Why in IH NMR ifwe use Chloroform as Solvent some times we wont get NH or OH signals?

Question

Because Some yomes in Chloroform we are getting signals for NH and OH

Answer 1

It all depends on the (chemical) exchange rate of the protons in NH and OH moieties in order to get signals from them or not.

If NH and OH protons are bound in hydrogen bonds, it is likely to see sharp signals. You can even see the fine splitting. Amide protons in peptides always show doublets if they are hydrogen-bonded.

However, it is important that your solvent doesn't contain traces of DCl or HDO (or D2O), since that enhances the exchange rate of exchangeable protons a lot. So, using either fresh CDCl3 or keep it dry over molecular sieves is an absolute prerequisite.

A question about NMR .. charming :-)

Answer 2

it's dependent on the compound. if the compound reacts slightly with the water in chloroform than signals will disappear or the peaks are simply too broad. try using dry chloroform (molecular sieves in the CDCl3) or you might be able to get away with changing solvents, but if you go to alcohol there is no way you'll see OH or NH peaks

Answer 3

Maybe the peaks are present, but they're so broad that you do not observe them. The Chloroform is probably diluted in water, and so there is fast exchange between the Hs in the OH groups and NH groups and the Hs in the water molecules. This causes the peaks to broaden, and are sometimes unobservable.

Answer 4

Three things

in NMR deuterated chloroform (CDCl3) is used

OH and NH are normally seen in CDCl3

Only if deuterated water (D2O) is added does the OH and NH disappear

Answer 5

Maybe because of their favored hydrogen-bonded dimeric association with the solvent of the compund you are looking at the resonance signal is significantly shifted down-field. So the signals are really there just not where they are expected.

Answer 6

They are there, just too broad to be seen. They are exchanging too rapidly. If you use really dry CDCl3 and you sample is dry, you should see them. DMSO, d6 usually gives the best opportunity to see exchangable signals. In MeOH, d4, D2O, and other protic solvents, you'll never see them because they are exchanging with the duterium.

Answer 7

You're lucky. You're three hours ahead of me. Don't bogart the chloroform eh. I think I'll need some myself to get through the day.