Imagine you are a chemist. You spend your days surrounded by test tubes, flasks and beakers, mixing substances together in a myriad of different chemical reactions. One day, after hours spent poring over lab notes and functional groups, you think you’ve had a breakthrough. You combine 50 ${\mathrm{cm}}^3$ of this with 10 ${\mathrm{cm}}^3$ of that, add two drops of an acid catalyst and heat in a water bath for 90 seconds. The mixture bubbles and froths; a purple precipitate forms. You think you’ve done it - you’ve created a new compound. But how can you be sure that it is actually new and previously unknown to scientists? Well, you can use a technique known as NMR spectroscopy. Understanding NMR spectroscopy is key to this process.

NMR spectroscopy stands for nuclear magnetic resonance spectroscopy. It has a wide range of applications, from analysing proteins to MRI scans in hospitals and identifying new molecules.

• We’ll look at how NMR works and explore ideas such as spin and resonance frequency.
• We'll then introduce two specific types of NMR, known as carbon-13 NMR and proton NMR.

What is NMR?

To understand what NMR actually is, let’s look at its name a little more closely: nuclear magnetic resonance spectroscopy. 'Nuclear' refers to nuclei, as you can probably guess. Remember that a nucleus is a dense mass found in the centre of an atom, containing protons and neutrons. 'Resonance' is a type of energy emitted by molecules. If we put that all together, we can work out that NMR measures the resonance energy of nuclei using magnetic fields.

How does NMR work?

Let’s take a look at how exactly NMR works. To do this, we need to investigate several different ideas.

• Nuclear spin.
• Parallel and antiparallel states.
• Resonance.

Nuclear spin

Cast your mind back to the start of your days learning chemistry, to the model of the atom. You know that an atom contains protons and neutrons in a central nucleus, surrounded by orbiting electrons. Protons and neutrons are collectively called nucleons.

Fig. 1 - An atom. The pink and yellow particles represent protons and neutrons, collectively known as nucleons. The green particles represent electrons

You might also remember that electrons have a property called spin. This takes two different directions: up and down. When electrons pair up in orbitals, one electron has an up spin and the other has a down spin.

Nucleons have spin too. Each proton or neutron has a spin of ½. If there is an even number of either protons or neutrons, their spins cancel out and there is no spin at all. But if there is an odd number, we are left with a net spin of ½. If there is an odd number of both protons and neutrons, we are left with a net spin of ½ + ½ = 1.

Fig. 2 - A table showing the net spins of different atoms, using hydrogen, helium and nitrogen as examples

Fig. 3 - Helium, shown on the left, has an even number of both protons and neutrons. It has no overall spin. Hydrogen, right, has an odd number of protons. It has an overall spin of 1/2

Parallel and antiparallel states

So, nuclei can have spin. That’s all well and good, but what does spin actually do? Well, spin can be influenced by magnetic waves and makes nuclei behave a little bit like bar magnets. If you put a magnet in an external magnetic field, it tends to line up with the magnetic waves surrounding it. Imagine a compass needle, for example. If you let it swing naturally, it follows the Earth’s magnetic field and points to the north pole. You can take hold of it and twist it so that it points to the south pole, but if you let go, it will quickly spin back around. Pointing towards the north pole is a more stable state than pointing towards the south pole.

If we place our nuclei with spin in an external magnetic field much like the Earth’s, they can have two states as well.

• If a nucleus’s spin is parallel to the external magnetic field, we say that it is spin-aligned.
• If their spin goes against the magnetic field, we say that it is spin-opposed.

Fig. 4 - Nuclei with spin that are placed in an external magnetic field can take two states: parallel (spin-aligned, left), or antiparallel (spin-opposed, right)

Resonance

The parallel (spin-aligned) state is much more stable than the antiparallel (spin-opposed) state. It therefore has lower energy. We call the energy difference between the two states ΔE.

We normally find most nuclei in the parallel state because it is more energetically stable. However, we can force nuclei to flip from parallel to antiparallel by supplying them with a certain amount of energy. This energy is equal to ΔE and varies according to the strength of the magnetic field surrounding the nuclei. We call this process of switching from the parallel to antiparallel state magnetic resonance. The energy needed to switch states is known as magnetic resonance frequency.

What determines magnetic resonance?

The magnetic field a nucleus with spin ‘experiences’ varies depending on which other atoms or groups surround it.

For example, carbon-13 has a nucleus with spin. If you put it in a magnetic field, most of its nuclei will fall into the parallel state. But if you supply just the right amount of energy, some will flip to the antiparallel state. This is called magnetic resonance. As we mentioned above, its resonance frequency is equal to ΔE, which varies according to the magnetic field surrounding the nuclei. This means that resonance can vary if the carbon-13 nuclei all experience the magnetic field differently. But why would that happen?

Well, electrons actually manage to shield or block out external magnetic fields. Imagine it is a windy day and you are surrounded by other people. If they are clustered closely around you, they’ll block out the breeze - but if they step away from you, you’ll get blown about by gusts of wind. In this metaphor, the electrons act like the other people shielding you from the wind, which represents the external magnetic field.

Take a carbon atom bonded to an oxygen atom. We know that oxygen is a lot more electronegative than carbon. It attracts the shared pair of electrons towards itself more strongly, tugging them away from the carbon atom. This carbon atom therefore experiences the magnetic field a lot more strongly, and so has a higher resonance frequency.

Fig. 5 - The shared pair of electrons in the C-O bond are pulled closer to the oxygen atom because oxygen is more electronegative than carbon

On the other hand, some groups of molecules are electron-releasing. These include methyl groups. If a carbon atom is bonded to three methyl groups, for example, it will have a lower resonance frequency than a carbon bonded to no methyl groups. This is because the electrons released by the methyl groups shield the nucleus from the magnetic field. They act like the other people, shielding the nucleus from the wind, and so the nucleus experiences the force of the field less strongly.

In summary:

• Each carbon atom needs a different amount of energy for it to flip to its antiparallel state, depending on the different atoms and groups attached to it.
• Carbon atoms attached to more electronegative groups have a higher resonance frequency than those attached to electron releasing groups.

What is the importance of resonance?

We can use computers to detect the resonance frequencies of different parts of a molecule and match them up to known structures. We supply energy to the sample of our molecule in the form of radio waves, and a detector detects the frequencies absorbed by the molecule. Because atoms attached to different groups have different resonance frequencies, we can then compare these frequencies to those of known structures in a database and work out what exactly the molecule looks like.

For example, the C=O bond found in aldehydes and ketones has a shift value of 190-220 ppm. We’ll explore what exactly that means later, but you should know that if we analyse a molecule using NMR spectroscopy and the results also include that value, there will be a C=O present.

Fig. 6 - A table showing chemical shift values for carbon atoms in different environments

How do we carry out NMR?

To carry out NMR spectroscopy on an organic sample, we follow the following steps.

• Dissolve the sample in a suitable solvent.
• Add a small amount of tetramethylsilane.
• Apply radio waves so that some of the nuclei in the sample flip to their higher energy antiparallel state.
• Plot a graph showing the energy of the radio waves absorbed against chemical shift.

What is chemical shift?

We give chemical shift the symbol δ and measure it in parts per million, or ppm. It is actually to do with the differences in resonance frequency between the nuclei in the sample and a reference molecule called tetramethylsilane. Also known as TMS, it gives a peak at the value 0 on the graph, and all other molecules take values greater than this. The more a nucleus is shielded from the magnetic field by electrons, the lower its resonance frequency and chemical shift value, so the nearer it’ll be to TMS in the spectrum.

Fig. 7 - An NMR spectrum for propanone, showing energy absorbed against chemical shift

What is tetramethylsilane?

Tetramethylsilane, or TMS, is a molecule that we use as a reference in NMR spectroscopy. As explained above, it has a chemical shift value of 0. All other nuclei have chemical shift values higher than this. TMS consists of a silicon atom bonded to four methyl groups, giving it the formula $\mathrm{Si}{\left({\mathrm{CH}}_3\right)}_4.$

Fig. 8 - Tetramethylsilane, or TMS

Why do we use TMS?

TMS is useful as a reference molecule for the following reasons.

• It has four carbon atoms in identical environments. We’ll explore the significance of this in the article Carbon-13 NMR, but you should know that this means it gives a strong, clear signal.
• It has a low boiling point and so is easy to remove from the sample after analysis is complete.
• It is inert and nontoxic. You don’t have to worry about it reacting with anything in your sample.

Types of NMR spectroscopy

We learnt that any atom with an odd mass number has nuclear spin and so can be used in NMR spectroscopy. However, this isn’t always very helpful when working out the structure of molecules. For example, what is the point of looking for peaks produced by ${}_{}{}^{35}\mathrm{Cl}$ atoms if your molecule only contains one chlorine atom? You might find out information about the groups bonded to that particular atom, but not the rest of the molecule! There are other atoms which are much more common in organic molecules - carbon and hydrogen.

Carbon-13 NMR

Carbon-13 is an isotope of carbon.

The most common isotope of carbon is carbon-12. It has six protons and six neutrons in its nucleus. However, a small proportion - about one percent - of all carbon atoms are carbon-13. They also have six protons in their nucleus but contain seven neutrons, giving them an atomic mass of 13.

Carbon is an important element in organic chemistry. It makes up the backbone of many different molecules. Its abundance makes it perfect for analysis using NMR, as it can give us information about many different parts of the target molecule.

But if only one percent of all carbon atoms are carbon-13, how do they produce peaks on the graph? Well, we know that carbon-12 atoms have an even mass number and so don’t have a nuclear spin. They won’t absorb any of the radio waves as they don’t have an antiparallel state to flip to. That means that any of the peaks on a carbon-13 NMR spectrum must be caused by carbon-13 atoms. Even though only a tiny proportion of all carbon atoms are carbon-13, there are so many molecules of the target compound in a sample that there will always be at least some carbon-13 atoms present. The detector picks up the peaks they produce, no matter how small.

To carry out carbon-13 NMR, we typically use the solvent ${\mathrm{CCl}}_4$. The peaks produced take values ranging from 0 to about 200. We’ll look at carbon-13 NMR in more detail in Carbon-13 NMR.

Proton NMR

There is another atom we can use in NMR that is even more common than carbon-13. ${}_{}{}^1\mathrm{H}$ is the main isotope of hydrogen, and its nucleus is actually just a proton. These nuclei give much smaller chemical shifts than carbon-13 nuclei, ranging typically from 0 to 10. Proton NMR spectroscopy, also known as hydrogen-1 spectroscopy, is a lot more useful than carbon-13 spectroscopy because the peaks produced are proportional to the number of nuclei in that environment. (For more detail, look at Hydrogen-1 NMR).

Imagine that you have a methyl group, ${\mathrm{CH}}_3$. There are three hydrogen atoms attached to the central carbon atom. Now imagine you have a $\mathrm{CH}$ group in the middle of a molecule. It has just one hydrogen atom attached. The three hydrogen atoms that are a part of the methyl group are all in the same environment and will produce a peak three times as high as the single hydrogen atom joined to the $\mathrm{CH}$ group.

Fig. 9 - A proton NMR spectrum