- Proton nuclear magnetic resonance questions
- Magnetic resonance imaging (MRI)
- Introduction to proton NMR
- Nuclear shielding
- Chemical equivalence
- Chemical shift
- Electronegativity and chemical shift
- Diamagnetic anisotropy
- 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
How peaks in a proton NMR can get split into multiplets due to coupling with neighboring protons. Created by Jay.
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- At6:54, why are the two hydrogens in red equivalent if one is next to a Cl and a H, and the other is next to two Chlorine atoms?(29 votes)
- At the temperature at which NMR is done, all single bonds are free to rotate. The C-C bond rotates at a high enough speed that the two hydrogens spend an equal amount of time near the Cl atom, and are therefore equivalent(56 votes)
- How do we know which protons interact (and result in coupling)?(8 votes)
- Coupling will usually happen between protons that are separated by 3 bonds (or less). Coupling is not normally observed when protons are 4 or more bonds away from each other.(14 votes)
- Does the splitting mean that the spin state of a proton switches between up and down multiple times during its exposure to the applied magnetic field?
At6:13: why is one peak of the doublet larger than the other? Does this indicate that the larger peak is closer to the original frequency that proton would experience without a neighbor?(3 votes)
- We say that the inner peaks of the two doublets are "leaning" toward each other (i.e., they are more intense than the outer peaks).
The explanation comes from (you guessed it!) quantum mechanics.
Let's call the four peaks A₁, A₂, B₂, and B₁.
When the chemical shifts of A and B are close together, the energies of the ↑↓ and the ↓↑ states begin to mix. Each state acquires some of the character of the other (sort of like hybridization)
In addition, as Δν becomes smaller, the probability of transitions (i.e. line intensities) changes.
The outer A₁ and B₁ transitions become weaker and eventually disappear (become forbidden).
If Δν > 10J (10 times the splitting), all lines have about the same intensity. But as the difference in chemical shifts between A and B decreases and the A and B signals come closer together, the inner lines become stronger and the outer lines become weaker until eventually A and B become identical and give just one signal.(5 votes)
- in general, how do the hydrogens affect each other? like when it's 6;00, why is the H with a ppm of 5.77 a triplet. Because of spin-spin coupling? I'm not sure how the different H really affect each other. At 7;00 minutes it talks about the different spin combinations, and I'm confused to why those 4 combinations are for the two protons but affect the one proton with a ppm of 5.77, causing it to be a triplet.(1 vote)
- A given proton will "resonate" when it is experiences a certain magnetic field.
This magnetic field is affected by the magnetic orientations of the H atoms on adjacent carbon atoms.
The proton at 5.77 ppm has two adjacent H atoms. The combinations of their orientations are "up-up", "up-down", "down-up" and "down-down".
"Up-down" and "down-up" cancel each other out. They have no effect on the magnetic field experienced by the H at 5.77 ppm. The H will resonate at an external field strength B₀.
"Up-up" increases the magnetic field experienced by the H. So you don't have to make the external field strength quite as high to get resonance.
And the "down-down" combination lowers the magnetic field experienced by the H. So you have to make the external field a little bit higher to get resonance.
So you get resonance at three different values of B₀ (a triplet).
The areas are in a 1:2:1 ratio because the middle state has twice as many chances of occurring.(8 votes)
- What about interaction between chemically equivalent hydrogens? I know it sounds a bit silly but I am just curious....(3 votes)
- Chemically equivalent hydrogens couple to each other very strongly, but this does not give rise to multiplet splitting.
Consider two protons A and X with a large Δν. We get an AX spectrum with a pair of doublets.
As Δν decreases we get an AB spectrum, we get a four-line spectrum with two more intense inner peaks coming closer together and the outer lines becoming weaker.
Finally, when Δν = 0, we get a single-line A₂ spectrum.
The reason for this comes from some complicated quantum mechanics.
As Δν/J approaches zero, the energy states begin to mix and the probabilities of transitions change.
The outer lines become weaker and eventually disappear (become forbidden), and the inner lines occur at the same place because each proton has the same value of ν.(4 votes)
- Is it possible for a group like F or H to induce coupling on a H present on the same carbon atom ?(3 votes)
- Yes. Coupling between H and F is very strong.
For H and F on the same carbon atom, J(HF) = 40 Hz to 60 Hz.
For example, in the spectrum of fluoroacetone (CH₃COCH₂F) the CH₂ signal is a doublet
at 4.63 ppm with a splitting of 48 Hz.(4 votes)
- Why don't all protons have doublet peaks? Can't all protons have 1 of 2 spins, either contributing or interfering with the applied magnetic field, and thus making all protons have 2 (ish) signals making our doublet peak?(3 votes)
- because the splitted distance is too small to hold the two peaks (if any) apart for one single hydrogen. thus they merge into 1 signal
on the other hand, with enough space between them, two different hydrogens can hold two separate peaks as we see in the video(1 vote)
- at0:40, Why does the proton closer to the oxygen atom (in red) have a lower chemical shift compared to the proton near the chlorine atom (in blue)? Chlorine is more electronegative than oxygen so I would expect the opposite.(2 votes)
- still confused how the blue hydrogen has only two possible spin states while the red has 3.(2 votes)
- in short, it's not depending on one's own number of hydrogen, but on the other ones'
a blue H is affected by two H, which are two
and their possible spin states
up-up, up-down, down-up, down-down
but up-down and down-up is equivalent
thus there are 3 possible effects a blue H can feel
the two red Hs are affected by only one blue H
it's possible spin state is
up-down (down-up is equivalent as above)
thus there are 2 possible effects the two red Hs can feel
# i believe you are confused with the colors (blue H has 3 peaks, red Hs have 2 peaks)
# signal peaks are different from spin states(3 votes)
- Why does the proton have two magnetic vectors in different directions ? Isn't it supposed to be only in the "up" direction since it is at the edge of the ring ?(2 votes)
- The energy difference between the two orientations is so small that there are almost equal numbers of nuclei in each state.
There are only a few more protons in the lower energy state than in the higher energy state.
Their location on the ring does not change this.(2 votes)
- [Voiceover] If we look at this molecule, we would expect three signals on an NMR spectrum. So this proton has a chemical shift of 7.25 parts per million. This proton is in a slightly different environment and so we get a slightly different chemical shift at 6.7. This three protons are equivalent and therefore, they give us one signal at a chemical shift of 3.9 parts per million. So based on what we know so far, we would expect the top NMR spectrum. So these three protons give us a signal at a shift of 3.9. So that would be this signal right here. Let's make the next proton red. So this proton right here in red has a shift of 6.7 so we would expect this signal. And then the last proton here I'll make it blue. This one is at 7.25. So this is the expected NMR spectrum based on what we know so far but this is not the actual NMR spectrum. So the actual NMR spectrum is this one down here. We still see this one signal with one peak for the protons in green. But if we look at the signal for the red proton, so on the top version we had one signal with one peak. In the bottom version here, that one signal has been split into two peaks. Let me go ahead and highlight those. So there's one peak and then there's the other peak. So the signal was split into two peaks. The exact same thing happened for the proton in blue. So we had one signal with one peak in the top version and down here that signal for the blue proton was split into two peaks. So we call this spin-spin splitting or spin-spin coupling and let's look in more detail about what's happening with that red and the blue proton. So let me go down here. We have some more room. Let's go ahead and draw in the spectrum with no interaction between the proton. So the first version that we talked about we expected one signal with one peak at 6.7 parts per million. That was the proton in red. And then we expected one signal with one peak at 7.25 parts per million and that was the proton in blue. So this top version here is the spectrum with no interaction between our two protons. But in reality there is an interaction because remember the red proton, the magnetic moment of the red proton can be up or down. It can be aligned with the external magnetic field or it can be aligned against the external magnetic field. So the red proton has a magnetic moment or a magnetic field that's going up or it's going down. Let's think about the example where the magnetic moment or the red proton is aligned with the external field first. We have our red proton and let's say the magnetic moment is aligned with the external magnetic field. Let me go ahead and draw in the external magnetic field like that. We called this B knot. And these two vectors are going in the same direction. So the magnetic field, the red proton adds to the external magnetic field and let me go ahead and draw in a larger vector here because now the effective magnetic field felt by the blue proton has increased. So the effective magnetic field is larger than the applied magnetic field because the red proton's magnetic field is adding to it. And so the proton in blue feels a larger effective magnetic field. And remember what that does, the energy difference between the alpha and the beta spins states. If you increase the magnetic field you increase the difference in energy between the alpha and the beta spin states. Therefore, you get a higher frequency signal and a higher chemical shift than expected. So this has the effect of increasing the shift, the chemical shift for the blue proton. We can draw in the blue proton at a higher chemical shift than expected. All right, let's do the same sort of thing except this time let's think about the red proton's magnetic moment aligned against the applied magnetic field. So this is the situation where the magnetic field of the red proton is going down. That's in the opposite direction of the applied magnetic field. So I draw in the applied magnetic field here. And so, the red proton's magnetic field is going to cancel out some of that external magnetic field and the proton in blue feels a smaller effective magnetic field. So I'm exaggerating here just to get the point across. But the proton in blue feels a smaller effective magnetic field that decreases the energy difference between the alpha and the beta spin states. Therefore, you get a lower frequency signal and a lower value for the chemical shift than expected. So a lower value for the chemical shift for the blue proton. So I go ahead and draw in the signal for the blue proton at a lower value for the chemical shift. And so, the end result is the signal for the blue proton is split into two. The signal for the blue proton is split into two because of the two different magnetic fields of the red proton. The blue proton also has a magnetic field pointing up or down and so the blue proton splits the signal for the red proton in the same way. So we can go ahead and draw, we can go ahead and draw the red proton being split in the same way. This down here represents the spectrum of where both protons are coupled to each other. We call this coupling. And so we end up seeing the signal split into two different peaks which we call a doublet. Let me go over here and let's look at our protons again. The proton in red is split into two peaks. The signal is split into two peaks like we talked about here. And the proton in blue, the signal is split into two peaks like this which we talked about right here. That's the idea of spin-spin splitting. Next we're gonna look at another example which is just a little bit more complicated but uses this exact same idea. Let's look at this molecule. So this proton has a chemical shift of 5.77 parts per million. And the protons here in red are equivalent and we would expect a signal at 3.95 parts per million. So we expect a signal due to two protons at 3.95. This represents two protons here and we expect a signal for one proton at 5.77. This signal isn't as intense because it's only representing one proton. If the red and blue protons did not interact with each other, we would expect that this for the proton NMR spectrum but they do interact with each other. And so we need to think about the magnetic moments of the two red protons. I could think about the first red proton having a magnetic moment that's pointing up and the second red proton having a magnetic moment that's also pointing up. So this is one possible combination of magnetic moments. Another possible combination, I could have the first red proton having a magnetic moment pointing up and the second red proton having a magnetic moment pointing down. Or the first proton can be pointing down and the second proton pointing up. That's two more possible spin combinations. The last possible spin combination would of course be the first protons magnetic moment is down and so is the second protons. This represents the possible combinations of magnetic moments or magnetic fields for the two protons in red. Let's take the first possible combination. Let's think about both protons having magnetic moments pointing up. Let's say our applied magnetic field, our external magnetic field points in the same direction. When you think about the magnetic field experienced by the proton in blue, we would add the magnetic fields for the protons in red. And so the effective magnetic field is increased, it's higher than the applied magnetic field. So we've increased the magnetic field experienced by the proton in blue. Therefore, increasing the energy difference between your alpha and your beta spin states increasing your frequency, and we get a higher chemical shift. So we get a higher value for the chemical shift. We could think about the chemical shift being higher. Let me go ahead and draw that in like that. So past 5.77. Let's look at the next two possible combinations. So this one and this one. We had magnetic moment up for the first proton, magnetic moment down for the second proton, and then down for the first and up for the second. What effect do those magnetic fields have on the effective magnetic fields felt by the proton in blue? Well, let's think about the first combination. So this combination right here. We have one, one magnetic field up, one magnetic field down, they would cancel. Same thing with this combination, one down and one up. Those magnetic fields cancel. And so, the effective magnetic field felt by the proton in blue is equal to the external magnetic field because the magnetic fields of the protons cancel each other out. And so we would expect, we would expect a signal at the proper, at the correct chemical shift. We would expect a signal at the correct chemical shift at 5.77 and this signal is actually going to be more intense than this signal and that's because of probability. There's a greater probability of having one of these combinations of magnetic fields than having this combination. So twice the probability gives you a doubly intense signal at the correct chemical shift. Finally, let's look at the last combination. So both magnetic fields pointing down. Both magnetic fields for the protons in red pointing down and we have our applied magnetic field pointing up. The effective magnetic field felt by the proton in blue is smaller. So we have a smaller value for the effective magnetic field. So we've decreased the magnetic field felt by the proton in blue. Decrease in the energy difference, decrease in the frequency, decreasing the chemical shift, right? So we'd expect a lower chemical shift. So we can go ahead and draw in a lower chemical shift so like that. Let's think about this signal up here. We said we'd expect one signal for the proton in blue but that one signal is affected by the magnetic fields of the different protons in red. And the possible combinations of the magnetic moments of the red protons take the signal for the proton in blue and give you three peaks. So we have one peak at a higher chemical shift, one peak of double intensity at the correct chemical shift, and one peak of lower intensity. And so, if we look at the NMR spectrum, so this is the signal for the proton in blue. The signal is split into three peaks, one, two and three and we call this a triplet. This is a triplet. What about the protons in red? The protons in red are affected by the magnetic field of the proton in blue. And the magnetic field of the proton in blue can be aligned either with the external magnetic field or against the external magnetic field. It's like the previous example we saw. Two possible magnetic fields for the proton in blue therefore, the signal for the protons in red is split into two. So we get a signal for the protons in red split into two, I'm attempting to draw that here. The signal is split into two peaks. The signal for the protons in red is split into two peaks and we call this a doublet. All right. More about spin-spin splitting in the next video.