1H-NMR: How does the oxygen in ethers act as a spin coupling barrier while the oxygen in hydroxy groups doesn't?

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 $^4\mathrm{J}$ coupling constant is just too small to be observed?

[Edit]: 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 $\ce{sp^2}$ 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 $^5\mathrm{J}$ 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?).

Here is the original adamantane paper for anyone whos interested: https://doi.org/10.1021/ja00958a011

Quote from the paper: "The coupling systems in I and VII differ from these in that rotation of 180 and 120° about the central bond has taken place. If one mechanism is responsible for all the five-bond couplings cited, it must be independent of the geometry of the central bond."

So let me ask a slightly better question: Is there any difference between $^4\mathrm{J}_{\ce{HCCCH}}$ and $^4\mathrm{J}_{\ce{HCOCH}}$ coupling in acyclic, unstrained, $\ce{sp^3}$ hybridized molecules (i.e. do things like lone pairs play a role in spin-spin coupling and if so, why)?

Answer 1

Hans Reich's website remains a treasure trove of NMR information that you can consult for theory and values of J couplings. All of the following comes from that site.

First, on the origin of through-bond couplings:

The scalar coupling J is a through-bond interaction, in which the spin of one nucleus perturbs (polarizes) the spins of the intervening electrons, and the energy levels of neighboring magnetic nuclei are in turn perturbed by the polarized electrons. [....] Because the effect is usually transmitted through the bonding electrons, the magnitude of J falls off rapidly as the number of intervening bonds increases.

On 4-bond proton J-couplings

Proton-proton couplings over more than three bonds are usually too small to detect easily (< 1 Hz). However, there are a number of important environments where such couplings are present, and can provide useful structural information. Coupling across π-systems are the most frequently encountered 4J couplings

As the following examples show, the oxygen nucleus in an ether or ester does not inherently interfere with a long-range coupling, and does not account for the sparsity of reported $^4\mathrm{J}_{\mathrm{HH}}$.