Lately I'm synthesizing a compound,
(the structure of this compound is in the picture)
I found out the 1H-NMR of this product are all broad peaks. I suppose it might be due to the present of mixture of rotamers (conformational isomers), as stated by the related literature. (Bournaud, C.; Chung, F.; Pérez Luna, A.; Pasco, M.; Errasti, G.; Lecourt, T.; Micouin, L. Synthesis 2009, 869– 887.)
So, I decided to do VT-NMR to see if I can get sharp peaks in my spectra, and luckily, most of the peaks became sharp at 75 degree(as shown in the picture). I choosed dmso-d6 as my D-solvent, because high temperature(> 60 degree) is needed to observed sharp peaks. (I've tried chloroform-d6 before, the peaks remained broad at 60 degree)
But a singlet is observed around 8.24 ppm, I suppose it might be the signal of chloroform-d6 that I used before, (or the signal of CHCl3, because I often use CHCl3 to get rid of n-hexane and ethyl acetate after column, in order to have a clean 1H-NMR spectra without any solvent peak).
I've put my compound under vacuum about one week, before I conduct the VT-NMR,(and there's still solvent remain...),and I'm sure that the compound is pure, without any impurity.
I'm asking how can I get rid of chloroform or other solvents before I conduct VT-NMR?
It's most likely CHCl3. To be sure, you could go for an HSQC and HMBC measurement and further verify your compound's structure by the way.
Why do you need to remove this residual peak in your NMR? If you want to remove it, try co-evaporation using a more volatile solvent (such as CH2Cl2).
Benedict Elvers thanks for your reply. I need to remove the residual peak in my NMR because I use dmso-d6 as d-solvent to conduct the VT-NMR experiment, and this solvent peak will be very obvious on my spetra (my advisor cares "A LOT" about that). I've found out that my compound dissolves in various kind of solvents easily, and I find it hard to remove them, even letting my compound stay at vacuum for a long time, solvents still remains. This problem has embarrassed me for a long time, since I began this project one year ago...
1) Do you use chloroform to clean the NMR tubes and caps? The signal could came from that
2)Do you reference the spectra to DMSO? It appears at 2.5 and I'm not sure of seeing that signal...
3) You could try freeze-drying to dry the product, it usually gives good results.
Anyway, the CDCL3 in DMSO appears at 8.32 ppm w.r.t. DMSO. (Article NMR Chemical Shifts of Trace Impurities: Common Laboratory S...
)
It is CDCl3 and not chloroform-d6. Did you try to record the spectrum of DMSO-d6 solvent alone prior to recording the solution? If the singlet is from the DMSo-d6 solvent then you can safely ignore this peak. If the solvent is exposed to air for a long period then dmso absorbs moisture from the air and leads to the display of water peak.
Marcos Martínez Fernández
Thanks for your reply,
1) No, I don't use chloroform to clean my NMR tube, instead, I use acetone and deionized water . I use chloroform in order to co-evaporate hexanes in my compound, because without doing that, hexanes will still remain in my sample after staying for a long time in vacuum.
And this works well when I use chlorform-d6 as my d-solvent, I see no hexane peaks(and no chlorform peak, because the peak of chloroform shares the same peak of that of the peak of chlorform-d6). But when I do VT-NMR, I must change the d-solvent to DMSO-d6 , and I can see the peak of chloroform in my spectra, that makes me embarrassed...
2) That's indeed the peak of DMSO, the peak at 2.5 ppm overlaps with the signal of the compound.GOOD FIND! :)
Thanks for your reply Max.
Does the compound crystallize during the chloroform evaporation? Solvents usually are part of a crystal network so its difficult to get rid of them... Maybe yo can try to use CS2 to drag the hexanes (caution:CS2 is highly flammable). Also you can try to use cyclohexane and freeze-dry the remanent solvent (sublimation)
My bests
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