Understanding Carbon-13 NMR Spectroscopy

Hey everybody, this is video 9-7 and this will be the fifth and final topic on NMR spectroscopy. And what we're going to be looking at here is carbon-13 NMR. So this is going to be the NMR not of the hydrogens or protons in an unknown compound, organic compound, but about the carbon atoms. And the big advantage of carbon NMR over proton is that there are fewer carbon atoms in a molecule. So the NMR spectra are very simple. and you get one signal for every equivalent carbon atom. We also have a very broad chemical shift range. Remember, in proton NMR, the chemical shift range went from 0 to about 13 parts per million. In carbon NMR, it goes from 0 to 220 parts per million. What this means is that if you have carbons that are very similar to each other, like let's say you have... three different types of methyl carbons in a molecule. In carbon NMR, it's very unlikely that these will have the same chemical shift, or that the chemical shifts will at least be spread out enough so you can actually see those three independent signals. In proton NMR, if you have three methyl groups, it's very likely that those signals are going to be right on top of each other and be very difficult to break them apart. So the broad chemical shift scale really helps in carbon-13 enumar. Now, one problem we do have is that the most abundant carbon in nature is actually the carbon-12 nucleus. The carbon-13 isotope, which is a stable isotope of carbon, only 1% of all of the carbon atoms in nature are carbon-13. So this means that if you have an organic compound, only 1% of those carbons will be detected by carbon-13 NMR. So this technique is many times less sensitive than proton NMR. We get around that by using more sample and by running the NMR experiment for a longer period of time. So with proton NMR, you can collect a really good NMR spectrum in a couple of minutes. Carbon-13 NMR spectra, however, may take a couple of hours, and it's not unusual to actually run these experiments overnight. But we also take advantage of this low natural abundance of carbon-13. Because only 1% of your carbon atoms in your organic compound are the carbon-13 nucleus, we don't see splitting by adjacent carbons. This is because the chances of having two carbon 13 atoms next to each other is very, very low. So what this means is you can run an NMR spectrum of your organic compound, your unknown, and every carbon atom that is non-equivalent to each other will give its own unique signal. In other words, a bunch of singlets. So these are much easier to interpret than proton NMR. But it... is possible to observe splitting caused by the hydrogen atoms that are bonded to a carbon. The typical experiment that we run is called a proton decoupled carbon-13 NMR. This means that we don't see splitting by the protons that are attached to a carbon atom. A unique experiment, however, is proton coupled carbon-13 NMR. In this case, we do see carbon-hydrogen splitting. And that splitting follows the N plus 1 rule. So, for example, a methyl group, a CH3, the proton-coupled signal will be a quartet. So, what we would typically do is first run a proton-decoupled spectrum, look at the total number of non-equivalent carbons, and then run the proton-coupled NMR spectrum and see what the splitting looks like. A quartet means a CH3. A doublet means a CH and on from there. So here's a correlation table just like we have for IR and for proton NMR. And you can take a look at this. Again, we don't want to go crazy with these correlation tables. We want to use them to identify specific types of functional groups. And there's only going to be two groups I'm going to point out to you. First, the carbons of the benzene ring. Those carbons are uniquely situated between about 100 and 170 parts per million. And a carbonyl carbon. The carbonyl carbons are the furthest downfield. And in carbon NMR, between about 160 and 200 parts per million. So that's the only place where chemical shift comes in for carbon atoms. So here's an example we'll take a look at. This is the structure of 2-butanol. I've got it color-coded this time, and we're going to try to figure out what the carbon-13 NMR looks like. So first, let's look at the number of non-equivalent carbon atoms. You can see that the molecule is not symmetrical, meaning that the CH3 on the right is non-equivalent from the CH3 on the left. So each of those should give... a carbon-13 signal. Plus, we have a CH2, and we have a CH that also is bonded to an alcohol functional group. So, we would expect that there will be four signals in the proton decoupled carbon-13 NMR spectrum. And there it is. Four signals, also nicely color-coded. The methyl group A is furthest upfield, next to it. is B, the methyl group B, then the CH2, and then the CH. Now, just notice the two methyl group signals, A and B. They're both methyl groups, but notice that B is taller than A is. It has nothing to do, the size or the height of the signal in carbon-13 NMR really has nothing to do with anything. In proton NMR, it's very different, where the size or the area of a signal is directly proportional to the number of hydrogens. In carbon NMR, the height of the signal has nothing to do with the number of those particular types of carbons. It has to do with something else. It has to do with what we call relaxation time. That's the time it takes for excited nuclei to flip their north and south poles. We're not going to worry too much about that. So don't let the height of the signals really bother you about what that might mean in terms of interpreting the structure of the unknown. Now, the way we're going to use carbon-13 NMR is as a backup. We're going to use it to confirm a structure that we have already come up with using IR and using proton NMR. All right, now let's look at the uncoupled carbon-13 NMR for the same compound. And there it is. So the same, we're going to do the same analysis. Okay, oh, by the way, this little triplet that we see on the far left, that is actually the solvent. It's most likely deuterated chloroform. That's CdCl3. And notice what the deuterium is doing is kind of interesting. It's splitting the carbon, but by an N plus 2 rule. Kind of interesting. Anyway. Back to the NMR, the carbon-13 proton-coupled NMR spectrum. So in this experiment, the protons are allowed to interact with the carbon nuclei and to cause N plus 1 splitting. So the methyl group on the right that we call A, there are three hydrogens there. Notice that it's split to a quartet. Methyl group B also is a quartet, so methyl groups show up as quartets. because of the three hydrogens and the n plus one rule. The CH2 shows up as a triplet because of the two hydrogens, and the CH shows up as a doublet because of the one hydrogen. So carbon-13 NMR is very useful. It's simple to interpret. It is used to back up the structure we come up with from IR and NMR. The only real disadvantage is that it has low sensitivity because there is such a low natural abundance of carbon-13 in nature. So at this point, I think this is probably the last video for Chapter 9. If necessary, I may add an additional video if we're having some trouble in putting together IR, proton NMR, and carbon NMR to solve structures. But I think we'll be ready to move on to our next chapter after this. And that's the end of this recording.

So the NMR spectra are very simple. and you get one signal for every equivalent carbon atom. We also have a very broad chemical shift range. Remember, in proton NMR, the chemical shift range went from 0 to about 13 parts per million.

In carbon NMR, it goes from 0 to 220 parts per million. What this means is that if you have carbons that are very similar to each other, like let's say you have... three different types of methyl carbons in a molecule. In carbon NMR, it's very unlikely that these will have the same chemical shift, or that the chemical shifts will at least be spread out enough so you can actually see those three independent signals. In proton NMR, if you have three methyl groups, it's very likely that those signals are going to be right on top of each other and be very difficult to break them apart.

So the broad chemical shift scale really helps in carbon-13 enumar. Now, one problem we do have is that the most abundant carbon in nature is actually the carbon-12 nucleus. The carbon-13 isotope, which is a stable isotope of carbon, only 1% of all of the carbon atoms in nature are carbon-13.

So this means that if you have an organic compound, only 1% of those carbons will be detected by carbon-13 NMR. So this technique is many times less sensitive than proton NMR. We get around that by using more sample and by running the NMR experiment for a longer period of time. So with proton NMR, you can collect a really good NMR spectrum in a couple of minutes.

Carbon-13 NMR spectra, however, may take a couple of hours, and it's not unusual to actually run these experiments overnight. But we also take advantage of this low natural abundance of carbon-13. Because only 1% of your carbon atoms in your organic compound are the carbon-13 nucleus, we don't see splitting by adjacent carbons. This is because the chances of having two carbon 13 atoms next to each other is very, very low. So what this means is you can run an NMR spectrum of your organic compound, your unknown, and every carbon atom that is non-equivalent to each other will give its own unique signal.

In other words, a bunch of singlets. So these are much easier to interpret than proton NMR. But it...

is possible to observe splitting caused by the hydrogen atoms that are bonded to a carbon. The typical experiment that we run is called a proton decoupled carbon-13 NMR. This means that we don't see splitting by the protons that are attached to a carbon atom.

A unique experiment, however, is proton coupled carbon-13 NMR. In this case, we do see carbon-hydrogen splitting. And that splitting follows the N plus 1 rule.

So, for example, a methyl group, a CH3, the proton-coupled signal will be a quartet. So, what we would typically do is first run a proton-decoupled spectrum, look at the total number of non-equivalent carbons, and then run the proton-coupled NMR spectrum and see what the splitting looks like. A quartet means a CH3. A doublet means a CH and on from there. So here's a correlation table just like we have for IR and for proton NMR.

And you can take a look at this. Again, we don't want to go crazy with these correlation tables. We want to use them to identify specific types of functional groups. And there's only going to be two groups I'm going to point out to you. First, the carbons of the benzene ring.

Those carbons are uniquely situated between about 100 and 170 parts per million. And a carbonyl carbon. The carbonyl carbons are the furthest downfield. And in carbon NMR, between about 160 and 200 parts per million. So that's the only place where chemical shift comes in for carbon atoms.

So here's an example we'll take a look at. This is the structure of 2-butanol. I've got it color-coded this time, and we're going to try to figure out what the carbon-13 NMR looks like. So first, let's look at the number of non-equivalent carbon atoms.

You can see that the molecule is not symmetrical, meaning that the CH3 on the right is non-equivalent from the CH3 on the left. So each of those should give... a carbon-13 signal. Plus, we have a CH2, and we have a CH that also is bonded to an alcohol functional group.

So, we would expect that there will be four signals in the proton decoupled carbon-13 NMR spectrum. And there it is. Four signals, also nicely color-coded.

The methyl group A is furthest upfield, next to it. is B, the methyl group B, then the CH2, and then the CH. Now, just notice the two methyl group signals, A and B. They're both methyl groups, but notice that B is taller than A is.

It has nothing to do, the size or the height of the signal in carbon-13 NMR really has nothing to do with anything. In proton NMR, it's very different, where the size or the area of a signal is directly proportional to the number of hydrogens. In carbon NMR, the height of the signal has nothing to do with the number of those particular types of carbons. It has to do with something else. It has to do with what we call relaxation time.

That's the time it takes for excited nuclei to flip their north and south poles. We're not going to worry too much about that. So don't let the height of the signals really bother you about what that might mean in terms of interpreting the structure of the unknown.

Now, the way we're going to use carbon-13 NMR is as a backup. We're going to use it to confirm a structure that we have already come up with using IR and using proton NMR. All right, now let's look at the uncoupled carbon-13 NMR for the same compound.

And there it is. So the same, we're going to do the same analysis. Okay, oh, by the way, this little triplet that we see on the far left, that is actually the solvent.

It's most likely deuterated chloroform. That's CdCl3. And notice what the deuterium is doing is kind of interesting. It's splitting the carbon, but by an N plus 2 rule.

Kind of interesting. Anyway. Back to the NMR, the carbon-13 proton-coupled NMR spectrum.

So in this experiment, the protons are allowed to interact with the carbon nuclei and to cause N plus 1 splitting. So the methyl group on the right that we call A, there are three hydrogens there. Notice that it's split to a quartet. Methyl group B also is a quartet, so methyl groups show up as quartets.

because of the three hydrogens and the n plus one rule. The CH2 shows up as a triplet because of the two hydrogens, and the CH shows up as a doublet because of the one hydrogen. So carbon-13 NMR is very useful. It's simple to interpret.

It is used to back up the structure we come up with from IR and NMR. The only real disadvantage is that it has low sensitivity because there is such a low natural abundance of carbon-13 in nature. So at this point, I think this is probably the last video for Chapter 9. If necessary, I may add an additional video if we're having some trouble in putting together IR, proton NMR, and carbon NMR to solve structures.

But I think we'll be ready to move on to our next chapter after this. And that's the end of this recording.

Transcript for:Understanding Carbon-13 NMR Spectroscopy

Transcript for:
Understanding Carbon-13 NMR Spectroscopy