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Chemistry Forums for Students => Organic Chemistry Forum => Organic Spectroscopy => Topic started by: insertwittyname on October 21, 2017, 06:51:37 PM

Title: missing proton peak in proton NMR
Post by: insertwittyname on October 21, 2017, 06:51:37 PM
 I prepared this compound in the lab C=CC(NC1=CC=C(C(C2=CC=CC=C2)=O)C=C1)=Oby a reaction between C=CC(Cl)=O and O=C(C1=CC=CC=C1)C2=CC=C(N)C=C2. I then took the 1H-NMR in CDCl3. While I've managed to find the peaks corresponding to the various protons, I can't see the one for the proton on N in the amide group. Mestrenova predicts the missing peak at a δ value of 8.84, but the only unidentified peak I have (which might be proton I'm looking for since all other peaks have been assigned) is at a δ=5.23
However on integrating the protons, it only shows 0.15 protons, which makes me doubt if it really is the amide peak. Would be glad to hear any suggestions.
PS: I used triethylamine to neutralise the HCl formed. Could the basic nature of the amine abstract the amide proton, thus returning a value much less than expected?
Title: Re: missing proton peak in proton NMR
Post by: Flatbutterfly on October 21, 2017, 06:59:19 PM
N has a nuclear spin I = 1 and this results in quadrupolar broadening often into the baseline.
Title: Re: missing proton peak in proton NMR
Post by: wildfyr on October 21, 2017, 10:59:37 PM
I agree with butterfly, amide proton peaks can be broad or missing depending on exact conditions
Title: Re: missing proton peak in proton NMR
Post by: insertwittyname on October 22, 2017, 04:01:33 AM
Flatbutterfly and wildfyr, thank you both for your help :D
Title: Re: missing proton peak in proton NMR
Post by: kriggy on October 22, 2017, 08:14:53 AM
You might try to measure it in different solvent or add little bit of acid
Title: Re: missing proton peak in proton NMR
Post by: Flatbutterfly on October 22, 2017, 10:24:18 AM
It is counter-intuitive but quadrupolar broadening increases at lower temperatures and hence running the spectrum at low T may bring the broadening into the fast NMR timescale domain and result in a sharper singlet. Hence run the spectrum at -90°C in CD2Cl2.
Title: Re: missing proton peak in proton NMR
Post by: clarkstill on November 16, 2017, 11:20:43 AM
Try passing your CDCl3 over anhydrous basic alumina before dissolving your sample. Often amide broadening is due to hydrogen bonding with water and/or the presence of HCl (or DCl) in old bottles of chloroform.