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 determined from the spinning sideband intensities in a 2D spectrum obtained from a 13C rotor-synchronized 2D CPMAS experiment, 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 our fitting procedure, we combined their multilinear regression analysis for the determination 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 molecule. The magnetic shielding principal elements of the nonprotonated aromatic carbon in poly(ethylene terephthalate) (PET) fibres and of the aromatic carbons in the central ring of an oriented liquid crystalline polymer network are reported. We estimate that the accuracy of the principal values is within 5 ppm.