Proton NMR & Analyzing NMR Spec - What to Do?

[Moogie]

Hi

Are there any protons that do not show up on proton NMR as I am analysing an NMR spec (of a known molecule but I don't know what it is!) and I can't get my integration to equal the number of Hs in the empirical formula of the molecule

Many thahks

 

[espen180]

If a proton is hydrogen bonded to another molecule (-OH-groups, COOH, even -NH₂) it may not show up on the spectrum.

Check for example the spectra of ethanol, acetic acid and ethyl amine.

What is the empirical formula?

 

[Ygggdrasil]

Many times, if you are using protic solvents, acidic protons, like the proton is -OH, -COOH, and -NH₂ groups will not show up in proton NMRs because they are being exchanged with the solvent.

 

[Moogie]

It must be some kind of alcohol or acid then. I didnt know some protons didn't show up. I'll be ok now :) The empirical formula is C9H8O2. Perhaps some sort of carboxylic acid from the 2 x oxygen.

 

[espen180]

depending on the number of missing protons, it might be a carboxylic acid, ether alcohol or diol. It might even be a hydroperoxide, who knows?

 

[chemisttree]

It must be some kind of alcohol or acid then. I didnt know some protons didn't show up. I'll be ok now :) The empirical formula is C9H8O2. Perhaps some sort of carboxylic acid from the 2 x oxygen.

In addition to exhcangeable protons (protons alpha to a carbonyl haven't yet been mentioned) you can have other effects. If your sample has several proton types that have significantly different T₁'s you can saturate those with the longer T₁'s in a multi pulse experiment. If you use too large of a tip angle, that becomes problematic in multi pulse experiments. If the offset frequency is for some reason off to one side or the other, integrations are affected. I've seen bad phase correction affect integration for protons near the low and high sides of the spectrum.

If you want to get as accurate as you can, perform a single pulse experiment on a sample of 33% concentration using a non-exchangeable solvent like CDCl3. Use a 45 degree pulse.