Why do we assume that the circulating pi electrons induce B

[FTNMR]

Why do we assume that the circulating pi electrons induces another magnetic field, B_in, that adds to the external mag field, B_ext, instead of opposing B_ext? Couldnt the pi electrons just as easily oppose B_ext?

This lecture says the protons on benzene will experience greater effectual field, B_eff, because the circulating pi electrons induces another field that adds to the B_ext.
But if ring is flipped, then wouldn't that mean that B_in then opposes B_ext.. Also what if pi electrons circulate other direction? then according to RHR B_in would again oppose B_ext.

Im wondering if this has something to do with the fact that I am looking at this in 2 dimensions.. maybe not important idk..
Thanks for any explanation or help

 

[Simon Bridge]

Well I'm thinking: an opposing field with be an extra negative field.
Do the pi electrons ever circulate to oppose the external field? i.e. if you flipped the ring, would the pi electrons start circulating the other way?
i.e. what makes them circulate?

 

[FTNMR]

is it the actual external mag field that causes the circulation to begin with? If it does then I think I am starting to understand, but if not then I think I am more confused. please answer again, thanks

 

[FTNMR]

OK i think it is the field that causes the pi electrons to circulate to begin with..so whether it opposes or adds to external field is just the right hand rule then.. is that correct understanding?

 

[Simon Bridge]

You "think"? You should go to your notes and check. I'm just asking you questions.

 

[Simon Bridge]

Just checking - it looks to me like a QM effect similar to Larmor precession.
Looking at something like ethyne - you can think of the atoms all lined up along the z-axis, but there is an ambiguity to how the triple bond electrons are oriented ... the time-average will be something like a cylinder. That should start you off ... or you can just use the result.

i.e. you are not "assuming" the result, it is the result because someone looked and discovered it.
Just like you don't assume that the Sun is a big ball of burning gas a long way away.
All the QM explanation will do is provide a math model for the result.

 

[DrDu]

Whether the field increases or decreases depends on the position of the protons. There are some exotic aromatic compounds where one proton points inside the ring instead of outside and the field they feel is decreased instead of increased.

 

[Simon Bridge]

"Position of protons"? "... proton points..."?
Is this chemistry talk?

Guessing: location of atom in the molecule, and orientation direction of the nuclear magnetic moment?

 

[Charles Link]

It would be interesting to get some more information on the application. Is this perhaps an MRI (NMR) and the proton experiences a shift in the nuclear resonant frequency (at which r-f absorption takes place) from that of a bare proton because of local molecular magnetic fields(from the pi electrons) that add to the applied (static) magnetic field? It appears the subject might be chemical shifts in Nuclear Magnetic Resonance. A google of the topic shows quite a lot of studies have been done for the shifts found for various substances.

 

[DrDu]

"Position of protons"? "... proton points..."?
Is this chemistry talk?

Well, 1H-NMR talk, to be precise.

 

[TeethWhitener]

Whether the field increases or decreases depends on the position of the protons. There are some exotic aromatic compounds where one proton points inside the ring instead of outside and the field they feel is decreased instead of increased.

Shameless self-advertisement (from my bygone days as a fullerene chemist): http://pubs.acs.org/doi/abs/10.1021/ja901383r

Whereas in benzene, the circulation of the pi electrons acts to deshield the protons, shifting their resonances downfield, in this case, the circulation of the pi electrons on the fullerene cage shields the methane protons (and the ¹³C nucleus) inside the fullerene, shifting their resonances upfield by a large amount ($\delta_H = -12.32$ ppm and $\delta_C = -20.62$ ppm in case you don't have access to the paper). You see the same effect when H₂, H₂O, and NH₃ are put into fullerenes.