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Author Topic: Carbon NMR DEPT problem  (Read 6495 times)

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green-goblin

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Carbon NMR DEPT problem
« on: July 25, 2008, 01:20:21 AM »

Hi all,

I have run a C13 dept 90 and dept 135 NMR on my sample (which is a modified water soluble polymer). The dept 90 shows a group of positive peaks which, as it is a dept 90, should indicate CH groups. However the dept 135 spectrum shows the same peaks as negative, meaning they are CH3 groups. Which is correct

Does anyone have any idea what is happening. Is this a regular occurrence in C13 NMR? The spectra have been repeated on a different spectrometer and the same pattern is seen. 

The sample is about 8 wt% in D20 and the spectrometer is at 500 MHz. Any suggestions would be great.

Thanks.
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Yggdrasil

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Re: Carbon NMR DEPT problem
« Reply #1 on: July 25, 2008, 04:00:51 AM »

You said this is a polymer.  Is there the possibility of overlapping peaks?
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green-goblin

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Re: Carbon NMR DEPT problem
« Reply #2 on: July 25, 2008, 04:09:40 AM »

Possibly. However I presume that if there is both CH and CH2 groups with similar chemical shifts, then the dept 135 would still show both, one negative and one positive.
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muse

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Re: Carbon NMR DEPT problem
« Reply #3 on: August 18, 2008, 12:53:41 AM »

Hi
Just a little comment.
DEPT135 gives CH and CH3 peaks as positive and CH2 as negative.

Does your spectrum count for all carbon and hydrogen that your molecule have?
You could run 2D experiments to get more information.
-m
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macman104

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Re: Carbon NMR DEPT problem
« Reply #4 on: August 18, 2008, 02:25:50 AM »

However the dept 135 spectrum shows the same peaks as negative, meaning they are CH3 groups. Which is correct
You cannot differentiate CH and CH3 peaks in the DEPT 135 (they both point in the same direction).  The CH3 peaks are those that are in the DEPT 135, but are not in the DEPT 90 (assuming no overlap).

Look here for an image. 

http://www.usm.maine.edu/~newton/Chy251_253/Lectures/DEPT/DEPT.html (figure 2)

If the ppm is the same for the peak in the DEPT 90 and 135, then assuming no overlap, they are the same peak.
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