Hydrogen -1 NMR

Imagine you have two test tubes filled with different unknown compounds. How would you find out what they are? We know that we can use time-of-flight spectroscopy to find out the relative masses of ions. We can use this technique to work out the relative molecular mass of our compounds. We can also carry out some simple tests to check for different functional groups - for example, adding Tollens reagent to see if the compound is an aldehyde.

But what if we wanted to know the exact structure of these samples? For example, we might know that the molecule contains an -OH hydroxyl group and a C=C double bond, but where exactly on the molecule can we find them? This is where we can use hydrogen-1 NMR spectroscopy.

• In this article, you'll discover how hydrogen-1 NMR is carried out before learning how to interpret hydrogen-1 NMR spectra.
• You'll explore ideas such as spin-spin coupling, integration traces, and the n+1 rule, and find out about the difference between low- and high-resolution NMR.
• In addition, you'll learn about the importance of deuterated solvents and heavy water (D₂O) in hydrogen-1 NMR.
• You'll also be able to practice using hydrogen-1 NMR spectra to infer the structure of a molecule.

What is hydrogen-1 NMR?

You should know that certain nuclei possess a property called spin, and that this determines how they behave in external magnetic fields (see Understanding NMR). You should also know how we can use their behaviour to identify different functional groups in molecules (see Carbon -13 NMR). Hydrogen-1 NMR takes spectroscopy to a whole new level by allowing us to work out the exact structure of molecules. It gives us information not only about the number of hydrogen atoms in each environment, but also the number of hydrogen atoms in adjacent environments.

Hydrogen-1 NMR process

Hydrogen-1 NMR works in just the same way as carbon-13 NMR. However, whilst in carbon-13 NMR we examined carbon-13 atoms, in this technique we look at hydrogen-1 atoms. Like carbon-13 atoms, hydrogen-1 atoms have an odd mass number and so have spin, meaning they show up in NMR spectra.

Fig. 1 - Hydrogen-1 atoms have one proton and no neutrons in their nucleus, giving them a net spin of 1/2

You’ll remember that carbon-13 is a relatively rare isotope of carbon - only one percent of all carbon atoms are carbon-13. However, hydrogen-1 is the most common hydrogen isotope. This makes obtaining hydrogen-1 spectra much easier.

Here's a quick recap of the NMR process.

• We dissolve our sample and add in a small amount of TMS, a reference molecule.
• We apply radio waves to the solution.
• Some of the hydrogen atoms in the sample absorb the energy from the radio waves and flip to their antiparallel, spin-opposed state.
• A spectrum is produced. It shows chemical shift, a property related to resonance frequency.
• We compare chemical shift values to those in a data table to work out the environment of the hydrogen atoms present in our sample.

Interpreting hydrogen-1 spectra

Now that we’ve revisited how we carry out NMR, we can look at how we analyse the spectra it produces. To fully understand these spectra, we need to consider environment and chemical shift values, as well as these new terms.

• Integration traces.
• Spin-spin coupling.
• The n+1 rule.
• Singlet, doublet, triplet, and quartet.

Environment

You should remember that the number of peaks on a spectrum shows the number of different environments that the atoms we are looking at, in this case, hydrogen-1 atoms, are found in. In hydrogen-1 NMR, all the hydrogen atoms attached to the same carbon have the same environment. However, hydrogen atoms on different carbons can also be in the same environment, if the carbon atoms they are attached to are bonded to exactly the same chemical groups as each other.

Look at ethanol, for example. The three hydrogen atoms circled in red are all in the same environment. This is because they are all attached to the same carbon. The two hydrogens circled in blue are in the same environment, but a different environment to the ones circled in red. Likewise, the green-circled hydrogen atom is in its own new environment.

Fig. 3 - Ethanol: the different hydrogen atoms are circled according to their environments

However, if we look at propan-2-ol, there are hydrogen atoms from multiple different carbon atoms all found in the same environment. This is because the carbon atoms that they are bonded to are joined to exactly the same groups. In this case, all of the hydrogen atoms in both -CH₃ groups are in the same environment: the carbon atoms in the CH₃ groups are each bonded to three hydrogen atoms and one -CH(OH)CH₃ group.

Fig. 4 - Propan-2-ol. Again, the hydrogen atoms are coloured according to their environment

Chemical shift

As we mentioned above, chemical shift is a property related to magnetic resonance frequency - the energy required to flip a nucleus from its parallel state to its antiparallel state. We measure it in parts per million, or ppm.

Hydrogen atoms in different environments have different chemical shift values depending on how well-shielded they are from the external magnetic field. We know that electrons shield nuclei. An atom bonded to an electron-releasing group, such as the methyl group, is better shielded from the magnetic field, and so has both a lower resonance frequency and a lower chemical shift than one bonded to an electron-withdrawing group.

This all means that chemical shift values vary depending on the atom’s environment. We can compare chemical shift values on a spectrum to those in a data table and use them to infer the environments of hydrogen atoms in our molecule.

Fig. 5 - A hydrogen-1 NMR spectrum for ethanol

Look at the above spectrum for ethanol. The small right-hand peak is given by TMS, our reference compound. The next peak along has a value of about 1.2. Looking at our data table below, we can work out that this peak belongs to hydrogen atoms in a methyl group, -CH₃. The next peak has a value of around 3.4. It belongs to hydrogen atoms on a carbon atom that is attached to an oxygen atom, as this gives values in the range 3.1 - 3.9 ppm. The leftmost peak has a value of about 4.8 and represents the hydrogen atom in ethanol’s -OH group.

Fig. 6 - A data table for hydrogen-1 NMR spectroscopy

Integration traces

You might remember that the peaks in carbon-13 NMR spectra have varying heights. They aren’t related to the number of carbon atoms present in each environment. However, the peaks in hydrogen-1 spectra are directly related to the number of hydrogen-1 atoms present. The area under each peak is proportional to the number of hydrogen atoms in each environment. For example, a taller peak shows there are more hydrogen atoms in that particular environment, whilst a shorter peak shows that there are fewer hydrogen atoms.

Judging the size of peaks by eye can be tricky, so the computer creates an integration trace. This is a line placed over the top of the spectrum. It goes up in steps. The relative height of each step tells you the ratio of numbers of hydrogen atoms in each environment. You can find this ratio by measuring these heights.

Fig. 7 - The NMR spectrum for ethanol. The integration trace is shown in red. If you measure the height of each step, you’ll find the ratio of hydrogen atoms in each environment

To make life easier, the computer often also places a number above each peak. This also tells you the ratio of hydrogen atoms in each environment - it saves you from having to measure each step of the integration trace!

In the example above, we can see that methanol has three hydrogen atoms in one environment, two hydrogens in a second environment and one hydrogen atom in a third environment.

Spin-spin coupling

Hydrogen-1 NMR spectroscopy can be further split into two types:

• Low-resolution spectroscopy uses peaks, chemical shift values and integration traces to tell you the number of different hydrogen environments, the relative number of hydrogen atoms in each environment, and the probable identity of each environment.
• High-resolution spectroscopy takes the process that bit further and tells you the number of hydrogen atoms in adjacent environments.

Let's explore that in more detail.

If we zoom in a little closer to a high-resolution hydrogen-1 NMR spectrum, we notice something a little bit odd. Take a look at the high-resolution NMR spectrum for ethanol, for example.

Fig. 8 - The high-resolution NMR spectrum for ethanol

Some of the peaks have split into a number of smaller peaks. This is because of something called spin-spin coupling, also known as spin-spin splitting or simply just splitting.

We know that each peak gives us information about hydrogens in a certain environment. Spin-spin coupling gives us information about the number of hydrogen atoms on the neighbouring carbon atom to the one responsible for the peak we are studying. These are known as hydrogen atoms in adjacent environments. That’s a bit of a mouthful, but it is easy to understand. If there are n hydrogen atoms on neighbouring carbons, the peak will split into n + 1 smaller peaks.

Let’s break it down. We know that ethanol has three different hydrogen environments. They’re repeated below to help your understanding.

Fig. 9 - The different environments in an ethanol molecule

Look at the hydrogen atoms circled in red, all part of a methyl group. They all belong to the same environment, so produce just one peak. Now, look at the adjacent carbon atom. It is bonded to two hydrogen atoms: there are two hydrogen atoms in an adjacent environment. Therefore, n = 2. If we use the n + 1 rule, we can predict that the methyl group peak will split into 2 + 1 = 3 smaller peaks.

Let’s take the carbon atom on the right now. Its hydrogen atoms produce the middle peak. Look at all the groups attached to it. There is just one attached carbon atom, our methyl group from above. The methyl group has three hydrogen atoms, so n = 3. Using the n + 1 rule, we can predict that this peak will split into 3 + 1 = 4 smaller peaks.

The smaller peaks all have names, shown in the table below.

There are a few further rules concerning spin-spin coupling that you need to know.

• If there are no hydrogen atoms attached to any neighbouring carbons, n = 0. This means that the peak won’t split - it will form 0 + 1 = 1 peak, a singlet.
• Spin-spin coupling only takes place if the hydrogen atoms bonded to any neighbouring carbon atoms are in different environments to the ones you are looking at. We call these equivalent hydrogens. Take a look at the example below for clarification.
• If there are multiple neighbouring carbon atoms with attached hydrogen atoms, we count n as the total number of hydrogens.
• The alcohol group (-OH) always forms just one peak, a singlet. It also has no effect on the splitting of other peaks - you can ignore it completely when working out spin-spin coupling.

Fig. 10 - An example of spin-spin coupling

Fig. 11 - A further example of spin-spin coupling

So, to summarise the high-resolution NMR spectrum for ethanol:

• Ethanol contains a -CH₃, a -CH₂- and an -OH group.
• The peak on the right is a triplet - it splits into 3 smaller peaks. There must be 2 hydrogen atoms in an adjacent environment. Therefore, this peak is caused by the -CH₃ group.
• The peak in the middle is a quadruplet - splits into 4 smaller peaks. There must be 3 hydrogen atoms in an adjacent environment. Therefore, this peak is caused by the -CH₂- group.
• The peak on the left is a singlet - it doesn't split. This peak is either caused by an -OH group, or there are no hydrogen atoms in an adjacent environment. We know that ethanol contains an -OH group, which therefore must be responsible for this peak.

Fig. 12 - The high-resolution NMR spectrum for ethanol, with the peaks labelled

Let’s put our knowledge to practice and have a go at finding out the structure of a molecule from its NMR spectrum.

When you first look at an NMR spectrum, it can be helpful to make a table, summarising all of the information that the table tells you. You should look at each peak in the spectrum and identify:

• Its chemical shift value, and hence the possible hydrogen environment.
• Its integration shift value, and hence the number of hydrogen atoms in that environment.
• The type of peak (for example a singlet or a doublet), and hence the number of hydrogen atoms in adjacent environments.

Here's a sample table for you to copy.

You can then use some logical thinking to put all of this information together and work out the molecule's structure. Here's an example.

Uses of hydrogen-1 NMR

Before we finish, let's consider the uses of hydrogen-1 NMR.

Hydrogen-1 NMR is primarily used to determine the structure of molecules. However, NMR in general has a variety of uses. These include:

• MRI scans for medical diagnosis.
• Mapping protein structure.
• Identifying carotenoids and other metabolites in food products.
• Studying organic molecules without damaging them.
• Detecting and analysing contaminants in environmental systems.