[clemi2310]

Hello everyone,

Since several days, I have some troubles to obtain a good NMR spetra of theses kinds of compounds. My solvent is DMSO-d6 (my compound is not soluble in other solvents).

I mean, I get very broad spectrum (see file attached). Impossible to integrate something... It happens only with these compounds (diketo acids)

I am an organic chemist and I don't know a lot about NMR parameters.

From what I've read it could be:

- a problem of poor shimming (solution not enough homogeneous, or dirty glass tube?)
- my compound with tautomeric forms
- my compound forming aggregates with H-bonds between each molecules

But here you have an example of my spectra, you also see the solvent peak very broad not sharp at all. I don't know what to do.

The spectrometer is a JEOL 400MHz (brand not so well known...)

Any suggestions?

Thank you!

[Babcock_Hall]

If the problem were shimming, all of the peaks would be broad. 

[clemi2310]

Thank you for your answer.

Indeed, aromatics are properly defined. However, we don't see it in the picture but the DMSO peak is broad we only see a singlet.

So, do you think self aggregation is something plausible?

[Babcock_Hall]

DMSO is a 1:2:3:2:1 pentet, because of deuterium splitting; if you cannot resolve the deuterium coupling, it will give a single broad peak.  I think some sort of chemical exchange is plausible, including aggregation/disaggregation.  There is a chance that you have a paramagnetic impurity (probably a metal ion) that is transiently binding to the oxygen atoms.

[discodermolide]

Try pyridine as the NMR solvent.

[Irlanur]

Well i am not an organic chemist, but at first sight I would guess that there are lots and lots of tautomeric forms here...? If the exchange rates are in the intermediate regime, the peaks will always broaden. You can try:

-higher (or lower) magnetic field.
-different temperature to see wether the lineform changes.
-try to increase the echange rates, e.g. add some acid?
-complexate with an diamagnetic metal.

-just use 13C-NMR. this could also be done in the solid state

[clemi2310]

Thank you for all your answers.

Indeed, this compounds have several tautomeric forms, some of them are predominant, that is why I was thinking of chemical exchange as a plausible explanation.

To obtain this molecule I hydrolyse the Diketo ester form (with a Methyl group instead of the OH), with NaOH 1M, then I acidify with HCl 1M to get the acid form. Maybe Na+ is involved.

I will first try temperature change. I do have deuterated Pyridine, I can also try.

[clemi2310]

Hello,

We solved the problem with the person responsible for the NMR.

First, there was a problem of shimming, the z5 and z6 parameters were out of the limit with theses compounds. By doing it manually we solved the problem.

Then, we started to heat the sample with the following temperatures:

-20°C, 40°C, 60°C, 80°C, 100°C

100°C happens to be the best temperature the signal is well improved ! I assume tautomeric forms are the explanation.


Thank you for all your help !

[Irlanur]

Glad it worked out.

100°C happens to be the best temperature the signal is well improved ! I assume tautomeric forms are the explanation.

most likely.