Question

It is possible to observe coupling between a hydroxyl proton and other protons. Why does the oxygen atom in ethers prevent any further coupling and act as a sort of barrier between spin systems? Or is coupling technically possible, but the resulting^4J

coupling constant is just too small to be observed?
I initially asked this question because we learned in our spectroscopy lecture that proton couplings across R-O-R, R-N-R and R-S-R (with R ≠ H) aren't really possible. It's nice to see that this is just a rule of thumb and that some coupling can occur.
Could you tell me if my reasoning for the coupling in the examples below is correct or not: Fermi contact interactions are usually the most important mechanism for spin-spin coupling. The carbon atoms in the furane derivate are sp^2 hybridized, so there should be additional σ-π-spin-polarization interactions. The carbonyl-carbon of the formate ester also has a lower s-character. The large^5J coupling in the trioxaadamantane derivate is the most surprising. In contrary to what I initially thought, the coupling mechanism seems to be different from the stereospecific sigma-bond contributions that are responsible for W-coupling (so maybe some through-space mechanism?).
Is there any difference between^4JHCCCH and^4JHCOCH coupling in acyclic, unstrained, sp^3 hybridized molecules (i.e. do things like lone pairs play a role in spin-spin coupling and if so, why)?

Answer 1

Oxygen atoms in ethers reduce proton-proton coupling due to their sp³ hybridization and electron distribution. Additional factors, including hybridization of carbon and the presence of lone pairs, further influence the observed spin-spin coupling in organic molecules.

Step-by-step explanation:

The observation of coupling between protons across ethers (R-O-R) is significantly reduced compared to other proton couplings due to the barrier created by the oxygen atom's lone pairs and the particular orbital hybridization in ethers. In ethers, the oxygen is sp³ hybridized, leading to a wider distribution of electron density and diminishing overlap with adjacent atoms, thus reducing through-bond coupling constants like ⁴J.

Additional factors such as the hybridization of carbon atoms and the electronic environment impact the coupling. In furane derivatives, sp² hybridized carbons can result in σ-π-spin polarization, affecting coupling. Similarly, the carbonyl carbon in a formate ester having lower s-character could influence coupling mechanisms.