Chemical Forums

Please login or register.

Login with username, password and session length

Sponsored links

Pages: [1]   Go Down

Author Topic: Difficulties to get a proper NMR spectra  (Read 1675 times)

0 Members and 1 Guest are viewing this topic.

clemi2310

  • Regular Member
  • ***
  • Mole Snacks: +0/-0
  • Offline Offline
  • Posts: 27
Difficulties to get a proper NMR spectra
« on: October 21, 2016, 03:16:31 AM »

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!

Logged

Babcock_Hall

  • Chemist
  • Sr. Member
  • *
  • Mole Snacks: +198/-14
  • Offline Offline
  • Posts: 3095
Re: Difficulties to get a proper NMR spectra
« Reply #1 on: October 21, 2016, 03:22:22 AM »

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

clemi2310

  • Regular Member
  • ***
  • Mole Snacks: +0/-0
  • Offline Offline
  • Posts: 27
Re: Difficulties to get a proper NMR spectra
« Reply #2 on: October 21, 2016, 03:30:06 AM »

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?
Logged

Babcock_Hall

  • Chemist
  • Sr. Member
  • *
  • Mole Snacks: +198/-14
  • Offline Offline
  • Posts: 3095
Re: Difficulties to get a proper NMR spectra
« Reply #3 on: October 21, 2016, 07:33:27 AM »

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.
Logged

discodermolide

  • Chemist
  • Sr. Member
  • *
  • Mole Snacks: +404/-70
  • Offline Offline
  • Gender: Male
  • Posts: 5038
    • My research history
Re: Difficulties to get a proper NMR spectra
« Reply #4 on: October 21, 2016, 06:57:37 PM »

Try pyridine as the NMR solvent.
Logged
Development Chemists do it on Scale, Research Chemists just do it!
My Research History

Irlanur

  • Chemist
  • Full Member
  • *
  • Mole Snacks: +28/-4
  • Offline Offline
  • Posts: 411
Re: Difficulties to get a proper NMR spectra
« Reply #5 on: October 23, 2016, 09:31:37 AM »

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
Logged

clemi2310

  • Regular Member
  • ***
  • Mole Snacks: +0/-0
  • Offline Offline
  • Posts: 27
Re: Difficulties to get a proper NMR spectra
« Reply #6 on: October 23, 2016, 10:13:39 PM »

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.
Logged

clemi2310

  • Regular Member
  • ***
  • Mole Snacks: +0/-0
  • Offline Offline
  • Posts: 27
Re: Difficulties to get a proper NMR spectra
« Reply #7 on: October 25, 2016, 11:58:19 PM »

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 !
Logged

Irlanur

  • Chemist
  • Full Member
  • *
  • Mole Snacks: +28/-4
  • Offline Offline
  • Posts: 411
Re: Difficulties to get a proper NMR spectra
« Reply #8 on: October 27, 2016, 02:01:10 AM »

Glad it worked out.

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

most likely.
Logged
Pages: [1]   Go Up
 

Mitch Andre Garcia's Chemical Forums 2003-Present.

Page created in 0.057 seconds with 24 queries.