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Carbon -13 NMR

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Carbon -13 NMR

In 2020, the program AlphaFold took home first prize at the biennial challenge known as CASP, short for Critical Assessment of Structure Prediction. The competition involved correctly predicting the structure of different proteins using Artificial Intelligence and algorithms. AlphaFold was developed by DeepMind, Google’s AI offshoot, and easily outperformed over 100 other teams to win the challenge. This is a real break-through for science because accurately knowing protein structure helps us develop drugs and understand the building blocks of cells. Previously, scientists have had to use fiddly experimental techniques to work out the structure of proteins. These include X-ray crystallography, cryo-electron microscopy, and NMR spectroscopy. Carbon-13 NMR is a type of NMR spectroscopy.

NMR spectroscopy stands for nuclear magnetic resonance spectroscopy. It is an analytic technique we use to identify molecules and determine their structure.

What is carbon-13 NMR?

There are two common types of NMR spectroscopy, known as carbon-13 NMR and proton NMR. We will focus here on carbon-13 NMR.

Carbon-13 NMR is a form of NMR spectroscopy that uses carbon atoms to work out the structure and identity of a molecule.

  • We will look at how carbon-13 NMR spectroscopy is carried out before learning how to interpret spectra.
  • We'll recap terms like spin, resonance frequency, and chemical shift.
  • You'll be able to practice spotting different carbon environments and identifying different molecules based on their spectra.

Before we go any further, let's first remind ourselves of how and why NMR works.

Spin

You should remember from Understanding NMR that nuclei with an odd mass number have a property called spin. Spin can be influenced by external magnetic fields and makes nuclei behave a little like bar magnets. If placed in an external magnetic field, these nuclei line up so their spin is either parallel to the field or antiparallel:

  • If they are in their parallel state, we call them spin-aligned.
  • If they are in their antiparallel state, we say that they are spin-opposed.

Carbon-13 NMR magnetic field parallel antiparallel StudySmarterBar magnets placed in an external magnetic field can take two states: spin-aligned (parallel) or spin-opposed (antiparallel). Anna Brewer, StudySmarter Originals

Resonance

Most nuclei with spin in a magnetic field are spin-aligned, in their parallel state. This is because it is more energetically stable than its antiparallel state. Think of it as swimming in a stream of water. It is a lot easier to swim with the current than to turn around and swim against it. However, if you put in enough energy, you can swim upstream. Flipping a nucleus from its parallel to its antiparallel state is called resonance. The energy required to do this is known as magnetic resonance frequency. If we supply a sample of nuclei with energy in a range of frequencies, some of them will absorb energy equal to their resonance frequency and flip to their antiparallel state.

Magnetic field strength

Different nuclei feel the strength of the magnetic field differently. This is because electrons shield nuclei from external magnetic fields. In the previous article we looked at the C=O bond. Oxygen is a lot more electronegative than carbon and so pulls the shared pair of electrons towards itself, leaving the carbon atom electron-deficient. The carbon atom feels the magnetic field much more strongly and has a higher resonance frequency.

Shielding means that the resonance frequency of nuclei of the same element varies depending on the atoms or groups surrounding them. A less well-shielded nucleus feels the strength of the magnetic field much more strongly and has a higher resonance frequency than a more well shielded nucleus.

Carbon-13 NMR C-O bond StudySmarterThe shared pair of electrons in a C-O bond is pulled closer to the oxygen atom because oxygen is more electronegative than carbon. Anna Brewer, StudySmarter Originals

Let’s put all this information together.

  • If we have a sample of a substance that contains nuclei with spin, we can supply it with energy and plot the energy absorbed against chemical shift, on a graph known as a spectrum.
  • Chemical shift is a value related to resonance frequency. We know that different nuclei will have different resonance frequencies depending on the groups surrounding them and so will have different chemical shifts.
  • By comparing chemical shift values to those in a data table, we can work out the structure of the substance.

How does carbon-13 NMR work?

Any old nucleus can't be analysed using NMR spectroscopy. It has to be a nucleus with an odd mass number. Carbon-13 is one such example. A carbon-13 nucleus contains six protons and seven neutrons, giving it a mass number of 13. This means it has spin. We can therefore analyse organic molecules containing carbon using carbon-13 NMR, as we mentioned earlier.

Carbon-13 NMR carbon-13 atom StudySmarterA carbon-13 atom, with six protons, seven neutrons and six electrons. Anna Brewer, StudySmarter Originals

Carbon-13 is a relatively rare isomer. It only makes up about one percent of all carbon atoms. However, we use samples containing large numbers of molecules. It’s extremely likely that at least some of the carbon atoms in the molecule are carbon-13 atoms and will therefore produce a peak on the graph.

To carry out carbon-13 NMR, we follow the following steps.

  • Dissolve the sample in a particular solvent such as CCl4.
  • Add a small amount of a reference compound such as TMS.
  • Analyse the spectrum produced of energy absorbed against chemical shift to work out the environments of different carbon-13 atoms.

Let’s explore some of these terms a little more closely.

TMS

TMS, systematically known as tetramethylsilane, is an organic molecule used as a reference in NMR spectroscopy. It takes the chemical shift value 0. We use it because it is cheap, inert, non-toxic, easy to remove, and gives a clear signal.

Environment

We’ve mentioned this term a couple of times now, but what does it actually mean?

An atom’s environment is simply all the other atoms or groups of atoms surrounding it.

When looking at environments, we don’t just look at the species directly bonded to the atom in question - we look at the molecule as a whole. Atoms are only in the same environment if they have exactly the same atoms, groups and side chains bonded to them. We'll have a go at working out environments in just a minute.

Chemical shift

As explored above, chemical shift is a value related to resonance frequency compared to the reference molecule TMS. We measure it in parts per million, ppm. In carbon-13 spectra, it typically ranges from 0-200.

Each carbon atom produces a specific chemical shift value. The most important thing to note is that chemical shift varies depending on the atom's environment. In other words, depending on the other atoms or groups attached to the carbon atom. Carbon atoms in different environments have different chemical shifts - a less well-shielded atom has a higher chemical shift value than a more shielded atom. In fact, chemical shift values always fall in certain ranges for carbon atoms in certain environments and these show up on spectra.

Spectra

Spectra are graphs produced showing chemical shift plotted against energy absorbed by the molecule.

By looking at spectra, we can infer the structure of our molecule.

Working out environments

Here’s an example of an organic molecule, propanal. How many different carbon-13 environments do you think this molecule has?

Carbon-13 NMR propanal StudySmarterPropanal. Anna Brewer, StudySmarter Originals

The carbon atom on the left, shown below circled in green, is bonded to three hydrogen atoms and a group. The middle carbon, circled in red, is bonded to a methyl group and a group. The carbon on the right, circled in blue, is bonded to an oxygen atom with a double bond, a hydrogen atom and a group. These three carbon atoms are all bonded to different species. We can therefore say that they are in different environments.

Carbon-13 NMR propanal environments StudySmarterThe different carbon environments in propanal. Anna Brewer, StudySmarter Originals

How about this next molecule, propanone?

Carbon-13 NMR propanone StudySmarterPropanone. Anna Brewer, StudySmarter Originals

The carbon in the centre, shown below circled in red, is bonded to two methyl groups. The carbon on the left is bonded to three hydrogen atoms and a group. The carbon on the right is also bonded to three hydrogen atoms and a group. Because they are both bonded to exactly the same atoms and groups, the two carbon atoms are in the same environment. Both are circled in green.

Carbon-13 NMR propanone environments StudySmarterPropanone's different environments. Anna Brewer, StudySmarter Originals

In general, if a molecule is symmetrical, it contains multiple carbon atoms in the same environment.

How do we interpret carbon-13 spectra?

Now we know what carbon-13 NMR spectroscopy is, we can have a go at interpreting a spectrum. To do this, we need a data table. This table shows chemical shift values produced by carbon atoms in certain environments.

Carbon-13 NMR carbon-13 chemical shift data table StudySmarterA typical data table for carbon-13 chemical shift values. Exact ranges can vary from table to table. Anna Brewer, StudySmarter Originals

Let’s look at a typical carbon-13 NMR spectrum. Take this one, produced using propanal.

Carbon-13 NMR propanal NMR spectrum StudySmarterThe NMR spectrum for propanal. Anna Brewer, StudySmarter Originals

There are four distinct peaks present. Remember, the peaks show frequencies of energy absorbed by carbon-13 nuclei as they flip from their parallel to their antiparallel states.

The peak on the right-hand side of the spectrum represents our reference molecule, TMS. We can ignore this when analysing the graph. This leaves us with three other peaks. This means that there are carbon atoms in three different environments.

The left-hand peak has a chemical shift value of about 190 ppm. Looking at our table, we can see that this falls into the range of chemical shift values produced by groups that belong to aldehydes or ketones. We know that propanal has an aldehyde group. So far, so good.

The next peak has a value of around 40 ppm, and the one to the right of that has a value of about 10 ppm. These fall into the range of carbons bonded to or groups.

Let’s go back to our molecule, propanal. We explored it earlier and know that it has carbon atoms in three different environments. Here is the molecule again for you to refer to.

Carbon-13 NMR propanal StudySmarterPropanal. Anna Brewer, StudySmarter Originals

Pulling together what we’ve learnt, we can conclude the following things:

  • The peak at 5 ppm shows the carbon atom circled in green, a methyl group.
  • The peak at 40 ppm shows the carbon atom circled in red. We know this because we can see it is bonded to a methyl group.
  • The peak at 190 ppm shows the carbon atom circled in blue. Again, we know this because it contains a bond.

Let’s now look at another example, the carbon-13 NMR spectrum for but-1-en-3-one.

Carbon-13 NMR but-1-en-3-one NMR spectrum StudySmarterThe carbon-13 spectrum for but-1-en-3-one. Anna Brewer, StudySmarter Originals

We can see the following things.

  • The peak at 190 ppm again shows a double bond.
  • The two peaks at 130 and 140 ppm show carbon atoms at either end of a double bond. Because there are two separate peaks, we know these carbon atoms are in two distinct environments.
  • The peak at around 25 ppm shows a methyl group. In this case, it is bonded to a group. Looking at the table, we can see that it falls into this range of 20-50 ppm quite nicely.

Carbon-13 Analysing But-1-en-3-one NMR StudySmarterBut-1-en-3-one. The carbons circled in green and red give the peaks at 130 and 140 ppm, the carbon circled in yellow gives the peak at 190 ppm and the carbon circled in blue gives the peak at 25 ppm. Anna Brewer, StudySmarter Originals

You might have noticed that the peaks produced are all different heights. In carbon-13 NMR, the height of the peaks has no correlation with the number of carbons in that environment.

Carbon -13 NMR - Key takeaways

  • NMR spectroscopy is an analytical technique used to identify and find the structure of different molecules.

  • Carbon-13 NMR detects the chemical shift value of the carbon isomer carbon-13. Chemical shift is related to magnetic resonance frequency. Carbon-13 nuclei show resonance because they have an odd mass number, meaning they have spin.

  • Different nuclei have different resonance frequencies depending on their environments. Nuclei better shielded by electrons feel the external magnetic field less strongly, and have lower resonance frequencies.

  • Tetramethylsilane, known as TMS, is used as a reference in carbon-13 NMR because it is cheap, inert, non-toxic, easy to remove, and provides a clear signal.

  • We can deduce the different environments of carbon-13 atoms using chemical shift values, which we compare to a data table. We can then use this information to work out our sample’s structure.

Frequently Asked Questions about Carbon -13 NMR

Proton NMR looks at the environments of hydrogen-1 atoms whilst carbon NMR looks at the environments of carbon-13 atoms.

Carbon-13 NMR is an analytical technique used to identify and work out the structure of molecules. It produces graphs called spectra, which contain various peaks that show the different environments of carbon atoms in a molecule.

Carbon-13 is used in NMR because it has an odd mass number. This means that it has a property called spin and behaves a bit like a bar magnet when placed in an external magnetic field. Because of this, carbon-13 atoms show up in NMR spectra.

Carbon-13 atoms have an odd mass number. This means that they have a property called spin. When placed in an external magnetic field, they act like bar magnets and line up with the magnetic field. Supplying them with enough energy causes them to flip in the opposite direction, but this energy varies depending on the other atoms and chemical groups bonded to the carbon atom in a molecule. By plotting a graph of energy against a value called chemical shift, we can identify which groups the carbon atom is bonded to and work out the structure of the molecule. 

Carbon-13 NMR tells you the different environments of carbon atoms and helps you work out the structure of an organic molecule.

Final Carbon -13 NMR Quiz

Question

What does NMR spectroscopy stand for?

Show answer

Answer

Nuclear magnetic resonance spectroscopy.

Show question

Question

Nuclei with spin behave a little like bar magnets. This means that when you put them in an external magnetic field, ________.

Show answer

Answer

Most rotate so that they are parallel to the magnetic field but some rotate so that they are antiparallel.

Show question

Question

Spin-aligned nuclei are _______ to an external magnetic field.


Show answer

Answer

Parallel

Show question

Question

What is magnetic resonance frequency?


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Answer

The energy required to flip a nucleus from its parallel state to its antiparallel state.

Show question

Question

A better shielded nucleus will have a _______ resonance frequency.


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Answer

Higher

Show question

Question

Explain why carbon-13 is suitable for use in NMR.


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Answer

It has an odd nuclear mass because it has six protons and seven neutrons in its nucleus.

Show question

Question

Name the reference molecule used in NMR.


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Answer

Tetramethylsilane, or TMS.

Show question

Question

Give some reasons why TMS is commonly used as a reference molecule in NMR.

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Answer

  • It is cheap.
  • It is inert.
  • It is nontoxic.
  • It is easy to remove.
  • It provides a clear reference point.

Show question

Question

What is chemical shift?


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Answer

A value related to an atom’s magnetic resonance frequency.

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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 range of chemical shift values is commonly seen in carbon-13 NMR?

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Answer

0-200 ppm

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Question

What does each peak in an NMR spectrum show?


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Answer

A different carbon environment.

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Question

The height of a peak in a carbon-13 NMR spectrum correlates to the number of carbons in that environment. True or false?


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Answer

False

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Question

How many different peaks will ethanol produce in a carbon-13 NMR spectrum?


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Answer

Two

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