NMR spectra and coupling patterns?

Question

I am supposed to deduce structures from given molecula formulae and the NMR spectra described. I have to talk about individual peaks to protons and coupling patterns.

eg: C2H3BrCl2

3.1 doublet 2H

5.5 triplet 1H

I don't know if this a lot to ask but can someone explain this all to me? I want to know how you find which peaks are what and how you find coupling patterns as I have no clue!

Please explain the example given!

Thank you in advance!

Answer 1

Review the theory of proton-proton coupling in NMR. You should quickly be able to convince yourself that if you have a group of equivalent protons coupled to one other proton, you will get a doublet, whereas if they are coupled to two other protons, you will get a 1:2:1 triplet.

The relative intensities also give information.  So you have one proton (signal at 5.5) coupled to two protons (signal at 3.1).  The intensities and the splitting patterns both make sense.

So you do not have the isomer with all three hydrogens on the same carbon. I do not know enough to say if it is possible to distinguish between the different isomers with two H on one carbon and the remaining H on the other.

Importantly, there are two different conventions for reporting peak positions; delta and tau. You need to tell us which convention you are using if you want someone more knowledgeable than me to interpret the positions of the peaks.

Hope this helps.