University of Ottawa NMR Facility Web Site

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Thursday, November 20, 2008

The Importance of Grinding Solid Samples

When the heteronuclear dipolar coupling interaction has been removed by high power decoupling, the NMR spectra of dilute spin I = 1/2 nuclei in a single crystal give rise to relatively sharp lines. The frequencies of the lines depend on the chemical shift tensor and the orientation of the single crystal with respect to the magnetic field. Finely powdered samples have many thousands of crystallites and all orientations of the crystallites with respect to the magnetic field are represented equally. As a result, for powders, one obtains a broad powder pattern. Samples that are not ground into a powder contain many fewer crystals than crystallites in a powder and will yield spectra with partially resolved lines. The envelope of lines for all of the crystals will approximate the true powder spectrum. An example of this is shown in the figure below.

Thank you to Victor Terskikh of the National Ultrahigh Field NMR Facility for Solids. for suggesting this post and kindly providing the data for the figure.

3 comments:

Vinod Chandran said...

Powder lines are broad. Single crystal lines are narrow. I am wondering how quasi-crystal lines may look.!?

vinod

Glenn Facey said...

Vinod,

Very good question. I don't know the answer but would guess that the lines would be reasonably sharp (but not as sharp as those from a classical crystal). Quasicrystals have local order but lack long range periodic order. NMR is sensitive to local order.

Glenn

Vinod Chandran said...

Thanks, Glenn.

It will be highly educational for NMR people, if somebody does a comparison of NMR responses of single-crystal, quasi-crystal and powder forms of the same material.

Have a nice weekend, Glenn.

Regards
Vinod