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Hydrogen -1 NMR

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Chemistry

Imagine you have two test tubes filled with two 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.

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 have also learnt 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 how many hydrogen atoms are in adjacent environments.

  • In this article, you'll discover how hydrogen-1 NMR is carried out before learning how to interpret spectra.
  • You'll explore ideas such as spin-spin coupling, integration traces and the n+1 rule.
  • By the end of this article, you should be able to compare hydrogen-1 NMR to carbon-13 NMR, and use hydrogen-1 spectra to infer the structure of a molecule.

How does hydrogen-1 NMR work?

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, these have an odd mass number and so have spin, meaning they show up in NMR spectra.

Hydrogen-1 NMR hydrogen-1 net spin atom StudySmarterHydrogen-1 atoms have one proton and no neutrons in their nucleus, giving them a net spin of 1/2. Anna Brewer, StudySmarter Originals

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.

Let's have a quick recap of how NMR actually works.

  • 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 . We can also use this in hydrogen-1 NMR. Another common solvent is . This is a solvent based on , but each molecule’s hydrogen atom has been replaced by the hydrogen isotope deuterium. Deuterium has an even mass number, so doesn’t have spin. This means that it won’t show up on the spectrum.

Hydrogen -1 NMR Deuterium StudySmarterDeuterium. Anna Brewer, StudySmarter Originals

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.

Hydrogen -1 NMR Ethanol and its environments StudySmarterEthanol: the different hydrogen atoms are circled according to their environments. Anna Brewer, StudySmarter Originals

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 are both bonded to exactly the same groups. In this case, each carbon atom is bonded to two hydrogen atoms and a group.

Hydrogen -1 NMR Propan-2-ol StudySmarterPropan-2-ol. Again, the hydrogen atoms are circled according to their environments. Anna Brewer, StudySmarter Originals

If a molecule is symmetrical, it will have 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.

Hydrogen-1 NMR ethanol spectrum StudySmarterA hydrogen-1 NMR spectrum for ethanol. Anna Brewer, StudySmarter Originals

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, . 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 group.

Hydrogen -1 NMR Data table for H NMR StudySmarterA data table for hydrogen-1 NMR spectroscopy. Anna Brewer, StudySmarter Originals

Hydrogen-1 spectra show much lower chemical shift values than carbon-13 spectra. This is because its bonded electron pair is much closer to its nucleus than carbon’s bonded pair, 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 had varying heights. They weren’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 in each environment. The area under each peak is proportional to the number of hydrogen atoms present. For example, a taller peak shows there are more hydrogen atoms in that particular environment than a shorter peak.

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.

Hydrogen-1 NMR ethanol spectrum StudySmarter
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. Anna Brewer, StudySmarter Originals

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

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

Hydrogen -1 NMR H NMR spectrum for Ethanol StudySmarterThe hydrogen-1 spectrum for ethanol. Anna Brewer, StudySmarter Originals

Some of the peaks have been 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. In other words, 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.

Hydrogen-1 NMR ethanol environments StudySmarterThe different environments in an ethanol molecule. Anna Brewer, StudySmarter Originals

Look at the hydrogens 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 contains 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. Looking at our graph, we can see that this is what actually happens.

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.

Hydrogen-1 NMR splitting StudySmarterA table showing names for smaller peaks that arise from spin-spin coupling. Anna Brewer, StudySmarter Originals

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 on any neighbouring carbons 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, , always forms just one peak, a singlet. It also has no effect on adjacent carbons - you can ignore it completely when working out spin-spin coupling.

Hydrogen-1 NMR spin-spin coupling example StudySmarterAn example of spin-spin coupling. Anna Brewer, StudySmarter Originals

Hydrogen-1 NMR spin-spin coupling example StudySmarterA further example of spin-spin coupling. Anna Brewer, StudySmarter Originals

Working out structure from hydrogen-1 NMR spectra

Let’s look at an example, to find out the structure of a molecule from its NMR spectrum.

Hydrogen-1 NMR spectrum StudySmarterThe hydrogen-1 spectrum for an unknown molecule. Anna Brewer, StudySmarter Originals

What can we infer from this spectrum? It can be helpful to make a table.

Let’s start at the peak with a chemical shift of 1.2. Its integration trace value is 3, so it has 3 hydrogen atoms. It must be a methyl group. Because it is a triplet, its neighbouring carbon must have 2 hydrogen atoms attached.

The next peak along is a quartet, so it must have a total of 3 hydrogen atoms on neighbouring carbons. It has a trace value of 2 so has just two hydrogen atoms itself. From the data table earlier, we can see that its chemical shift value of 2.2 means it is .

The final peak is a singlet and has a shift value of 10.5. This means it must be part of a carboxylic acid group, . Remember that the group, also found in carboxylic acids, always produces a singlet.

Let’s put this molecule together. We have two ends: a carboxylic acid group and a methyl group. In the middle we have some sort of group with . Our molecule is propanoic acid.

Hydrogen-1 NMR propanoic acid spectrum StudySmarterPropanoic acid with its hydrogen-1 NMR spectrum. The peaks are labelled accordingly. Anna Brewer, StudySmarter Originals

What are 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 or 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 can use an integration trace to find out the ratio of hydrogen atoms in each environment.

  • Peaks in 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 hydrogens in adjacent environments.

Hydrogen -1 NMR

Hydrogen-1 NMR is mostly used to identify molecules and work out their structure.

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.

Yes. Hydrogen NMR and proton NMR spectroscopy are different names for the same technique.

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. 

Hydrogen is used in NMR because it has an odd mass number. This means it has spin and is affected by external magnetic fields.

Final Hydrogen -1 NMR Quiz

Question

Give another name for hydrogen-1 NMR spectroscopy.

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Answer

Proton NMR spectroscopy.

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Question

What property must atoms have to show up in NMR?

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Answer

Spin

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Question

 Nuclei with spin must have __________________.


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Answer

An even mass number.

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Question

State the number of each type of subatomic particle in a hydrogen-1 atom.


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Answer

  • 1 proton.
  • 1 electron.
  • 1 neutron.

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Question

What sort of energy is used in NMR?


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Answer

Radio waves.

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Question

Name the molecule used as a reference point in hydrogen-1 NMR.


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Answer

TMS

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Question

 A nucleus with spin can be in two states when placed in an external magnetic field: _____ or ______.


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Answer

Parallel, antiparallel.

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Question

What is magnetic resonance frequency?


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Answer

The energy required to flip a nucleus from its spin-aligned state to its spin-opposed state.

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Question

What are the units for chemical shift?


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Answer

Parts per million, ppm.

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Question

What does an integration trace show?


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Answer

The number of hydrogen atoms in each environment.

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Question

In a hydrogen-1 NMR spectrum, peak A has a trace value of 2 and peak B has a trace value of 3. What can you infer from this information?


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Answer

Peak A and B have the hydrogen atom ratio 2:3.

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Question

Give another name for spin-spin coupling.


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Answer

Spin-spin splitting.

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Question

What is the n+1 rule?


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Answer

Hydrogen-1 spectra show n+1 peaks, where n is the number of hydrogen atoms.

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Question

A peak in a hydrogen-1 NMR spectrum has split into four smaller peaks, which is also known as a _____.


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Answer

Quartet.

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Question

A peak in a hydrogen-1 NMR spectrum has split into four smaller peaks, meaning that _________.

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Answer

There are three hydrogen atoms in adjacent environments.

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