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Friday, August 22, 2008

The Fourier Transform of a Single Rotational Echo

The spinning sidebands in MAS NMR spectra are the result of rotational echos in the free induction decay. These echos show up as spikes in the free induction decay (and can be used conveniently to set the magic angle). The intensity envelope of the spinning sideband manifold in the NMR spectrum mimics the static wideline spectrum. One can obtain an NMR spectrum similar to the wideline spectrum from MAS data by Fourier transforming a single rotational echo rather than the entire FID. This is illustrated in the figure below for the 27Al MAS data for kaolinite at 21.1 Tesla.

3 comments:

Philip said...

Cool post Glen. Check out the TOP processing trick for turning the 1 pulse signal of rotational echoes into a 2D spectrum. http://www.grandinetti.org/resources/Publications/070-jcp_138_174203_2013/070-jcp_138_174203_2013.pdf

philip

Reza Siavashi said...

Dear Glen,

Thanks for this beautiful and always helpful blog.

I have a question regarding the rotational echo. Do you know what is the physical or chemical origin of rotational echoes? In MAS NMR usually the signal from the top of the first rotational echo is Fourier transformed, why the rest of the FID is not used?

Thanks again,

Reza

Glenn Facey said...

Reza,
The origin of the rotational echo is related to a full rotor cycle at the magic angle averaging out an interaction (dipolar, CSA or first order quadrupolar). Take a look at Addrew's paper from 1958. In MAS NMR, one usually Fourier transforms the entire FID containing all of the rotational echos. The rotational echos are the origin of the spinning sidebands in the Fourier transformed NMR spectrum.

Glenn