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

In summary, identical protons in the same environment do not undergo spin-spin splitting because they have the same total spin, making transitions between singlet and triplet states impossible. This results in equidistant splitting, but without any observable transitions.
  • #1
RKlintworth
7
0
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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  • #2
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?
 
  • #3
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?
 
  • #4
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.
 
  • #5
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.
 
  • #6
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.
 

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

1. Why do "identical" protons not experience splitting?

Identical protons do not experience splitting because they have the same chemical environment, meaning they are surrounded by the same types of atoms and have the same electron density. As a result, they experience the same magnetic field and have the same energy levels, leading to a single peak on an NMR spectrum.

2. How does NMR distinguish between "identical" protons?

NMR distinguishes between "identical" protons by utilizing different magnetic field strengths and radio frequencies. The slight variations in energy levels of these protons result in different resonance frequencies, allowing them to be distinguished and appear as separate peaks on an NMR spectrum.

3. Can "identical" protons ever experience splitting?

Yes, "identical" protons can experience splitting if they are in different chemical environments. This can occur if they are part of a molecule that has symmetry or if they are in close proximity to another atom or functional group that affects their magnetic environment.

4. How can NMR be used to determine if protons are "identical" or not?

NMR can be used to determine if protons are "identical" or not by analyzing the number of peaks on the spectrum. If there is only one peak, it indicates that the protons are truly identical. However, if there are multiple peaks, it suggests that the protons are not identical and are experiencing different magnetic environments.

5. Are "identical" protons always in the same chemical environment?

No, "identical" protons are not always in the same chemical environment. They can have different chemical environments if they are part of different molecules, have different functional groups attached to them, or if they are in different positions within the same molecule. These variations in their chemical environment can result in different resonance frequencies and lead to splitting on an NMR spectrum.

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