Hi everybody, this is video 9-5 and the third of our topics on proton NMR is going to be signal splitting. So signal splitting, up to this point, IR and NMR have given us about the same amount of information, a little bit different for each, but nothing that conclusively gets us to the final structure of an unknown organic compound. Signal splitting.
is extremely valuable. It is unique to NMR and essentially it allows us to look at an NMR spectrum and to come up with pieces of the molecule. Then all we have to do when we're done is take these pieces as if they were pieces of a jigsaw puzzle and literally put them together to build a molecule.
So the way signal splitting works is that We know that we have a magnetic field for some hydrogen nucleus. Let's say we're looking at NMR spectrum. We see a signal. We know that that signal is due to the spinning hydrogen nucleus and that signal that we're observing is what I'm going to have in red on these slides. Now there are other hydrogen nuclei in the molecule.
The hydrogen nuclei that are on the directly adjacent carbon atoms will also have magnetic fields. these magnetic fields are going to in some way influence the magnetic field of the hydrogen signal that we are observing. And the way we see this influence is that the signal gets split. So you get a signal.
If you have adjacent hydrogen nuclei, that signal might be split into two lines, three lines, four lines, or even more. And that's what signal splitting is about. It tells you what hydrogens are next to or adjacent to a hydrogen signal that you are focusing on. So the way this works is we have a n plus one rule that helps us to predict proton NMR signal splitting.
So let's start out with this very simple example. This is just a piece of an unknown molecule and we are looking at the signal for the hydrogen that's in red. Now notice that directly adjacent to that hydrogen.
is a methyl group and the magnetic field of the hydrogens on that methyl group are going to affect the signal for the hydrogen red. That's the one that we're looking at. The way that we can predict where that hydrogen red signal is in the NMR spectrum is we look for its splitting and its splitting will reflect how many hydrogens are next to it by the N plus 1 rule. So we have Three hydrogens that are adjacent to the hydrogen signal we're observing, 3 plus 1 equals 4. So the red hydrogen signal will appear as four lines, which we call a quartet.
Now, on the flip side of this, the methyl hydrogens are also going to give a signal in the NMR. So now we move on to that signal, and that's going to be three hydrogens. Again, using the n plus 1 rule, we have one adjacent hydrogen.
n plus 1 tells us that the CH3, those hydrogens, should appear as a doublet, a signal that's split into two. That's how signal splitting works. It's going to take a little while to get used to, so we'll be doing a bunch of examples.
All right, let's do an overview now of the bigger picture for signal splitting. All right, so we've got a table here. This is an example of the different numbers of hydrogens that are adjacent to a hydrogen we're observing, which, again, is a red hydrogen, and also what we call these split signals from a singlet, doublet, triplet, etc.
So here's a piece of some organic molecule. The hydrogen in red is the one we're observing the NMR signal of. Now we're going to start out by assuming that there are not any hydrogens on the adjacent carbons.
Well, that means N is 0. 0 plus 1 is 1. The red hydrogen appears as a singlet, or a one-line signal. Now here's a piece where we have one adjacent hydrogen. Okay, there's one hydrogen adjacent to the red one.
N is 1. 1 plus 1 is 2. The red hydrogen will appear as a doublet. And on we go. With two adjacent hydrogens, the red hydrogen appears as a triplet.
With three adjacent hydrogens, three plus one is four, the red hydrogen appears as a quartet. Of course, it's also possible that you have adjacent hydrogens on more than one carbon. Here we have one, two, three, four.
A total of four adjacent hydrogens. Four plus one is five. Notice that I'm not calling that a pentette. Once we get past a quartet, in other words, once we have more than three adjacent hydrogens, in other words, four or more adjacent hydrogens, Sometimes it's tough to actually see the splitting because the lines are close together. We know it's more than a quartet, but we're really not sure if it's a pentet, a sextet, a septet.
So we just call it a multiplet. Multiplet meaning that we have four or more adjacent hydrogens. That can be a little tricky, all right? But fortunately, that's not a common theme or a common problem or example.
that we're going to observe. All right, so let's apply this now to an example of isopropyl methyl ether. So we want to know, consider how many signals do we expect in the NMR, and what is the signal splitting going to look like?
So in other words, we think that the structure that we see on the screen is our unknown. We're going to run an NMR spectrum of it, but let's predict first what that NMR spectrum is going to look like. So let's first identify the equivalent hydrogens. So notice that the two methyl groups on the right are both bonded to the same carbon.
We're going to label those as A because they are identical. We then have this one CH that's unique. We're going to call that B and then we also have a CH3 on the left.
and we'll call that C. So we should have three unique signals in the NMR spectrum. And I'm going to put that information in the table, A, B, and C. Now, how many protons do we have for each signal?
Well, A are those two identical methyl groups. That's six protons. B is one proton, and C is three protons.
Now let's predict the splitting for each signal. So for A, if we want to know what these six protons will look like in the NMR, we see that they are both adjacent to one hydrogen. Applying the n plus one rule, one plus one is two, so A will appear as a doublet.
Now B. B is adjacent to one, two, three, four, five, six. Six plus one is seven. We don't see a septet.
We're just going to call that a multiplet. We might not be able to actually count up all seven lines. It will be more than a quartet.
All right, and finally, we have C on the left here. Notice that there are not any adjacent hydrogens. So, N is 0, 0 plus 1 is 1. C will appear as a singlet. So, we predict that the NMR spectrum for this compound should have a doublet.
a multiplet, and a singlet. Well, here is the NMR spectrum for isopropyl methyl ether. The structure is there. Notice I have everything labeled.
Now remember, so for signal A, those two methyl groups are adjacent to that one hydrogen, and we said A was a doublet. Notice that right around 1 ppm, we have a doublet. So I'm going to label that signal as A. Signal B, is adjacent to the six hydrogens. Okay, six plus one is seven.
Since it's always, it's hard to see that many lines, we're looking for a multiplet. And right about four ppm, we see a multiplet. Now, if you're really careful, you can see one, two, three, four, five, six, seven.
So we're actually got resolution here that's pretty good. We can actually count those seven lines. And finally, signal C is not adjacent to any hydrogens.
It should be a singlet, and there it is. Also notice that we have numbers by each signal for A, B, and C. And the NMR spectrometer can actually give us the area under these signals, and that area is going to be proportional to the number of protons.
So, for example, if... if we actually ran the sample in our NMR spectrum, spectrometer here at BSU, we may get numbers such as 0.5, 1.5, and 3. And then we'd have to figure out some type of a multiplier to convert it to 1, to 3, to 6. So that gives you one way in which we are going to use signal splitting. Okay, so this is the end of the recording.
What I'm doing here is at the, you know, for now on, at the end of each video, I'm going to try to leave you with a question that you should be able to answer if you understood what was going on in the video. And I'll leave the answer to this up to you. And that is the end of this recording.