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Monday, February 11, 2008

High Resolution CPMAS NMR of 13C Bonded to 14N

In 13C CPMAS spectra, the resonances of 13C bonded to 14N can give unusual line shapes. They often show up as 1:2 doublets which should not be interpreted as seperate resonances. This effect is observed because the dipolar coupling between the spin I=1/2 13C and the quadrupolar, spin I=1 14N is not fully averaged by the magic angle spinning as is the case for a pair of spin I=1/2 nuclei. The appearance of the spectrum is a function of the quadrupolar coupling constant for the 14N, the orientation of the electric field gradient tensor with respect to the internuclear vector, the distance between the 13C and the 14N, the strength of the magnetic field and the 14N T1 relaxation time. This effect is not specific to 13C and 14N, but a general effect between and spin I=1/2 and quadrupolar neulei. In the figure below, the 50 MHz 13C CPMAS spectrum of D,L-alanine is presented as an example. The resonance shaded and expanded in yellow is the carbon directly bonded to nitrogen.

2 comments:

Anonymous said...

Hi,

I notice that cp-mas is typically done at very high field for increased sensitivity and resolution.

However, how can one determine or compute the minimum field that would still give satisfactory spectra? I see that some folks publish 13C NMR spectra using strengths of 20 MHz or so, but can in theory one push the limits and go to a lower field? What information should be considered?

thanks so much,
Andy S.

Glenn Facey said...

Hi Andy,

Thank you for your question. High fields do indeed give higher sensitivity however the chemical shift resolution may not improve as much as one might initially think. This is due to the fact that the chemical shift dispersion for each site (in units of ppm) is constant and therefore larger (in frequency units) at higher fields. See an example here:

http://u-of-o-nmr-facility.blogspot.ca/2009/09/field-dependence-of-chemical-shift.html

When a spin I=1/2 nuclide is attached to a quadrupolar nuclide, the CPMAS signal for the I=1/2 nuclide gets narrower at higher fields as described in the post. You can certainly find the lowest acceptable field to get sufficiently narrow lines. This can be done by simulating the spectrum as a function of Larmor frequency with the readily available program "Simpson".

http://nmr.au.dk/software/simpson/

-Glenn