Anisotropic - Isotropic Correlation The Measurement of CSAs

In section 3.1, it was stated that it is possible to extract the anisotropy and asymmetry of the CSA by fitting the observed MAS sideband intensities. It is, however, necessary to be able to resolve, at a low vr, the sidebands of the different resonances. The problem of the ID spectrum becoming increasingly more crowded as the number of distinct resonances increases can be overcome by extending the NMR experiment to a second dimension. 

For the original MAH and MAT techniques, a significant drawback was the long measuring time (1—2 days) that was required even when up to 5 g of sample was used. However, modified versions of the MAT approach employing 180° rather than 

90° pulses and using a faster % have been presented, which offer a better experimental sensitivity [112, 113] For example. Fig. 9.23 shows the anisotropic CSA patterns for each resolved resonance in a selected region of the isotropic C spectrum of the terpene verbenol [114]. Six resonance lines are resolved for both the C2 and C3 carbons, with the CSA tensor spinning-sideband patterns being remarkably similar. Supported by quantum-chemical calculations of the CSA tensors, the solid-state NMR analysis demonstrated that verbenol exhibits polymorphism, with, however, only minor conformational variations in the distinct 

An alternative means by which the isotropic and anisotropic chemical shift interactions can be separated is the 2D PASS (phase-adjusted spinning sidebands) experiment due to Levitt and co-workers [115]. By changing the timings of the application of five 71 pulses in the tj dimension, it is possible to separate the spinning sidebands by order. As a specific example, Fig. 9.24 shows the 2D PASS spectrum for the antibiotic, penidUin-V. [116] An analysis of this spectrum allowed the determination of the CSA prindpal values for aU the resonances. A distinct advantage of this approach is that only very few (typically 16) increments must be made in the indirect dimension. 

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