Skip to main content
Pearson+ LogoPearson+ Logo
Start typing, then use the up and down arrows to select an option from the list.

Organic Chemistry

Learn the toughest concepts covered in Organic Chemistry with step-by-step video tutorials and practice problems by world-class tutors.

15. Analytical Techniques:IR, NMR, Mass Spect

NMR Spectroscopy


General NMR Features

Play a video:
Was this helpful?
Now we're going to discuss a very powerful analytical tool called nuclear magnetic resonance, or NMR. And while there's many different types of any Mars that we could learn, we're going to start off with one that's most important for this course. And that's called Proton NMR. So Proton NMR is an instrumental method that is gonna allow us to identify and distinguish protons Okay, in slightly different Elektronik environments. Okay, we're gonna use magnetic fields to actually generate magnetic fields around these atoms and see what the strength of the magnetic fields are that we get back from them. Okay, Now, I'm actually gonna leave the entire scientific explanation of how nuclear magnetic resonance works up to your professor or up thio YouTube because you could definitely spend a good 10 15 minutes learning all about that. And that's not really the most important part. What I'm going to really focus on is how to read it, how to understand it and what you need to know to pass your exam, right? That's what we do here. So, um, let's go ahead and jump right into what the spectrum looks like as you see this spectrum. Um, you know, has units that you have to get familiar with. It has shapes they have to get familiar with. Let's just talk about the basics in terms of navigating the spectrum. Well, first of all, we're gonna see that on the X axis. We have this unit of P P. M. Okay, Ppm stands for parts per million. And it's really just an arbitrary unit of measurement that we use for this. Just some scientists decided that he wanted to measure the magnetic response of these atoms where these nuclei through parts per million. So that's what we go with. Okay, notice that it starts at zero, and it usually goes up to here. Have it about 2 11. It actually usually ends around 13. Okay, so 13 14 somewhere around there, you're going to get your entire spectrum. Okay. Now, notice that we have these words at the top that are kind of our navigation words. Okay? And it's important that you are able to associate thes. So, first of all, we have the words downfield and upfield. Notice that upfield is close to zero and downfield is close to the high number of 13. Okay, now, this might make sense while I'm telling you right now. But you'd be surprised once you shut off this video. How confusing that could be. It's like upfield. Is that the towards the bigger number or the smaller number? So we have to figure out a way to memorize that, because I want you to know, make sure that you have that for the rest of your life, you know, kind of a life changing definition. Let's see, Let's talk about something else, which is shielded and d shielded. So shielded is a word that we're gonna discuss in a second D Shielded would be the opposite of that word. Correct. Okay, so I'm going to define what it means to be shielded in a second. But notice that downfield and D shielded happen to be the same direction of the spectrum. So if you say something's downfield, that's the same thing is saying it's Dee shielded. If you say it's up field, that's the same thing of saying it's shielded. So the way I like to remember this is I'm sorry. I'm a guy Double ds. All right, so you got your downfield, you got your d shielded super inappropriate, and it worked. Now you're gonna remember that. Okay, so now you know, kind of. If you can remember which direction they go, then you're fine. You've got everything else. Okay, so let's go ahead and talk about just some general features of the NMR spectrum. Well, first of all, there actually is no natural zero for NMR spectrum. The zero is basically gonna be a molecule. That is our test molecule that we run all of the other molecules against. And that test molecule, the one that we use as our zero is going to be called T. M s. Okay, that stands for Tetra methods silent. Okay. It's basically a silicon molecule that has four methyl groups around it. Okay. And that happens to be remembered that we said zero is around shielded. That happens to be an extremely shielded molecule. We'll discuss what that means. So basically, electrons are what shield protons from the effects of NMR. Okay, so basically, what that means is that the more electrons that I can have around my hydrogen, the less it's gonna experience the magnetic field that I'm in producing from the NMR. Okay, so the more electrons, the less it the lesson experiences it, the more shielded it is, the more stripped of electrons it is, the less electrons it has, the more it's gonna feel that magnetic field. And it's gonna actually, um, it's going to actually result as a consequence of that and the more down field or D shield that it's gonna be OK. So one way you can think of it, is that the further downfield your your proton is the more getting man super inappropriate, the more naked the proton is. Okay, so you're going in the direction of your double DS. Now you're naked. I don't even know where this is going. I didn't even plan that, by the way. But basically what I'm trying to say here is imagine that you're going out into the middle of, like, Michigan winter or something like that, right? It's bitter cold. Um, you have a big wool coat. You're not going to really feel the cold that much. Right? So, um, that would be the kind of the idea of being shielded. You're not gonna feel that cold, so you would kind of show up in the shielded area, kind of like the Tetra methods silent. It's very shielded. It's got all these metal groups, right? But if all of a sudden I were toe take off that big wool coat and all I have is boxers on case I've given you a great visual. Now I'm gonna feel super cold, and I'm going to result much more down field or D shielded. Okay, so in general, the less electrons I have around me or the less the thinner my coat is the more down field, I'm gonna result. Okay, so now we guys, we understand kind of the shifting of the right toe left idea here, but it turns out that there's a lot more information that we can gain than just that. There's actually four different types of information that we get from an NMR from a proton NMR spectra. And we're going to discuss kind of introduce with those four types of information are Okay, so the first type of information that we're gonna be ableto learn is we're gonna learn the number of signals, the total number of signals and the total number of signals is then going to describe how many different types of hydrogen are present because basically, if you have Let's say that you have three different peaks or let's look at our example here. This one has four different peaks. Notice that it has peak a peak. Be peak. See, and peak D. Okay, Our signal A through D, right. All of these protons are wearing different layers of clothing. You could imagine that Proton a is the guy with the big heavy jacket. And Proton D is the one that he's in the process of stripping down. Okay, so when the other guys air somewhere in between. So what that's telling us is that we have four distinct types of protons on this molecule just by telling me that that's already a lot of information. Okay, so it tells me how many different types of hydrogen protons are present. Okay, then we've got the chemical shift. Okay, The chemical shift is the actual part. That's the parts per million that's going to tell me how shielded or d shielded the actual protons are. So I'm going to know that there's four different types of protons and this one has a shift of four. This one has a shift of two that's going to tell me kind of what kind of functional groups they're attached to see where we're going with this. It's already telling me a lot of information, but there's more than that. We're also going to be able to use the height of the signal. So it turns out that on this sample NMR that I showed you notice that some of them are really short and some of them are really tall. That's actually for a reason. It turns out that the ones that are taller, the ones that take up more area, more area under that curve, that's going to represent that there's actually more of that type of hydrogen. Okay, so just, for example, taking a versus B Okay, I noticed that I have A and B next to each other. I could safely say that there's more of the coded hydrogen, the wool coat than the ones that air be the ones that have a slightly less clothing honor that air slightly more D shielded because there seems to be a lot more area underneath that curve than there is below beat. Get what I'm saying. So it turns out that we call that the relative ratio of hydrogen so basically tells us that we have more of one type of hydrogen and less of another. Again, that's a lot of information. But finally we have spin splitting or what's called multiplicity and spin Splitting is that final piece of information that really just is really just helps us determine the structure of a molecule. And what it does is it describes to us how close or how far the different molecules are from each other. Now spin splitting is represented by the fact that instead of just being one peak, noticed that a lot of these have, like multiple little peaks in them. For example, D the guy that's most stripped down appears tohave four little tiny peaks in it. Those what we call the multiplicity or the splits, and that's going to tell us what type of hydrogen he's next to. Okay, so tons of information here, what we're gonna do now is we're going to spend lots of time going through every single piece of this so that you really understand. We're gonna do every single piece of this in depth so that you can take one of these NMR spectra and understand perfectly what's going on. Okay, At least that's the goal. So let's go ahead and move on to the next topic