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Monday, January 5, 2009

1H / 27Al TRAPDOR NMR of Kaolinite

TRAPDOR (TRAnsfer of Populations in DOuble Resonance) NMR (Grey and Vega, JACS 117, 8232 (1995)) is a solid state NMR technique where the effects of dioplar coupling between a quadrupolar nucleus and a spin I = 1/2 nucleus can be observed in the spectrum of the spin I = 1/2 nucleus. The technique relies on a rotor synchronized spin echo of the spin I = 1/2 nucleus with CW irradiation of the quadrupolar nucleus during the first echo delay period. The CW irradiation during a single rotor cycle behaves like an adiabatic frequency sweep as the quadrupolar frequencies vary over the course of the rotor cycle. The effects of dipolar coupling between the quadrupolar nucleus and the spin I = 1/2 nuclei, which are normally averaged by MAS, are reintroduced in the TRAPDOR measurement and the complete refocusing of the spin I = 1/2 NMR signal is prevented. The technique therefore can be used to determine whether or not a spin I = 1/2 nucleus is close in proximity to a quadrupolar nucleus. The figure below shows the 1H / 27Al TRAPDOR NMR spectrum of kaolinite at 11.7 Tesla. The top two traces are conventional rotor synchronized 1H Hahn echo spectra acquired with MAS rates of 12 kHz and 2.8 kHz, respectively. The bottom trace was acquired at the same spinning speed as the middle trace with CW irradiation of the 27Al during the first echo delay. One can see a very much reduced 1H echo indicating the presence of heteronuclear 1H - 27Al dipolar coupling.
This technique can be used to "find" quadrupolar neuclei which are "invisible" by direct detection due to their very large quadrupolar coupling constants.

4 comments:

Anonymous said...

Nice post. Thank you. Could you tell us how do you do the phasing for the TRAPDOR spectrum? Did you use the same parameters as the Hahn echo? Sometimes people use the "difference" spectrum, by substracting the TRAPDOR from Hahn echo, but often substracting two spectra generates sharp negative peaks, any suggestions?

Glenn Facey said...

Anonymous,

Thank you for your comment. You raise a very important (and in my opinion often overlooked) point. Indeed, there is a significant phase difference between the Hahn echo spectrum and the TRAPDOR spectrum. These phase differences make difference spectroscopy very problematic and subjective as each of the spectra must be phased independantly. The most sensible thing to do to avoid subjective errors due to phasing differences is to consider the difference in the overall signal intensity.

Glenn

Anonymous said...

Thank you Glenn. Your answer is very helpful. By the way, how do you measure the overall signal intensity? Do you use the intensity of the echo top in FID (real+imaginary) or you do the integration in the spectrum? Thank you.

Glenn Facey said...

Anonymous,

In this case I did not measure the signal intensity at all however either of the ways you propose should be OK - or perhaps even calculating a magnitude spectrum.

Glenn