[Nescafe]

Hello,

I was reading about the coupling constants of alpha v.s. beta glucose and read that according to Karplus curve, for the Beta conformation where the Anomeric proton is at a 180 degree to the neighboring proton (axial-axial) the coupling is larger than that of the alpha-glucose when the anomeric proton is equitorial and the neighboring proton axial. The dihedral angle in this case would be 60. How is it so? Correct me if I am wrong but when two protons are further apart the coupling constant is larger reflecting their distance which makes sense in this case but how is it that two protons further apart give rise to a larger coupling constant, should it not be weaker? Maybe I can try to explain this and someone can confirm or correct me. If a proton is split by a neighbouring proton (so in this case the anomeric proton will be a doublet) the coupling (how far apart the two teeth of the doublet are) is purely an indication of how this other proton which is splitting it is?

Thanks in advance,

Nescafe.

[discodermolide]

Try looking in Google. There are some calculators out there that may help you here.

[Nescafe]

Try looking in Google. There are some calculators out there that may help you here.

Thanks for the response. I am more interested in the fundamental reason as to why a proton that is at a further distance away results in a larger coupling constant rather than calculating it though.

[Dan]

It's not a simple distance relationship - notice that the ³J maxima occur at 0° (minimum separation) and 180° (maximum separation).

³J coupling is a "through bond" interaction - i.e. through molecular orbitals. Thinking about molecular orbitals, can you think of a reason why coupling might be greatest when the dihedral angle is 0° or 180°?

[Nescafe]

Hmm, does it have to do with the fact that at 0 degrees and 180 degrees the orbitals of the neighbouring hydrogens are parallel and therefore they "feel" eachothers magneic fields more? But how so? Why would they feel eachothers magnetic field more so than if they are not parallel? I am going to try to answer my own question. Perhaps when parallel the local magnetic fields interact with eachother better than if they were not so they communicate with eachother where as in the opposite scenario where they are at an angle magnetic fields are not in the right geometry to fully influence eachother and communicate less.

[orgopete]

This is beyond my pay grade to answer. The electrons in a bond are already antiparallel. Although I had not thought of this example, I think there are many examples in which one might argue for an electron-electron interaction. The bond connecting a conjugated diene is shortened. I argue this shows the electrons are interacting in some manner. We often illustrate this as resonance structures. I would argue the Karplus curve indicates there are stereochemical constraints to the interactions.

[Dan]

Hmm, does it have to do with the fact that at 0 degrees and 180 degrees the orbitals of the neighbouring hydrogens are parallel and therefore they "feel" eachothers magneic fields more? But how so?

The qualitative understanding in my head (I'm not an expert in this area) is that if coupling is propagated through orbitals, the communication should be maximum when the orbitals have maximum overlap - when they are periplanar.