H-NMR spectrum of my compound is very clear. All peaks present on spectrum (and their integrals) are as expected, except:
1) A peak corresponds to protons from a primary amine is missing. I considered this as reasonable, taking into consideration the fact that solvent mix consists MeOH.
2)There is an unknown tall peak at 3.82ppm. The shape and the integral doesn't match to be for primary amine's protons.
The solvent is CDCl3+0.3% TMS/MeOD (7:1)
Could you please give me any possible explanation?
Mr. Pach, thank you very much for your answer. I had already excluded two peaks (I didn't mention them) that corresponds to solvents, for CDCl3 (7.4ppm) and for CD3OH (3.4ppm). So, could the ''unknown'' to be due to water from humiditity maybe?
It may be due to water..... Are your compounds crystalline.? Then it may be water of crystallisation. Well, in my compounds also, water peak goes quite down field, but I haven't come across such a case... Peak at 3.82. You can try changing your Nmr solvent, and use anhydrous one.
Likely water. CDCl3 can be very hygroscopic, depending on how the solvent was stored.
Dear Eleni,
the easiest and fastest way to check if it is residual water is to run a hsqc - if there is no crosspeak at 3.82 ppm than it corresponds to exchangeable protons, i.e. water.
best greetings, markus
You will always have water in solvents even in NMR solvent ampoules. The chemical shift of H2O at 25 degrees in CDCl3 is ~1.6 ppm and in MeOD ~4.9. In a mixed solvent the chemical shift will be a weighted average. However, as you have a primary amine the exchangeable NH2 signals will likely coalesce with the water signal and if the sample is concentrated (multiple mgs of material) then the signal could again move.
To directly observe the NH2 you would need a reasonably dry aprotic solvent (CDCl3, DMSO etc preferably from an ampoule or a new bottle) and it should be observable.
I agree with Kenneth. The chemical shift of H2O in CDCl3 is ~1.6 ppm and in MeOD ~4.9! Measure the spectrum of the solvent without substance. You have few methanol in your mixture (1/8 from 0.3%), chemical shift of H2O should be ~1.6 ppm. Typically we see both peaks amine and water in CDCl3. But their position is in this case lower than 3.8 ppm. Probably peak at 3.82 ppm is the result of proton exchange amine groups with methanol and water. You are sure that your substance is amine? It can be an amide?
Dear Mrs. Elena Yu. Ladilina, thank you for your answer. Yes, I am sure that is a primary amine group..!
You've got a lot of labile protons (NH2, HDO, H2O, CD3OH) in yor sample which - in a fast exchange regime - may to more or less extend average to a new peak that does not match neither by chemical shift nor by integral (obviously)
You could try to lower the temperature to slow down the exchange. In this case the signal of amine may appear separately
put some Al2O3 (neutral or basic) for chromatographic usage in your tube, shake well, let it settle on the bottom of your tube and meassure again....
I agree with 1st answer, that of Dr. Kenneth Cameron. It is dissolved moisture peak in solvent mixture of CDCl3 + CD3OD. As he suggests, you may have to use aprotic solvent (fresh solvent free from moisture) and preferably lower temperature (say 278 K) to observe the NH2 peak.
Correction in your question:
when you say : The solvent is CDCl3+0.3% TMS/MeOD (7:1),
I think it is 0.03% TMS, otherwise it would be huge TMS peak .
best wishes, Raghothama
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