Determination of the 13C magnetic shielding tensor in partially oriented polymer systems.
It is shown that, for an oriented sample, the 13C magnetic shielding tensor of a specific nucleus can be detd. from the spinning sideband intensities in a 2D spectrum obtained from a 13C rotor-synchronized 2D CPMAS expt., originally designed by Harbison and co-workers (G.S. Harbison, V. Vogt and H.W. Spiess, J. Chem. Phys., 86(3) (1987) 1206; G.S. Harbison, H.W. Spiess, Chem. Phys. Lett., 124 (1986) 128). In the presented fitting procedure, the authors combined their multi linear regression anal. for the detn. of the order parameters with a variation both of the principal values of the magnetic shielding tensor and of the direction of the principal axes relative to the mol. The magnetic shielding principal elements of the nonprotonated arom. carbon in poly(ethylene terephthalate) (PET) fibers and of the arom. carbons in the central ring of an oriented liq.-cryst. polymer network are reported. The authors est. that the accuracy of the principal values is within 5 ppm.
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