Hi everyone,
I would like first of all to thank everyone who answered my call for suggestions. I’ll summarize here what was suggested and what I have done so far:
Most people suggested that the lack of signal in 13C was due to the dynamic exchange between the tautomers which may cause broadening of the peaks.
Solutions:
Rising the temperature
Rising the magnetic field
Applying large LB (5-20Hz) to see the signal buried in the noise
Other noticed the lack of signal in HSQC and were surprised (as I was) since, even during exchange, the HSQC signal will be expected.
Some suggested checking pulses, tuning of both channels, offsets.
This is what I tried:
Higher temperature and field range: There’ still no methyl signal at 40, 60 and 70C. No difference neither when I ran the sample on 500MHz (instead of 300). If anything, other peaks disappeared as we’re heading toward coalescence (as expected)
Applying LB: no change
All pulses, tuning, etc... were checked and calibrated before running the samples
The RT 1H spectrum indicates the molecule is in the slow exchange rate, as the peaks from each tautomers are well separated. The methyl group is on a diazole ring. The HRMS confirms the expected structure.
Cheers,
Celine
>
> From: celine schneider <cmschnei_at_mun.ca>
> Sent: 15 February 2019 20:45
> To: ammrl
> Subject: AMMRL: peaks missing in 13C NMR
>
> Hi everyone,
>
> one of our user's sample is showing the weirdest lack of 13C peaks in the methyl region and I can’t think of a way to explain it. The solvent is DMSO and the compound has CH3, CH2 and aromatics. It is believed to be a tautomer. The 1H spectrum is clearly showing the presence of CH3 (2 peaks) as expected from the structure. However, when we run 13C, no methyl groups are showing. Same when we run HSQC or APT or DEPT. Since we have no problem seeing the quartenary carbons (or the aromatic and CH2), I don’t think it’s a problem of relaxation. The only peaks missing are the 2 CH3. This happened for several derivative of the same compound but only when run in DMSO. If the sample is run in CDCl3, the methyl groups are there…
>
> Any suggestion?
>
> Celine
>
>
> Celine Schneider, PhD
> Adjunct Professor in Chemistry
> Research Laboratory Associate
> NMR Facility / C-CART
> CREAIT Network
> Memorial University of Newfoundland
> St. John's, Newfoundland, A1B 3X7
>
> Tel: (709)-864-3134
> email: cmschnei_at_mun.ca <mailto:cmschnei_at_mun.ca>
>
> http://www.mun.ca/creait/CCART/
>
Celine Schneider, PhD
Adjunct Professor in Chemistry
Research Laboratory Associate
NMR Facility / C-CART
CREAIT Network
Memorial University of Newfoundland
St. John's, Newfoundland, A1B 3X7
Tel: (709)-864-3134
email: cmschnei_at_mun.ca
http://www.mun.ca/creait/CCART/
Received on Tue Feb 19 2019 - 07:08:58 MST