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# Diamagnetic anisotropy

Learn what diamagnetic anisotropy is and how it affects chemical shift in proton NMR. Explore the concept of diamagnetic anisotropy, focusing on how pi electrons in molecules like benzyne and acetylene respond to an applied magnetic field. The induced magnetic field created by circulating pi electrons affects the effective magnetic field experienced by protons, influencing their chemical shift in NMR spectroscopy. This key principle helps explain variations in chemical shifts across different molecular structures. Created by Jay.

## Want to join the conversation?

• Why do the pi electrons on benzene circulate in a particular direction? how can you tell?
• the pi electrons on a benzene ring circulate in a particular circular path under the influence of an external magnetic field. Whenever an external magnetic field is applied, there is an induced emf (electromotive force) that sets up in such a way that the magnetic field produced by the charge carriers opposes the external applied magnetic field at its center.
the direction of induced current is given by right hand thumb rule. If you point your thumb in the direction opposite to the external magnetic field then the direction in which your other fingers fold gives you the direction of induced current. And as you know the direction of flow of electrons is opposite to the direction of the induced current. (for more clarity about the direction of emf I suggest you to through the Lenz's Law )
• @ he states that it occurs for all pi electrons (and mentions that it can occur in the alkenes as well), if this is the case then I still don't understand why is it that the alkynes have a lower chemical shift compared to the alkenes.
• Alkynes have a cylindrical cloud of electron density surrounding the H-C≡C-C axis.
To get these electrons to circulate, the axis must be oriented vertically (not horizontally, as with benzene).
This puts the H atoms in the middle of the induced field Bin, where Bin is going "down" (not "up", as with benzene).
This reduces the effective field Beff experienced by the H atom, so the external field B₀ has to be increased to reach resonance.
Although aromatic protons are deshielded, alkyne protons are shielded.
• I will re-watch this video for clarity but can I just assume that in general the electron density will create/increase the shielding affect in molecules , whereas for cyclic molecules like benzene it actually decreases the shielding affect?
• In aromatic molecules, the induced ring current creates a magnetic field that deshields the outer protons but shields the inner protons.
• At , the proton below the triple bond experiences a reduced net magnetic field, so a lower chemical shift. Got that. BUT, does the proton above the triple bond (ie the one he doesnt circle in purple) experience a higher net magnetic field, so a higher chemical shift, since the induced field aligns with the applied field? Video didnt say anything specific, thanks!
• I thought the same thing initially. The hydrogens opposite each other on the giant ring experienced the induced magnetic field as pointing in opposite directions, so wouldn't the hydrogens opposite each other on acetylene also experience the induced magnetic field in opposite directions? But actually, the hydrogens on acetylene aren't really "opposite each other" in the way that matters. If we're concerned with the vertical component of the induced magnetic field, we need to look at the horizontal position of the hydrogens.*** On acetylene, the hydrogens are in the same horizontal position: they're both inside the loop of current, so they experience the same vertical direction of magnetic field. In the case of the ring, some hydrogens are inside the loop and some are outside the loop: their horizontal positions are different, so the vertical direction of magnetic field they feel is different.

*** This is because the magnetic field generated by a current is the cross product** of the current and the displacement* from the current, i.e., it is always perpendicular to the direction of the displacement (and the direction of the current). Thus, if you want to know about the vertical component of the magnetic field, it is the horizontal component of your displacement that is relevant.

** A "cross product" is a vector whose direction is perpendicular to the plane of the two vectors being multiplied. (This would be ambiguous, since there are two directions that are perpendicular to a given plane, but we have all collectively agreed to use a convention called the "right-hand rule" to determine which of those two directions we mean.)

* "Displacement" is a vector which points from the origin (wherever you define that to be) to where you are.
• At it says that the proton feels the induced magnetic field in combination with the applied magnetic field. My question is wouldn't the effect of the induced magnetic field cancel itself out (because it is going in the opposite direction in the inside of the ring)?
• The induced field doesn't cancel itself out. It is going around in a "circle".
The proton experiencing the field is outside the ring.
If there were a proton inside the ring, it would experience the same field but in the opposite direction.
• How do you know the electrons will move counterclockwise in benzene?
• We know that the electrons will move in a way that will counteract the applied magnetic field because over the course of thousands of experiments and hundreds of years, it has never not done that. It's called Lenz's Law, and you can think of it as similar to Newton's Third Law, which is the "equal and opposite reaction" one.

Electrons are basically like cats: they don't want you to meddle with their environment. So whenever you change the magnetic field through a loop of them, they do their very best to change it back. So in the case of the benzene ring in the video, we put in a magnetic field that pointed up, so the electrons are furiously counteracting that by making a magnetic field inside the loop that points down. (Because the hydrogens are outside the loop, they feel the opposite of what the electrons are doing to the inside of the loop, which is why the hydrogens end up feeling an even bigger net magnetic field.)

(Credit to Wikipedia for the comparison of Lenz's Law to Newton's Third Law)
• 1. why cant pi electron circulation (and thus induced magnetic field that opposes the applied magnetic field) happens in alkenes?

2. How can we know for sure that alkyne's orientation is always vertical and benzene's orientation is always horizontal?
• 1. It does. That's why alkenes are shifted downfield to 5-7 ppm.
2. It isn't. The molecules are always tumbling in the magnetic field. Benzene, for example, has the maximum effect when the molecule is perpendicular to the magnetic field. However, other orientations will have some component in that direction. We see the average of all these orientations.
• What is the common name for that "giant molecule" at ?