Hello.
Recently, NMR spectrum of some samples showed that the -CH3 peak is very small as shown in the figure I attached. The sample has three -CH3 functional groups, and two of these -CH3 in structure will be detected as one peak. Another -CH3 peak appeared at 27 ppm and two methyl groups were found at around 40 ppm (very small peak) and confirmed by 2D NMR.
However, I wonder if the -CH3 group can be so small. Do you have this experience? Do you have any ideas about this?
You have rather typycal pattern of chemical exchange with intermediate rate comparable with reciprocal of echangable signals deference expressed in Hz. It can be hindered rotation. You chould receive spectra at low (-10 - 20C) and should see splitting of the broad signal. If you find not a reasonable scheme of exchange you can send me the structure but I think you easealy find it yourself
Before you say a sensitivity of a peak is too low, always check the integrals. Peak heights have no quantitation meaning, only integrals do. And even though quantitation in decoupled 13C is not really possible, you will see that the integrals of your peaks tell a different story than the heights. At least I think so. In any case, the recommendation of Valentin Talsi is also to be followed.
It was mentioned that the signal is actually a CH3 because there was confirmation by 2D. Which 2D spectrum ? It can be a question that involves relaxation time. Maybe a longer delay will intensify the signal. It has not been clear whether it is a structure substance already known or not. Without seeing the 1 H and 2 D spectra it is difficult to rule out the possibility of impurity, or error of signal signaling.
Jabbar S Hadi , Sir, How many scans will you recommend for a 3mg sample so as to obtain a reasonable good carbon 13 signals?
There are two reasons.
1) Relaxation time given is very less which is very distinct possibility as one of your methyls is present with adequate response.
2) The presence of one or more quadrupolar nuclei in adjacent position. (sometimes broad signal of -CH3 groups observed in -N,N Dimethyl group.
If in your case, second possibility is true than do one thing, add a drop of H2O (demineralised water) to your sample and run a 13C NMR again. This will reduce effect of the quadrupolar nucleus.
Hope this helps.
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