Very fast MAS and MQMAS NMR studies of the spectroscopically challenging minerals kyanite and andalusite on 400, 500, and 800 MHz spectrometers
✍ Scribed by Lawrence B Alemany; Stefan Steuernagel; Jean-Paul Amoureux; Rhonda L Callender; Andrew R Barron
- Publisher
- Elsevier Science
- Year
- 1999
- Tongue
- English
- Weight
- 355 KB
- Volume
- 14
- Category
- Article
- ISSN
- 0926-2040
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✦ Synopsis
The well-characterized minerals kyanite and andalusite have long presented great challenges in using solid state 27 Al NMR to determine the isotropic chemical shift d , quadrupole coupling constant e 2 qQrh, and asymmetry parameter h for CS each of the inequivalent aluminum sites in these minerals. Indeed, these minerals have frequently been used to test advances Ž 1 . in instrumentation. Recent advances in magnet technology up to 18.8 T s 800 MHz H and in MAS probe technology Ž . spinning up to 35 kHz and considerably stronger rf and refinements of the two-dimensional, multiple quantum magic angle Ž . spinning MQMAS technique suggested that these developments could be profitably used to study kyanite and andalusite by solid state 27 Al NMR. The benefit of being able to study kyanite both by MAS and MQMAS techniques on 400, 500, and Ž . 2 800 MHz spectrometers is demonstrated. The two octahedral aluminum sites with the largest and nearly equal e qQrh values give overlapping 1D MAS or 2D 3QMAS signals at all three field strengths. Nevertheless, quantitatively accurate 3Q Ž 2 . signal intensities at 9.4 T for all four octahedral aluminum sites with e qQrh values up to 10 MHz allow more detailed Ž 2 analysis. Even if the 3Q signal intensities are not quantitative, their isotropic shifts provide an approach if accurate e qQrh . and h values are available other than deconvolution of the MAS spectrum for calculating d values. For andalusite, 34 CS kHz MAS on the 800 MHz spectrometer significantly narrows the extremely broad signal for the octahedral aluminum, and only slight difficulties are encountered in quantitating the relative amounts of AlO and AlO present. Even with 5 6
e 2 qQrh s 15.3 MHz, the octahedral aluminum in andalusite gives a signal in a MQMAS experiment, albeit of reduced intensity. As appropriate, we discuss some of the benefits and limitations of these advances in instrumentation and of different experimental approaches for studying non-integral spin quadrupolar nuclei in solids.