in an accompanying paper.
