Residual Coupling Effects due to Quadrupolar Nuclei in MAS Spectra

2 Residual coupling effects due to quadrupolar nuclei in mas spectra. It was seen that for both dipolar and spin-spin coupling between nuclei the first-order anisotropic effects are proportional to P2(cos0) so that under MAS if a nucleus experiences both these effects, only the isotropic effects of J remain. However, the 

This term is anisotropic and produces a powder pattern. It has been derived under the assumptions that first-order perturbation of the S-states is sufficient, that the J tensor is axially symmetric and that the unique axis of J is aligned with the intemuclear vector. Under MAS this term will be scaled but, as it is not proportional to P2(cos0), it cannot be completely removed. Hence the MAS spectrum will still have some residual width, but the most profound effect is to leave an isotropic term which can be calculated by averaging the powder lineshape. Hence for a J-coupled system with an axially symmetric quadrupole interaction, the spectrum is shifted from the isotropic chemical shift by  

This effect was first reported for (Opella etal. 1979, Frey and Opella 1980, 

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