NMR Spectrum - expected triplets appear as singlets

  • Thread starter Thread starter chemistryfourm
  • Start date Start date
Click For Summary

Discussion Overview

The discussion revolves around the unexpected appearance of singlets in the NMR spectrum of diethyl cyclopent-3-ene, 1, 1, dicarboxylate, where doublets and triplets were anticipated. Participants explore potential explanations for the observed multiplicity, considering both theoretical and practical aspects of NMR spectroscopy.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that the cyclic proton environments are observed as singlets instead of the expected doublets and triplets, raising questions about the reasons behind this discrepancy.
  • Another participant suggests four general possibilities for the unexpected results: incorrect specimen identity, insufficient plot resolution, the need for NMR machine servicing, or incorrect expectations of results.
  • A different viewpoint introduces the idea that resonance can alter the "average" observed structure, potentially affecting the spectra, although the participant expresses doubt that this applies in this case.
  • One participant proposes that the orientation of the hydrogens in the ring may influence the observed multiplicity, referencing the Karplus equation and suggesting that certain orientations lead to low coupling constants.
  • A participant shares a link to a proton spectrum that matches the observed results, indicating that similar spectra have been documented for the product in question.
  • A question is raised regarding the impact of molecular symmetry on the observed spectrum, inviting further exploration of this aspect.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the cause of the unexpected singlet appearance, with multiple competing views and hypotheses presented throughout the discussion.

Contextual Notes

Participants acknowledge various factors that could influence the NMR results, including potential issues with specimen identity, instrument resolution, and molecular orientation, but do not resolve these uncertainties.

Who May Find This Useful

This discussion may be useful for chemists and researchers interested in NMR spectroscopy, particularly those exploring molecular structure and spectral interpretation in cyclic compounds.

chemistryfourm
Messages
1
Reaction score
0
TL;DR
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
 
Chemistry news on Phys.org
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!)
 
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.
 
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.
 
What effect does the symmetry of the molecule have on the observed spectrum?
 

Similar threads

NMR: Why do "identical" nuclei not couple with each other?
  • · Replies 3 ·
Replies
3
Views
4K
Why Does the 31P NMR Spectrum of PF5 Depend on Temperature?
  • · Replies 4 ·
Replies
4
Views
3K
I'm having trouble reading proton NMR
  • · Replies 3 ·
Replies
3
Views
4K
What are the key steps to analyzing an NMR spectrum?
  • · Replies 1 ·
Replies
1
Views
4K
Chemistry H NMR Spectrum - Molecular Structure
  • · Replies 1 ·
Replies
1
Views
12K
Graduate Are Protons and Nuclei Transitioning Between Spin States in NMR and MRI?
  • · Replies 2 ·
Replies
2
Views
2K
Undergrad LHC run 2, does Supersymmetry worsen hierarchy fine tuning
  • · Replies 2 ·
Replies
2
Views
2K
H NMR of unknown sample (organic chem)
  • · Replies 3 ·
Replies
3
Views
3K
How Do You Apply Pople Notation to Complex Spin Systems Like in 1-Bromobutane?
  • · Replies 1 ·
Replies
1
Views
15K
Graduate GUT Mass Unification Predictions
  • · Replies 2 ·
Replies
2
Views
4K