NMR help

[glbxc]

Hi everyone,

 

I was wondering whether anyone could help me to explain why the chemical shifts in my proton and carbon NMR results do not exactly match with those reported in the literature?

The solvent used in my experiment is exactly the same as the one used in the literature of reference (CD3OD), however the frequency applied in my experiment was 500MHz as opposed to 400MHz by the study i am comparing my results with.

 

For example, in my proton NMR spectra, my results are usually 0.08 to 0.26 ppm higher than that in the literature (e.g: 5.16 x 5.00; 7.51 x 7.25). In the carbon NMR, the results, the variation goes from 0.01 to 3.58 ppm.

 

I wonder, is that acceptable? can I still mention that my results are in agreement with those reported in the literature?

 

I have looked everywhere and i couldnt find whether there are variation limits for comparison of NMR data.

 

THANKSSSSSSSSSSSSSS!!!!

[Mr Skeptic]

It's not so much the height that is your data. You need your relative heights, and the positions on the line. You'd get different heights by using a different concentration, for example, or a different exposure time.

[glbxc]

When you mention "heights", are you talking about the chemical shift (ppm)?

[Mr Skeptic]

Sorry, I misunderstood you. I was talking about intensity. Um, if you stretch or shift your graph, can the peaks be made to match the reference graph? I don't really know too much about NMRs.

[glbxc]

the problem is that I only have the reference results in a table format, not in a graph:(

[Mr Skeptic]

Tables can be graphed. You only really need the peak locations (though the intensity would be useful too).

[UC]

A small amount of variation happens sometimes. It probably has more to do with the instrument shimming and proper referencing than anything else. Concentration of sample may also be an issue as various kinds of intermolecular bonding change chemical shifts.

 

I'd say if you have:

 

1) the right number of peaks

2) roughly appropriate ratios of integration area/peak height (integration is better, but sometimes things overlap a bit) for proton spectra (this goes down the toilet for carbon). I say roughly appropriate, because different protons have different relaxation times and the integration/height ultimately comes down to instrument paramaters versus these relaxation times. As an example, phenolic hydrogens consistently come up "short" with a relative integration of about 0.6 on the NMR I usually use. I could increase the signal collection time and the integration would increase to ~1.0 relative to other hydrogens, but it's not worth it unless it's for publishing.

3) roughly proper locations

 

Then, you're fine.

[glbxc]

UC,

 

Many many thanks for your input on that!! Much appreciated::

[Horza2002]

Well sevral affects can affect where the peaks occur. A major one will be concentration of ur sample. I have observed signals that move up to 1ppm simply by changing the concentration. Temperature is another major influence on the position of the peaks.

[spin-1/2-nuclei]

Hi...

 

Unfortunately, the variations you've reported are too much to say that your compound is consistent with data observed in literature.

 

Changing the MHz from 500 to 400 should not effect the center of the peaks. Your signal to noise and resolution should improve, but the center of the peaks should not change, therefore they should be at the same ppm. Is it possible that you have a metal contaminant? Shimming should not change the ppm of your peaks, bad shimming can cause them to be mishapen, so you likely have a sample related problem. Another good way to check this is to see if the peaks for a standard sample are also off. If that is the case you might need to ask the NMR technician for help.