Carbon NMR - Online Tutor, Practice Problems & Exam Prep

Carbon-13 NMR is a specialized form of nuclear magnetic resonance that detects carbon-13 isotopes, which occur naturally at about 1% abundance. Unlike proton NMR, carbon-13 NMR does not exhibit splitting due to the low likelihood of adjacent carbon-13 atoms. The chemical shifts in carbon-13 NMR range from 0 to 210 ppm, maintaining the same order of functional groups as in proton NMR. Familiarity with these shifts is often sufficient, as detailed memorization is rarely required. Understanding carbon-13 NMR enhances analytical skills in organic chemistry.

13C NMR General Features

Now I want to briefly touch on another form of nuclear magnetic resonance except this type of NMR is going to detect carbon-13 isotopes instead of protons. This is fittingly called carbon-13 NMR. Carbon-13 NMR is a more limited type of nuclear magnetic resonance in contrast to proton NMR. There's actually less information that we can get from carbon-13 than we can from proton NMR. This is largely in part to the low natural incidence of the carbon-13 isotope. I'm not sure if you guys recall from general chemistry, but if you remember, carbon-13 has a natural abundance of about 1 out of every 100 carbon atoms will be a carbon-13. Because there are so few carbon-13s in molecules, that means that splitting, remember one of the major forms of information we get in proton NMR, is not observed at all. You do not get any splitting in carbon-13 NMR. If you do the math, it makes sense because if you think about it, the only way splitting could occur is if you have a carbon-13 that's next to another carbon-13 so that they can interfere with each other. But if the natural incidence of carbon-13 is one in a hundred, that means that the chances of getting two carbon-13s next to each other are one in a hundred times two. So that means that the chances that you would actually get these two carbons to split each other is actually one over ten thousand. Okay? So the math just gets too crazy. It's such a small percentage of our carbon-13s that will split that we just basically say that it's not observed at all.

Other than that, we're going to see all of the major themes that we learned in proton NMR carried over into carbon-13 NMR. The only differences being we're not detecting hydrogens anymore. We're detecting carbons. We're not getting any splitting and we are going to have to learn some new shift values in terms of our chemical shifts because the instrument is calibrated differently. Now what you're going to notice is that the same general pattern applies. If you were just to not look at these numbers and just look at the order of these groups, you see that they're all in the same exact order. We have alkane. We have alkyne. We have our electronegatives. We have our alkene. We have benzene. We have carbonyls. So, really the order hasn't changed at all. It's just the absolute values that have changed because the spectrum of a carbon-13 NMR goes from about 0 to about 210. So it's just a different set of values.

Now to make things a little bit better for you, just the fact that these values didn't really change a lot helps. But also, it's extremely rare for professors to ask you to memorize these values because they tend to not care as much about carbon-13 because it's just not as helpful of an analytical method. Many times, they'll tell you that you don't really need to know these shift values. You should just be familiar with them. In that sense, if you're familiar with proton NMR, you're already up to speed with shifts. We're going to go straight into some practice problems that kind of help us to solidify our knowledge of carbon-13 NMR. I want you to go ahead and answer this question and then I will go ahead and answer it for you. So take a shot at it.

Identifying 13C Signals

All right. So how many different signals did we get? Well, this is definitely going to depend on whether you saw symmetry or not. And actually, it turns out that there is symmetry in this molecule because we've got the benzene ring that's exactly the same on both sides, and we actually have symmetry along this bond. Now you might be wondering, maybe you're under the impression that there wasn't any symmetry because you see this CH₃ that's kind of off tilted to one side. You're thinking that this is an asymmetrical molecule. But, again, remember single bonds can rotate as much as they want. Really, even though it's drawn that way right now, remember that the CH₃ could easily go to the top, could easily go to the side, and it could easily rotate all around there. So, actually, there's a perfect plane of axis all the way through this molecule. Just imagine that the CH₃ rotated so that it's right underneath that dotted line. Sorry, I know that was tricky, but it's stuff you have to be able to visualize. Now recall, we're not concerned about hydrogens here, just different types of carbons. So that means that every carbon gets a peak, a signal, even the ones without hydrogen. So this is going to be my first type. This is going to be signal A. This, oops, different color. This is going to be signal B, as well as this one over here. This is going to be signal C along with this one over here. Now this is where things change a little bit from proton NMR. This one gets its own signal. That's signal D. This one is signal E, and then finally, we have signal F. So this one would have 6 signals. Many times what we'll see with carbon 13 NMR is that you get a different number of signals than you would with just proton NMR because you have a different type you're looking for a different type of atom. Awesome. Now let's move on to the next question. Just look at all 4 compounds and try to see which compounds would meet these criteria of only having one peak for both the proton NMR and the carbon 13 NMR. So go ahead, and then we'll just answer all 4 at once.

1H and 13C Joint Problem

The answer is only compounds B and will yield one signal each for proton NMR and carbon 13 NMR. If we look at compound A, compound A would only give us one signal for proton NMR. That would be the signal that's experienced right at the end. However, we would actually get 2 separate signals for carbon 13. Two signals. So that one was out. Now we look at is kind of in the same boat where we would get one signal for proton NMR, but we would get 2 signals for carbon 13.C13NMR. So these questions obviously do not work. These elusive answers do not work. That was just a quick overview of C13NMR. Let's go ahead and move on to the next topic.

Here’s what students ask on this topic:

What is Carbon-13 NMR and how does it differ from Proton NMR?

Carbon-13 NMR is a type of nuclear magnetic resonance that detects carbon-13 isotopes, which naturally occur at about 1% abundance. Unlike proton NMR, carbon-13 NMR does not exhibit splitting due to the low likelihood of adjacent carbon-13 atoms. The chemical shifts in carbon-13 NMR range from 0 to 210 ppm, maintaining the same order of functional groups as in proton NMR. While proton NMR provides more detailed information due to higher natural abundance and splitting patterns, carbon-13 NMR is still useful for identifying carbon environments in a molecule.

Why is there no splitting observed in Carbon-13 NMR spectra?

Splitting is not observed in Carbon-13 NMR spectra because the natural abundance of carbon-13 isotopes is very low, about 1%. For splitting to occur, adjacent carbon atoms must both be carbon-13 isotopes. The probability of this happening is extremely low (1 in 10,000), making it practically negligible. Therefore, carbon-13 NMR spectra do not show splitting patterns, unlike proton NMR, where splitting is common due to the higher natural abundance of hydrogen atoms.

What is the range of chemical shifts in Carbon-13 NMR?

The range of chemical shifts in Carbon-13 NMR is from 0 to 210 ppm. This range is different from proton NMR but maintains the same order of functional groups. For example, alkanes, alkenes, alkynes, and carbonyls will appear in the same relative order as in proton NMR, but their specific shift values will differ. Familiarity with these shifts is often sufficient, as detailed memorization is rarely required in academic settings.

How does the low natural abundance of Carbon-13 affect Carbon-13 NMR analysis?

The low natural abundance of Carbon-13, about 1%, affects Carbon-13 NMR analysis by reducing the likelihood of adjacent carbon-13 atoms, which in turn eliminates splitting patterns. This makes Carbon-13 NMR less detailed compared to proton NMR. However, it still provides valuable information about the carbon environments in a molecule. The low abundance also means that Carbon-13 NMR spectra are generally simpler and easier to interpret, focusing mainly on chemical shifts rather than splitting patterns.

Do you need to memorize chemical shift values for Carbon-13 NMR?

In most academic settings, detailed memorization of chemical shift values for Carbon-13 NMR is not required. Professors typically emphasize familiarity with the general range and order of functional groups rather than exact values. The chemical shifts in Carbon-13 NMR range from 0 to 210 ppm, and understanding this range is usually sufficient. This approach allows students to focus on interpreting spectra and identifying carbon environments without the burden of memorizing specific shift values.