NMR Spectroscopy: Videos & Practice Problems

Nuclear Magnetic Resonance (NMR), particularly proton NMR, is a vital analytical tool for identifying protons in various electronic environments. The spectrum displays chemical shifts in parts per million (ppm), indicating shielded (upfield) and deshielded (downfield) protons. Key features include the number of signals, chemical shifts, signal height representing relative hydrogen ratios, and spin splitting, which reveals molecular structure. Understanding these elements is crucial for interpreting NMR spectra and applying concepts like deshielding and shielding in organic synthesis.

General NMR Features

Now we're going to discuss a very powerful analytical tool called nuclear magnetic resonance, or NMR. While there are many different types of NMRs we could learn, we're going to start off with the one that's most important for this course, and that's called proton NMR. Proton NMR is an instrumental method that will allow us to identify and distinguish protons in slightly different electronic environments. We're going to use magnetic fields to actually generate magnetic fields around these atoms and see what the strength of those magnetic fields is that we get back from them. Now, I'm actually going to leave the entire scientific explanation of how nuclear magnetic resonance works up to your professor or up to 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. That's what we do here. Let's go ahead and jump right into what the spectrum looks like.

As you see, this spectrum has units that you have to get familiar with. It has shapes that you have to get familiar with. Let's just talk about the basics in terms of navigating the spectrum. First of all, we're going to see that on the x-axis, we have this unit of ppm. PPM stands for parts per million, and it's really just an arbitrary unit of measurement that we use for this. Just some scientist decided that he wanted to measure the magnetic response of these atoms or these nuclei through parts per million. That's what we go with. Okay? Notice that it starts at 0 and it usually goes up to, here I have it above to 11. It actually usually ends around 13. Okay? So 13, 14, somewhere around there, you're going to get your entire spectrum.

Notice that we have these words at the top that are kind of our navigation words. It's important that you are able to associate these. First of all, we have the words downfield and upfield. Notice that upfield is close to 0 and downfield is close to the high number of 13. 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 make sure that you have that for the rest of your life, a kind of life-changing definition.

Let's talk about something else which is shielded and deshielded. Shielded is a word that we're going to discuss in a second. Deshielded would be the opposite of that word, correct? I'm going to define downfield, that's the same thing as saying it's deshielded. Spectrum. So if you say something's downfield, that's the same thing as saying it's deshielded. If you say it's upfield, that's the same thing as saying it's shielded. So the way I like to remember this is, Double D's. Alright. So you got your downfield, you got your deshielded.

First of all, there 0 is basically going to 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 0, is going to be called TMS. That stands for tetramethylsilane. It's SiC4H12. That happens to be, remember that we said 0 is around shielded, an extremely shielded molecule. We'll discuss what that means.

So basically, electrons are what shield protons from the effects of NMR. The more electrons that I can have around my hydrogen, the less it's going to experience the magnetic field that I am producing from the NMR. The more electrons, the less it experiences it. The more shielded it is. The more stripped of electrons it is, the less electrons it has, the more it's going to feel that magnetic field and it's going to actually result as a consequence of that, and the more downfield or deshielded it's going to be. So one way you can think of it is that the further downfield your proton is, the more getting, the more naked the proton is. So you're going in the direction of your Double D's. 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. You have a big wool coat. You're not going to really feel the cold that much, right? So that would be the kind of the idea of being shielded. You're not going to feel that cold, so you would kind of show up in the shielded area, kind of like the tetramethylsilane. It's very shielded. It's got all these methyl groups, right? But if all of a sudden I were to take off that big wool coat and all I have is boxers on, case I've given you a great visual now, I'm going to feel super cold and I'm going to result much more downfield or deshielded. In general, the less electrons I have around me, or the thinner my coat is, the more downfield I'm going to result. Now, guys, we understand the kind of shifting of the right to left idea here.

Here’s what students ask on this topic:

What is proton NMR and how does it work?

Proton NMR (Nuclear Magnetic Resonance) is an analytical technique used to identify and distinguish protons (hydrogen atoms) in different electronic environments within a molecule. It works by placing a sample in a strong magnetic field and irradiating it with radiofrequency waves. The protons in the sample absorb and re-emit this energy at specific frequencies, which are detected and translated into a spectrum. The spectrum displays chemical shifts in parts per million (ppm), indicating the electronic environment of the protons. Shielded protons appear upfield (lower ppm), while deshielded protons appear downfield (higher ppm). Key features include the number of signals, chemical shifts, signal height, and spin splitting, which together provide detailed information about the molecular structure.

What is the significance of chemical shift in NMR spectroscopy?

The chemical shift in NMR spectroscopy is a crucial parameter that indicates the electronic environment of protons in a molecule. Measured in parts per million (ppm), it reflects how shielded or deshielded a proton is by its surrounding electrons. Shielded protons, which have more electron density around them, appear upfield (lower ppm), while deshielded protons, with less electron density, appear downfield (higher ppm). The chemical shift helps identify the types of functional groups present and their positions within the molecule, making it an essential tool for elucidating molecular structure.

How do you interpret the number of signals in a proton NMR spectrum?

The number of signals in a proton NMR spectrum corresponds to the number of distinct proton environments in a molecule. Each signal represents a set of protons that are in the same electronic environment. For example, if a molecule has four different types of protons, the NMR spectrum will show four distinct signals. This information helps determine the number of unique hydrogen environments, which is crucial for deducing the molecular structure. By analyzing the number of signals, along with their chemical shifts, one can infer the presence of different functional groups and their relative positions within the molecule.

What is spin splitting (multiplicity) in proton NMR and how is it interpreted?

Spin splitting, or multiplicity, in proton NMR refers to the splitting of NMR signals into multiple peaks due to interactions between neighboring protons. This phenomenon occurs because the magnetic field experienced by a proton is influenced by the spin states of adjacent protons. The number of peaks in a split signal follows the n+1 rule, where n is the number of neighboring protons. For example, a proton with two neighboring protons will show a triplet (2+1=3 peaks). Spin splitting provides information about the number of adjacent protons and their relative positions, aiding in the determination of the molecule's structure.

What is the role of TMS (tetramethylsilane) in NMR spectroscopy?

Tetramethylsilane (TMS) is used as an internal standard in NMR spectroscopy. It provides a reference point for chemical shifts, which are measured in parts per million (ppm). TMS is chosen because it has a single, sharp signal at 0 ppm, due to its highly shielded protons. This signal serves as a baseline for comparing the chemical shifts of other protons in the sample. By using TMS, chemists can standardize NMR spectra, ensuring consistent and accurate interpretation of chemical shifts across different experiments and instruments.