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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?
Hydrogen-1 NMR spectroscopy, also known as proton spectroscopy, is an analytical technique used in organic chemistry to analyse molecules and determine structure.
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.
In carbon-13 NMR, we used the solvent CCl4. We can also use this in hydrogen-1 NMR. Another common solvent is CDCl3. This is an example of a deuterated solvent, meaning that all of the molecule's hydrogen atoms are the isotope deuterium. Deuterium has an even mass number and so doesn’t have spin. This means that it doesn't show up on the spectrum.
Fig. 2 - Deuterium has no spin
We can't use solvents containing hydrogen-1 atoms, as they produce one giant signal that monopolises the NMR spectrum. Deuterated solvents, on the other hand, don't produce any peaks on the spectrum.
The energy needed for an atom to flip is known as its magnetic resonance frequency. It varies depending on the atom’s environment - all the other chemical groups surrounding it.
Atoms that are better-shielded from the magnetic field by electrons have a lower resonance frequency, and therefore lower chemical shift, than those less well-shielded. This means that hydrogen atoms bonded to electron-releasing groups such as methyl have lower chemical shift values than those bonded to electronegative groups like oxygen.
For a more detailed look into NMR, check out Understanding NMR and Carbon -13 NMR.
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
An atom’s environment is all the other atoms and chemical groups attached to it.
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
Here's a handy tip: If a molecule is symmetrical, then it has hydrogen atoms in the same 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
Hydrogen-1 spectra show much lower chemical shift values than carbon-13 spectra. This is because the bonded electron pair in hydrogen-1 is much closer to its nucleus than the bonded pair in carbon-13, and so hydrogen’s nucleus is much better shielded from the external magnetic field. We know from above that this gives atoms a lower resonance frequency and thus lower chemical shift values.
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.
| Name | Number of peaks | Number of hydrogen atoms bonded to adjacent carbon atoms |
| Singlet | 1 | 0 |
| Doublet | 2 | 1 |
| Triplet | 3 | 2 |
| Quarter | 4 | 3 |
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
As described above, the hydrogen atoms in the alcohol group (-OH) produce a peak that doesn't split, no matter the number of hydrogen atoms bonded to any adjacent carbon atoms. This peak also has a wide range of chemical shift values which vary dramatically depending on the conditions. For example, the -OH peak's chemical shift value changes if the sample is more concentrated, or in a slightly different solvent. Altogether, these two factors make the -OH group hard to identify. Fortunately, there is a solution: adding a small amount of deuterium oxide (D2O) to the solution.
Deuterium oxide is also known as heavy water, and its name gives you some clues about its identity. Take a molecule of water, replace the hydrogen atoms with heavier deuterium (2H) atoms, and you have D2O. It is useful because of its interaction with alcohols.
Alcohols are very, very slightly acidic. When you add them to water, you set up an equilibrium, in which some of the alcohol molecules ionise by giving up a hydrogen ion to water. At the same time, alcohol reforms by gaining a hydrogen ion.
ROH + H2O ⇌ RO- + H3O+
The same thing happens when you add an alcohol to heavy water. However, the alcohol ions gain a deuterium ion, instead of a standard H+ ion. Overall, this results in swapping the alcohol's -OH group for an -OD group.
ROH + D2O ⇌ RO- + HD2O+ ⇌ ROD + HDO
Remember that deuterium doesn't show up in hydrogen-1 NMR spectra, thanks to its even spin. This means that the -OD group doesn't produce a peak on the spectrum. When -OH groups are swapped for -OD groups, their corresponding hydrogen-1 NMR spectrum peak vanishes. Therefore, we can infer the following:
- Measure a hydrogen-1 NMR spectrum for a sample.
- Add a few drops of D2O and measure the sample again.
- Any peaks that are present in the first spectrum but not in the second spectrum are caused by -OH groups.
Here's an example. We saw the hydrogen-1 NMR spectrum for ethanol earlier; compare it to the spectrum produced when you add a small amount of D2O to the sample:
Fig. 13 - The high-resolution hydrogen-1 NMR spectra for ethanol, with (right) and without (left) heavy water. Note the absence of the -OH peak in the spectrum for the sample with heavy water
You should also note that this principle is true for -NH- groups as well. Like -OH groups, their respective peaks disappear from hydrogen-1 NMR spectra when you add D2O.
Working out structure from hydrogen-1 NMR spectra
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.
| Peak chemical shift value | Type of hydrogen environment | Integration trace value | Number of hydrogen atoms in environment | Type of peak | Number of hydrogen atoms in adjacent environments |
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.
Work out the structure of this unknown organic molecule from its high-resolution hydrogen-1 NMR spectrum, shown below.
Fig. 14 - The hydrogen-1 spectrum for an unknown molecule
Let’s start at the peak with a chemical shift of 1.2. Its integration trace value is 3, so there are 3 hydrogen atoms in this environment. It must be a methyl group (-CH3). Because it is a triplet, it must have 2 hydrogen atoms in adjacent environments.
The next peak along is a quartet, so it must have a total of 3 hydrogen atoms in adjacent environments. It has a trace value of 2 so there are two hydrogen atoms in this environment. From the data table earlier, we can see that its chemical shift value of 2.2 means it is some sort of carbonyl group (C=O) joined to a carbon atom that has one or more hydrogen atoms (-COCH-). But we know that this particular environment contains 2 hydrogen atoms, and so the peak must therefore represent -COCH2-.
The final peak is a singlet and has a shift value of 10.5. This means it must belong to the -OH bond in a carboxyl group (-COOH). Remember that the -OH group always produces a singlet.
| Peak chemical shift value | Type of hydrogen environment | Integration trace value | Number of hydrogen atoms in environment | Type of peak | Number of hydrogen atoms in adjacent environments |
| 1.2 | -CH3 | 3 | 3 | Triplet | 2 |
| 2.2 | -COCH- | 2 | 2 | Quartet | 3 |
| 10.5 | -COOH | 1 | 1 | Singlet | 0 |
Let’s put this molecule together.
- We have three different environments in total.
- We know that we have two groups which must be at the ends of the molecule: a carboxyl group (-COOH) and a methyl group (-CH3).
- The methyl group must be next to an environment with 2 hydrogen atoms.
- We also have a -CH2- group with 2 hydrogen atoms, which must be next to a C=O group. This works out well - it slots right in between the carboxyl group and the methyl group!
Our molecule is propanoic acid.
Fig. 15 - The high-resolution hydrogen-1 NMR spectrum of propanoic acid. The peaks are labelled accordingly
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.
Hydrogen-1 NMR - Key takeaways
Hydrogen-1 NMR, also known as proton NMR, is an analytical technique that helps us identify molecules and work out their structure.
Hydrogen-1 NMR looks at the resonance of hydrogen-1 atoms. It uses TMS as a reference molecule and CCl4 or CDCl3 as a solvent.
Hydrogen-1 NMR produces chemical shift peaks at values from around 0 to 10, which is a much narrower range than that found in carbon-13 NMR spectra.
We use an integration trace to find out the ratio of hydrogen atoms in each environment.
Peaks in high-resolution hydrogen-1 NMR spectra split into smaller peaks according to the number of hydrogen atoms in adjacent environments. This is known as spin-spin coupling or spin-spin splitting. Peaks split according to the n + 1 rule, where n is the total number of hydrogen atoms in adjacent environments.
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Frequently Asked Questions about Hydrogen -1 NMR
What is Hydrogen-1 NMR used for?
Hydrogen-1 NMR is mostly used to identify molecules and work out their structure.
What are the principles of H NMR?
Hydrogen-1 atoms have an odd mass number, meaning they show a property called spin. This means they are affected by an external magnetic field. By analysing their behaviour in such a magnetic field, we can find out what chemical groups the hydrogen-1 atoms are a part of and work out the structure of their parent molecule.
Is proton NMR the same as hydrogen NMR?
Yes. Hydrogen NMR and proton NMR spectroscopy are different names for the same technique.
What is hydrogen resonance?
When nuclei with spin are placed in an external magnetic field, they either line up parallel to the magnetic field, or antiparallel to the field. The parallel state is much more energetically stable than the antiparallel state, but if you give the nucleus enough energy, it can flip to the antiparallel state. This is known as magnetic resonance. Hydrogen resonance is therefore the name of the process where hydrogen nuclei flip from their parallel to their antiparallel state.
Why is hydrogen used in NMR?
Hydrogen is used in NMR because it has an odd mass number. This means it has spin and is affected by external magnetic fields.
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