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Topic: NMR: Acetylene - Circulation of pi electrons  (Read 12534 times)

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Offline Winga

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NMR: Acetylene - Circulation of pi electrons
« on: March 22, 2006, 03:52:05 AM »
According to reference books and lecture notes, they mentioned that the two pi-orbitals of acetylene overlap and become a cylinder (donut) in shape so that the pi electrons can circulate along the C-C bond axis.

If we figure out the overlapping between these two pi-orbitals, because of the sign of each lobe, there should be a nodal plane in each new orbital after the interaction. Therefore, circulation of e- will be blocked.

So, how to explain the continuity of circulation of e-?

Is it the signs of orbitals can be ignored? But why?
« Last Edit: March 22, 2006, 03:52:32 AM by Winga »

Offline green-goblin

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Re:NMR: Acetylene - Circulation of pi electrons
« Reply #1 on: March 22, 2006, 06:11:00 AM »
Would the signs of the orbitals be opposite?
If they are though I can't see how a pi bond will form, like you said the node would stop any
electron cirulation.

fishyuyue

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Re:NMR: Acetylene - Circulation of pi electrons
« Reply #2 on: March 22, 2006, 06:38:18 AM »
According to reference books and lecture notes, they mentioned that the two pi-orbitals of acetylene overlap and become a cylinder (donut) in shape so that the pi electrons can circulate along the C-C bond axis.(movement of e is not like the moving subject in macroscopic world. In the microscopic world, the electrons obery the Uncertainty Principle)

If we figure out the overlapping between these two pi-orbitals, because of the sign of each lobe, there should be a nodal plane in each new orbital after the interaction. Therefore, circulation of e- will be blocked.

So, how to explain the continuity of circulation of e-?

Is it the signs of orbitals can be ignored? But why?

Offline Winga

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Re:NMR: Acetylene - Circulation of pi electrons
« Reply #3 on: March 22, 2006, 07:56:13 AM »
So, how can the Uncertainty Principle explain it?

fishyuyue

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Re:NMR: Acetylene - Circulation of pi electrons
« Reply #4 on: March 22, 2006, 08:12:23 AM »
using the charge density to describe the movement of the electron is better.

fishyuyue

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Re:NMR: Acetylene - Circulation of pi electrons
« Reply #5 on: March 22, 2006, 08:14:42 AM »
So, how can the Uncertainty Principle explain it?


We can't say Yes or No simply, we'd better consider the possibility of this situation

Offline Winga

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Re:NMR: Acetylene - Circulation of pi electrons
« Reply #6 on: March 22, 2006, 09:04:46 AM »
using the charge density to describe the movement of the electron is better.
But e- density still cannot pass through a nodal plane.

Offline Winga

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Re:NMR: Acetylene - Circulation of pi electrons
« Reply #7 on: March 22, 2006, 09:35:55 AM »
Would the signs of the orbitals be opposite?
If they are though I can't see how a pi bond will form, like you said the node would stop any
electron cirulation.
I mean the overlapping of these two orbitals.

Offline Winga

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Re:NMR: Acetylene - Circulation of pi electrons
« Reply #8 on: March 22, 2006, 09:37:02 AM »
Another one

Offline movies

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Re:NMR: Acetylene - Circulation of pi electrons
« Reply #9 on: March 22, 2006, 12:32:15 PM »
In an alkyne there is practically no barrier to rotation about the two C-C bonds at the termini, so the time-average of these orbitals ends up looking like a cylinder, while at any instant it would look like two orthogonal p-orbitals.

Offline Winga

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Re:NMR: Acetylene - Circulation of pi electrons
« Reply #10 on: March 22, 2006, 03:25:16 PM »
In an alkyne there is practically no barrier to rotation about the two C-C bonds at the termini, so the time-average of these orbitals ends up looking like a cylinder, while at any instant it would look like two orthogonal p-orbitals.
You mean the rotation of the whole molecule along C-C bond axis.
However, this rotation seems that all the atoms are fixed in their own positions, only the pi-orbitals are rotating.
I think the anisotropic effect is due to the circulating of e- in the orbitals, but in this case, it becomes the circulation of orbitals, is it quite strange?

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Re:NMR: Acetylene - Circulation of pi electrons
« Reply #11 on: March 22, 2006, 04:35:35 PM »
You can rotate those orbitals without changing the hybridization of the carbons in the alkyne, so there isn't any barrier.  I don't mean the whole molecule, just think about the pi-orbitals spinning around; nothing else has to change.

Offline Winga

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Re:NMR: Acetylene - Circulation of pi electrons
« Reply #12 on: March 23, 2006, 10:38:21 AM »
You can rotate those orbitals without changing the hybridization of the carbons in the alkyne, so there isn't any barrier.  I don't mean the whole molecule, just think about the pi-orbitals spinning around; nothing else has to change.
Are there any reference books or journals mention about this phenomenon?

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Re:NMR: Acetylene - Circulation of pi electrons
« Reply #13 on: March 23, 2006, 12:13:09 PM »
My textbook from sophomore organic chem. says that the cylindrical shape is due to the fact that the pi-bonds are rather diffuse.  It's doesn't mention any kind of spinning.  I can't think of any good reason why the spinning wouldn't occur though.

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