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Organic chemistry
Course: Organic chemistry > Unit 14
Lesson 3: Proton NMR- Introduction to proton NMR
- Nuclear shielding
- Chemical equivalence
- Chemical shift
- Electronegativity and chemical shift
- Diamagnetic anisotropy
- Integration
- Spin-spin splitting (coupling)
- Multiplicity: n + 1 rule
- Coupling constant
- Complex splitting
- Hydrogen deficiency index
- Proton NMR practice 1
- Proton NMR practice 2
- Proton NMR practice 3
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Diamagnetic anisotropy
Explanation of diamagnetic anisotropy. How it affects chemical shift in proton NMR. Created by Jay.
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Why do the pi electrons on benzene circulate in a particular direction? how can you tell?(27 votes)
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 )(30 votes)
@8:58he 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.(12 votes)
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.(11 votes)
- 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?(4 votes)
- In aromatic molecules, the induced ring current creates a magnetic field that deshields the outer protons but shields the inner protons.(14 votes)
- At8:30, 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!(4 votes)
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.(5 votes)
- At2:52it 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)?(3 votes)
- 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.(5 votes)
- How do you know the electrons will move counterclockwise in benzene?(3 votes)
- 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)(5 votes)
- 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?(3 votes)- 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.(5 votes)
What is the common name for that "giant molecule" at3:38?(3 votes)
18-annulene is the name of the molecule.(1 vote)
- The NMR technique that these videos describe (FT-NMR) relies on a constant magnetic field, and electric current can only be induced in a conductor by a changing magnetic field. How is an opposing magnetic field being induced in a constant external magnetic field?(3 votes)
- This has puzzled me for a while as well – I think that what we were forgetting is that the molecules are moving!(3 votes)
The increase in chemical shift of the benzene molecule is due to the increase in the strength of the effective magnetic field, but it does not affect the shielding experienced by the protons. Is this correct?(4 votes)
8:58Video transcript
- [Voiceover] In this video
we're gonna talk about diamagnetic anisotropy. So some pretty fnacy words there. We talked about diamagnetism
in an earlier video and we used current in a
loop of wire as an analogy. So if current is moving in this direction in a loop of wire, so I represents current, a magnetic field is created. And at the very center of this loop, the magnetic field is
pointing straight down. As you move away from the center, I can draw in some more
magnetic field lines, so we didn't do this in the earlier video, and as you get closer to
the edge of this loop, inside of the loop the magnetic field will be pointing down, but outside of the loop of wire, the magnetic field will be pointing up. Same thing on this side,
so pointing down inside and the magnetic field points
up on the outside of the loop. So when you're talking about current you're thinking about
positive charges moving. But that's not what's happening. We know that electrons
are really what are moving and moving charges
create a magnetic field. The electrons are moving in a direction opposite to how we define current. So we have electron
density moving this way and we get a magnetic field. If we think about benzyne, benzyne has six pi electrons, so up here is benzyne. So let's go ahead and
identify the pi electrons. Two, four, and six. And if we put benzyne in
an applied magnetic field, so here is our applied
magnetic field B naught, so it's pointing up. Those six pi electrons of
benzyne are going to circulate to create an induced magnetic field. So let me go ahead and draw a picture of the pi electrons in
benzyne circulating. So the pi electrons are
going in this direction. If the pi electrons are
going in that direction, then we know the induced magnetic field will be pointing down here. So at the very center the
induced magnetic field will be pointing down. So induced magnetic field points down. As you move away from the center, once again we can draw in some
more magnetic field lines, and then as you get to
the edge of the ring, edge of the benzyne ring here, once again inside of the ring the magnetic field points down, but outside of the ring the magnetic field is gonna point up. It's the same thing on this side. Inside it points down, outside of the ring, the
magnetic field points up. So let's think about the magnetic field experienced by this proton. So that proton experiences the applied magnetic field B naught, but it also feels this
induced magnetic field, which is in the same direction as the external magnetic field. So this is the direction of
the induced magnetic field outside of the ring. So the effective magnetic
field felt by this proton, you'd have to add the
induced magnetic field to the applied magnetic field to find the effective magnetic field. So outside of the ring we get a larger magnetic field. So we get a large magnetic field, we get a large difference between the alpha and the beta spin states in terms of energy. And a greater difference
in terms of energy means a higher frequency absorbed. And therefore you get a
higher chemical shift. And so the proton on
benzyne has a chemical shift of approximately 7.27 parts per million. So this is just for any proton on any kind of benzyne ring here. Your general range is
gonna be 6.5 to eight. And so if there are several molecules that demonstrate this
effect very dramatically, and let's take a look at one of them. So how do we know that
this effect is even true? So if I look at this molecule, we're gonna have a giant ring here. So let me go around so you can see the outline
of this giant ring. So a much bigger ring than benzyne. We have a lot of pi electrons. So more pi electrons then benzyne, so I'll just highlight some of them. Two, four, six, eight, and so on. You can see we have alternating single, single double bonds
here in this molecule. And so if you put this molecule into an external magnetic field, you're gonna get the same
situation as benzyne. So let's think about
these inner protons here. So we have six inner protons. If we look at the diagram for benzyne, if you have an applied
external field B naught, in the center, right in
the center of the ring, the inner protons experience an induced magnetic field that's down. It opposes the external magnetic field. So let me go ahead and draw that out here. So if we apply an external
magnetic field B naught, the inner protons have
an induced magnetic field caused by the movement
of those pi electrons. The induced magnetic field
opposes the applied field. And so the effective magnetic field felt by those inner protons is smaller. So we get a smaller, we get a smaller effective magnetic field. Smaller effective magnetic field means a smaller energy difference between the alpha and
the beta spin states. Therefore we get a lower frequency signal and a lower chemical shift. And the chemical shift for
these six inner protons turns out to be negative
two parts per million. So think about what that means. Negative two is past TMS. So if I go back up here, TMS was at zero. So negative two would be to the right. I don't even have room to show it on this chemical shift right here. So way past TMS. So a pretty dramatic effect. We can look at the protons outside of the ring as well. So let me go and highlight those. So we have 12 protons outside of the ring. Since those protons are
outside of the ring, the induced magnetic field is now in the same direction as the applied magnetic field. So therefore we get a larger
effective magnetic field felt by one of those protons. A larger magnetic field means a greater difference in energy between your alpha and beta spin states, so you get a higher frequency signal and a higher chemical shift. The chemical shift is about
nine parts per million. So the dramatic difference between these chemical shifts for these inner and outer protons, shows you how powerful this effect can be. Let's use this effect to explain the shift for a proton on a triple bond. So if we think about acetylene, so here's acetylene, and we're thinking about
the signal for this proton. Let's think about the
carbon it's attached to. So this carbon right
here is sp hybridized. And in the previous video, we talked about the fact
that an sp hybrid orbital has more s character than an sp two or sp three hybrid orbital. And therefore the electron density is going to be closer to that carbon. So you can think about
an sp hybridized carbon as being more electronegative than an sp two or sp
three hybridized carbon. So the electron density is
closer to this carbon here, which you would think
would deshield this proton and give you a higher chemical shift than a proton on a double bond. But that's not what we observed. The shift for this proton turns out to be approximately two to 2.5. So it's actually a lower chemical shift than a proton on a double bond. And let's see if we can explain why. So if we apply an external magnetic field, so B naught is our applied
external magnetic field, we know that causes pi
electrons to circulate. And if we have an upright
orientation of acetylene, so the orientation of
the molecule matters, so if it's facing in this direction, the pi electrons are
gonna circulate like this. And just like we talked about in benzyne, if the pi electrons circulate like that we get an induced magnetic field down in this direction, like that. So we can draw a few more
magnetic field lines like that. And think about the magnetic
field experienced by let's say, this proton. So this proton is feeling
the applied magnetic field, it's also feeling the
induced magnetic field. But the induced magnetic field is in the opposite direction of the applied magnetic field. So we could draw the
induced magnetic field opposing the applied magnetic field. So that proton that I circled there, actually feels a smaller
effective magnetic field here. So if you have a smaller
effective magnetic field, you're decreasing the energy difference between your alpha and beta spin states. So you get a lower chemical
shift than expected due to this effect. And so that's currently how we explain the chemical shift of somewhere around 2.5 for a proton on a triple bond. And so this effect holds true
any time you have pi electrons that can circulate when you put a molecule in an applied magnetic field. And so we could also use this to explain, for example the proton on a double bond. So here's some pi electrons. Or the proton, here we have
next to a carbonyl here. So we have pi electrons here. So any time you have pi electrons, this effect can be present. And as we've seen, it can be a very powerful effect and really affect the chemical shift.