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

[RKlintworth]

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.

 

[mfb]

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?

 

[RKlintworth]

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?

 

[mfb]

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.

 

[DrDu]

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.

 

[RKlintworth]

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.