NMR Spectrum - expected triplets appear as singlets

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My NMR of diethyl cyclopent-3-ene, 1, 1 , dicarboxylate shows the cyclic proton environments both as singlets where they are expected to be a doublet and triplet
My NMR of diethyl cyclopent-3-ene, 1, 1 , dicarboxylate shows the cyclic proton environments both as singlets where they are expected to be a doublet and triplet.
Proton env B would be epxected to be a doublet and D to be a doublet. Everyone in the lab got the same result. Is there an explanation as to why it they are singlets and not multiplets as expected? The chemical shift and integration suggest they are the expected proton enviornment however the multiplicity is not as expected.
NMR exp 2.png
nmr exp 2.png
 
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  • #2
It seems there are only 4 general possibilities:
1) the specimens are not what you think they are
2) plot resolution is not sufficient to resolve close peaks
3) NMR machine needs servicing
4) you have an incorrect expectation of results

Cheers,
Tom

p.s. Please let us know what you find causing the effect.
(we all like to learn!)
 
  • #3
Tom.G said:
It seems there are only 4 general possibilities:

There is more, for example in some molecules resonance can change the "average" observed structure, making the spectra different from the one that looks obvious and logical. I doubt that's the case here though.
 
  • #4
I think it is the orientation of the hydrogens in the ring. Check the Karplus equation. You will see that certain orientations lead to low values for J(H-C-C-H). The protons at the double bond - D - are in the plane of the ring, the ones at position B are on tetrahedral carbons and not in the same plane.

So then I googled the proton spectrum of your product, and got the same spectrum as you have.
See this https://www.rsc.org/suppdata/cc/b6/b618341g/b618341g.pdf and look at the first spectrum.

I actually googled proton nmr of cyclopent-3-ene first and up came your product.
 
  • #5
What effect does the symmetry of the molecule have on the observed spectrum?
 

1. Why do expected triplets in an NMR spectrum appear as singlets?

This phenomenon usually occurs due to rapid molecular motion that averages the magnetic environment of the nuclei, effectively collapsing what would be a triplet into a singlet. This can be influenced by temperature, solvent viscosity, or the molecular structure itself. If the coupling between nuclei is averaged out by these factors, the distinct peaks of a triplet may not be resolved.

2. How can temperature affect the appearance of NMR peaks?

Temperature can have a significant impact on NMR spectra. As temperature increases, molecular motion typically becomes more rapid, which can lead to averaging of the magnetic environments of the nuclei. This increased motion can cause the peaks in an NMR spectrum, such as triplets, to collapse into singlets because the coupling between nuclei is averaged out over the rapid motion.

3. Can the choice of solvent influence whether triplets appear as singlets in an NMR spectrum?

Yes, the solvent can play a crucial role in the appearance of NMR peaks. Solvents with different viscosities or polarities can affect the relaxation times and molecular motion of the sample. A solvent that facilitates faster molecular motion can lead to the averaging of magnetic interactions and thus a simplification of the spectrum where triplets may appear as singlets.

4. What does it mean if all peaks in an NMR spectrum are singlets, even though multiplets are expected?

If all peaks in an NMR spectrum appear as singlets despite expecting multiplets, it could indicate that the sample is undergoing very rapid molecular motions, or there might be an issue with the experimental conditions such as temperature or incorrect sample concentration. It might also suggest that the magnetic field is not homogeneous or that there is an instrumental calibration error.

5. How can I confirm that the appearance of singlets instead of triplets is not due to an experimental error?

To confirm that the observation is accurate and not due to experimental error, you can try adjusting the experimental conditions. Varying the temperature, changing the solvent, or adjusting the concentration might help. Additionally, ensuring that the NMR instrument is properly calibrated and that the magnetic field is homogeneous across the sample can also eliminate potential errors. Running a standard known sample can further help in verifying the instrument's accuracy.

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