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Wednesday, March 26, 2008

1H NMR Spectra of Solids

Students will sometimes come into the lab with an insoluble organic material and ask me for a 1H NMR spectrum of the solid. My response is usually, "I'll draw it for you." after which I will draw a broad lump on the nearest piece of scrap paper and hand it to them with a smile. I will then explain why.

In the liquid state, molecules are randomly tumbling and the dipole-dipole coupling between the protons is averaged to zero allowing the observation of sharp resonances, isotropic chemical shifts and J coupling. In solids, the molecules are more rigid and each proton is dipolar coupled to many other protons. All of the protons are coupled together in a large network. Since the dipolar coupling is much larger than the chemical shift range and any J coupling, the spectrum appears as a broad lump, typically about 50 kHz or so in width. Magic angle spinning will help the situation but is not usually fast enough to average the dipolar coupling. "High" resolution 1H spectra of solids can be obtained (with great effort!) by combining magic angle spinning and multiple pulse decoupling (CRAMPS). In this method points are sampled during windows in the multiple pulse decoupling scheme. Typical resolution in a CRAMPS spectrum is still only about 1 ppm or so. In the figure below, 1H NMR spectra are compared for a static organic solid, an organic solid spinning at the magic angle at 12 kHz and the same solid dissolved in a solvent. In the MAS spectrum one can see that the 1H resonance is narrower with spinning sidebands at 12 kHz, however the resolution is insufficient to observe discreet resonances.

3 comments:

Victor said...

Hi Glenn,

welcome to the 21st Century!

http://nmr900.ca/images/Glycine-FastMAS.jpg

Glenn Facey said...

Hi Victor,

Yes, a VERY BEAUTIFUL 900 MHz 1H spectrum of solid glycine indeed. It really shows how far solid state NMR has come and highlights the ability of our National Ultrahigh-Field NMR Facility for Solids, however, even at the highest available fields and the incredible spinning speed of 70 kHz, the line widths are still ~ 1 ppm. This is hardly high resolution in comparison to the spectra of liquids where the line widths are typically 3 orders of magnitude smaller. Perhaps the 22nd century will have more to offer.

Glenn

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