The measurement of {sup 13}C chemical shift tensors in complex polycyclic aromatic compounds and coals by an extremely slow spinning MAS experiment
- Univ. of Utah, Salt Lake City, UT (United States)
- Chinese Academy of Sciences, Wuhan (China)
The {sup 13}C CP/MAS experiment has proven to be a powerful technique for obtaining high resolution spectra in complex solids such as coal. MAS narrows the chemical shift anisotropy (CSA) to its isotropic shift when the sample spinning speed is greater than the anisotropy. While the isotropic chemical shift is useful in characterizing chemical structure, the principal values of the chemical shift tensor provide even more information. These principal values are available from the powder pattern obtained from a stationary or slowly spinning sample, Unfortunately, the overlap of many broad powder patters in a complex solid often prevents the measurement of the individual principal values. In an effort to address this problem of spectral overlap, many 2D techniques have been developed to simultaneously obtain the dispersion by isotropic shift, such as produced by MAS, in one dimension and the tensorial information as separate powder patterns in the second dimension. A very successful technique is the slow spinning modification of the magic angle hopping experiment recently proposed by Gan, which we call the Magic Angle Turning (MAT) experiment. This experiment has a number of advantages over earlier 2D methods. The use of very slow spinning speeds (<50 Hz) favors the quantitative polarization of all carbons and allows the use of a large volume sample rotor resulting in a typical 2D spectrum acquisition requiring less than 24 hours. The mechanical device for slow spinning is very stable and high resolution in the isotropic chemical shift dimension can be easily obtained. The MAT experiment could be done on a suitably stable MAS probe. The only disadvantage of the original MAT experiment is that data acquisition starts right after the last pulse, causing distortion in the evolution dimension (the second dimension) even if a delay as short as 20 {mu}s is used.
- OSTI ID:
- 254675
- Report Number(s):
- CONF-940813--
- Country of Publication:
- United States
- Language:
- English
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