19F Shielding Tensors from Coherently Narrowed NMR Powder Spectra

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Abstract
Coherent line-narrowing methods are used to reduce the dipolar width of the 19F resonance in a number of powdered solids, disclosing the powder patterns arising from chemical shift anisotropy. The principal elements of the shielding tensors are reported and compared with previous experimental and theoretical results. In certain cases it has been possible to use the averaging induced by high-temperature restricted rotation to assign the principal elements of the shielding tensor to directions in the molecular framework. In C6F6 the shielding tensor is axially symmetric within experimental error; σ‖–σ⊥ = 155 ppm, with the unique (‖) axis parallel to the molecular sixfold symmetry axis. In CF3CO>2−Ag+ we find σXX = − 3 ppm, σYY = − 67 ppm, σZZ = + 70 ppm, and 〈σ〉 = − 96 ppm referred to liquid C6F6. Here the Z axis is approximately along the C–F bond and the Y axis lies in the CCF plane.