NMR: Why do "identical" protons not experience splitting?

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Discussion Overview

The discussion revolves around the phenomenon of spin-spin splitting in nuclear magnetic resonance (NMR) spectroscopy, specifically addressing why protons in identical environments do not experience splitting with one another. The scope includes theoretical explanations and conceptual clarifications related to NMR behavior of protons.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions why protons in identical environments do not couple with one another despite being in close proximity, seeking clarity on the underlying process.
  • Another participant notes that both protons see the same environment and questions where differences would arise that could lead to interaction.
  • A participant expresses confusion about why identical nuclei do not interact magnetically, despite having the same chemical shifts and being in similar environments.
  • One participant suggests that a small shift might be expected if a hydrogen atom were absent, but acknowledges the difficulty in observing such a scenario.
  • Another participant explains that total spin becomes a good quantum number for identical nuclei, leading to no transitions between singlet and triplet states, although splitting is still present in the form of equidistant triplet states.
  • A later reply reiterates the point about total spin and the absence of transitions, indicating that the splitting exists but is not observable in the same way.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and confusion regarding the interaction of identical protons, with some points of agreement on the nature of total spin and its implications, but no consensus on the reasons behind the lack of coupling.

Contextual Notes

Participants highlight the complexity of interactions in NMR and the specific conditions under which identical protons may or may not couple, indicating that assumptions about chemical environments and quantum states play a significant role in the discussion.

RKlintworth
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Hey, I've been trying to figure out why a protons in identical environments will not undergo spin-spin splitting with one another.

From what I understand splitting of a signal for a proton is due to the nucleus of another nearby either opposing the external magnetic field or acting in the same direction as this field. yet identical protons even on the same carbon will not couple with one another regardless of their close proximity.

If anyone could shed some light on this process it would be greatly appreciated.

Thank you.
 
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They both see the same environment: one carbon atom they are bound to, and one hydrogen atom somewhere nearby, and some other components of the molecule. Where do you expect a difference between the two protons to come from?
 
I don't expect the signals to have different chemical shifts at all, I realize that they would resonate at the same frequency. What I am confused about is why the nuclei no longer interact. Why the nucleus of the one does not effect the magnetic field experienced by the other? Why would a difference in chemical environment suddenly allow for this interaction?
 
I would expect a small, but non-zero shift compared to "no hydrogen atom there", but that would give a free radical so I guess it's hard to observe that.
 
The point is that total spin becomes a good quantum number if both nuclei are identical. So there is no transition between singlet and triplet states, but only within the triplet, which are equidistant. So the splitting is still there, only the transitions are missing.
 
DrDu said:
The point is that total spin becomes a good quantum number if both nuclei are identical. So there is no transition between singlet and triplet states, but only within the triplet, which are equidistant. So the splitting is still there, only the transitions are missing.
Okay thank you, That makes sense.
 

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