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.

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.

• 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.

Bar 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.

The 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.

A 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?

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

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?

Propanal. Anna Brewer, StudySmarter Originals

The carbon atom on the left, shown below circled in green, is bonded to three hydrogen atoms and a $-{\mathrm{CH}}_2\mathrm{CHO}$ group. The middle carbon, circled in red, is bonded to a $-{\mathrm{CH}}_3$ methyl group and a $\mathrm{CHO}$ group. The carbon on the right, circled in blue, is bonded to an oxygen atom with a double bond, a hydrogen atom and a ${\mathrm{CH}}_2{\mathrm{CH}}_3$ group. These three carbon atoms are all bonded to different species. We can therefore say that they are in different environments.

The different carbon environments in propanal. Anna Brewer, StudySmarter Originals

How about this next molecule, propanone?

Propanone. 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 ${\mathrm{COCH}}_3$ group. The carbon on the right is also bonded to three hydrogen atoms and a ${\mathrm{COCH}}_3$ 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.

Propanone's different environments. Anna Brewer, StudySmarter Originals

Carbon 13 NMR interpretation

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.

A 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.

The 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 $-\mathrm{RCO}-$ 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 $-{\mathrm{CH}}_3$ or $-{\mathrm{CH}}_2-$ 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.

Propanal. 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 $\mathrm{C}=\mathrm{O}$ bond.

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

The 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 $\mathrm{C}=\mathrm{O}$ double bond.
• The two peaks at 130 and 140 ppm show carbon atoms at either end of a $\mathrm{C}=\mathrm{C}$ 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 $\mathrm{C}=\mathrm{O}$ group. Looking at the table, we can see that it falls into this range of 20-50 ppm quite nicely.

But-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