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Nuclear Magnetic Resonance Spectroscopy: C-13 vs 1H

Instructor: Korry Barnes

Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.

The primary purpose of this lesson will be to get a foundational understanding of C-13 and H-1 NMR and how they can provide valuable structural information about organic compounds.

A Case of Unknown Identity

Let's say you're enrolled in your first organic chemistry laboratory course and your teacher says that today, you are going to be synthesizing aspirin (acetylsalicylic acid). As you begin your experiment, it hits you: how will you know your synthesis was successful? Are you just supposed to blindly accept that the reaction worked? Being able to tell if a reaction worked or more generally what the product(s) is of any organic reaction is a very important thing to be able to determine. An organic chemists' go-to method for doing this is known as nuclear magnetic resonance spectroscopy or NMR for short.

NMR is an extremely powerful tool for a chemist, and so its understanding is of paramount importance at least on a foundational level. Why do you ask? Because NMR can provide information about the structure of a molecule that otherwise wouldn't be possible. In this lesson, we are going to explore the two most common types of NMR, carbon-13 and proton, and see what kinds of structural information is possible from each one. Come along and let's make some sense of this!

C-13 NMR

Since carbon NMR is much simpler to understand, we will begin our discussion here. Let's say you have a bag of jelly beans, and they are only red, blue, green, and yellow in color. For the sake of simplicity, let's say that all the shapes of different colors are the same so that jelly beans of like color cannot be distinguished from one another. If someone were to ask you, how many different types of jelly beans do you have, what would you say? You would say 4 correct? That's exactly the type of information C-13 NMR tells us. It tells us how many unique or different carbons are contained in an organic molecule.

For example, if we consider ethyl acetate as an example. This molecule has 4 distinct or different carbon atoms, each in different chemical environments because of their bonding pattern and the other groups of atoms they are connected to. For this reason, we would expect to see 4 different peaks in the NMR spectrum for ethyl acetate.


Chemically distinct carbons in ethyl acetate
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Chemical Shift

An obvious question you might have is where would we expect to find the different peaks in the spectrum? Peaks appear in different regions of the NMR spectrum based on the types of chemical environments they are in, and peaks in the NMR are reported as chemical peaks and the units are in parts per million (ppm). This is known as the 'delta' scale.


Carbon-13 NMR spectrum of ethyl acetate
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Proton or 1H-NMR

Proton (abbreviated 1H) NMR is a lot like carbon in the sense that it tells us how many different types of hydrogens are in an organic compound (or different kinds of jelly beans). The spectrum is a bit more complicated; however, so let's address some important features of proton NMR.

Integration

The integration, or area under each peak in the proton NMR spectrum tells us the relative number of protons giving rise to that particular signal. For instance, each of the two -CH3 peaks in the spectrum of ethyl acetate would integrate to 3 hydrogens (or protons) since there are three hydrogens attached to each carbon. Similarly, the -CH2 peak in the spectrum would integrate to 2 protons.

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