Select your language

Suggested languages for you:
Log In Start studying!
StudySmarter - The all-in-one study app.
4.8 • +11k Ratings
More than 3 Million Downloads
Free
|
|

All-in-one learning app

  • Flashcards
  • NotesNotes
  • ExplanationsExplanations
  • Study Planner
  • Textbook solutions
Start studying

NMR Spectroscopy

Save Save
Print Print
Edit Edit
Sign up to use all features for free. Sign up now
Chemistry

Let us say you have isolated a new protein previously unknown to science. You know that it contains, say, hydroxyl groups and sulfur groups, but you don’t know where in the molecule they are, or how the protein chain folds up into its three-dimensional shape. Using NMR spectroscopy, you’ll be able to determine the location of these groups and the structure of the protein.

  • In this article, we will introduce the topic of NMR spectroscopy.
  • We will start by exploring what it is before learning how you interpret spectra.
  • We'll then look at the different types of NMR spectroscopy.
  • We'll finish by discussing NMR spectroscopy's uses.

How does NMR spectroscopy work?

NMR spectroscopy relies on some tricky concepts, but the process itself is relatively simple. You follow these steps:

  • Dissolve your sample in a suitable solvent, such as .
  • Add a small amount of a reference molecule, such as TMS.
  • Place the sample in an external magnetic field.
  • Fire radio waves at the sample.

NMR spectroscopy, short for nuclear magnetic resonance spectroscopy, is an analytical technique we use primarily to find out the structure of molecules. It is based on the behaviour of certain nuclei in an external magnetic field.

Nuclei in the sample absorb and emit radio waves according to the other atoms or groups bonded to them. These waves are detected by a detector. The detector produces a spectrum showing the energy absorbed against a property called chemical shift.

What is chemical shift?

You’ll learn more about the science behind NMR spectroscopy, including chemical shift, in Understanding NMR. However, we’ll take a quick look at it now to help you understand how NMR spectroscopy works.

As we mentioned, NMR spectra show the chemical shift of nuclei. This is a property related to something called magnetic resonance frequency. Chemical shift is measured in parts per million, or ppm.

To summarise briefly, certain nuclei act a little weirdly when placed in an external magnetic field. They take one of two states: parallel, known as spin-aligned; or antiparallel, known as spin-opposed. If we supply them with enough energy they can flip from their parallel to their antiparallel state. This energy is called their magnetic resonance frequency.

Magnetic resonance frequency is the energy needed for a nucleus to flip from its parallel to its antiparallel state in an external magnetic field.

Magnetic resonance frequency varies depending on the environment of an atom.

An atom’s environment is all the different chemical groups attached to it.

Identical nuclei from the same element can have different magnetic resonance frequencies and different chemical shift values if they are bonded to different groups, because they are found in different environments. This is the fundamental concept behind NMR spectroscopy.

Only nuclei with odd mass numbers can be used in NMR spectroscopy. This is because they have a property called spin. You'll learn more about spin in Understanding NMR.

NMR Spectroscopy NMR spectrometer StudySmarterA high-power NMR spectrometer. commons.wikimedia.org

Interpreting NMR spectra

As we mentioned above, NMR spectroscopy produces graphs called spectra, plotting energy absorbed by the sample against chemical shift. The graphs show a number of different peaks. Nuclei from identical atoms produce peaks at different chemical shift values depending on the other atoms or groups of atoms bonded to them. Notice the peak shown at 0 ppm. This is given by TMS, a reference molecule.

Tetramethylsilane, also known as TMS, is a molecule commonly used as a reference point in NMR spectroscopy.

NMR Spectroscopy NMR spectrum example StudySmarterAn example of an NMR spectrum, showing distinct peaks. Note the peak given at 0 ppm by TMS, the reference molecule. Anna Brewer, StudySmarter Originals

There are two important things to know:

  • Environments with certain functional groups produce chemical shift peaks that fall within a particular range.
  • Unique environments give unique chemical shift peaks.

How does this help us? Well, if you have two clear peaks on your spectrum, your sample must contain nuclei in two different environments. You can then compare the chemical shift value of the peaks to values in a data book, which will tell you what sort of environment the nuclei are in, and the different functional groups that are attached to them. This helps you work out the structure of the molecule in your sample.

Let’s say you have the following spectrum for an unknown molecule.

NMR Spectroscopy NMR spectrum example StudySmarterA carbon-13 NMR spectrum. Anna Brewer, StudySmarter Originals

You can see peaks at around 58 ppm, 18 ppm and 9 ppm. Let’s compare these values to a data table.

NMR spectroscopy NMR data chemical shift table StudySmarterA typical data table for carbon-13 NMR. Anna Brewer, StudySmarter Originals

The peak at 58 ppm matches the values for an group, which range from 50-90 ppm. We can therefore infer that this molecule contains that particular group. Similarly, we can see that the peak at 18 ppm falls into the range for an group, and the peak at9 ppm falls into the range for an group.

What molecule do you know that contains just these particular groups? Let’s put them together:

NMR spectroscopy interpreting spectra propanol StudySmarterOur mystery molecule is propan-1-ol. Anna Brewer, StudySmarter Originals

The molecule is propan-1-ol.

In summary, by comparing chemical shift values to ranges in a data book, we can infer the different groups within a molecule and work out its overall structure.

Different types of NMR spectroscopy

Not all nuclei can be used in NMR spectroscopy. Most aren’t influenced by an external magnetic field and can’t be detected. Two types of nuclei that do produce results in NMR spectroscopy are carbon-13 nuclei and hydrogen-1 nuclei.

Remember that carbon-13 shows that we have an isotope of carbon with a mass number of 13. Mass number is the combined number of protons and neutrons in an atom. Carbon has an atomic number of 6, meaning it has six protons, and so carbon-13 atoms must have 13 - 6 = 7 neutrons.

Both types of spectroscopy follow the general technique described above and detect the chemical shift of carbon-13 nuclei and hydrogen-1 nuclei respectively. However, the chemical shift peaks in hydrogen-1 spectra fall within a much smaller range.

Hydrogen-1 NMR spectroscopy is also known as proton spectroscopy. A hydrogen-1 nucleus doesn’t have any neutrons or electrons - it is just a proton.

NMR spectroscopy hydrogen-1 atom StudySmarterA hydrogen-1 atom. If you take away the electron you are left with only the nucleus, which contains just one proton. Anna Brewer, StudySmarter Originals

Hydrogen-1 NMR spectroscopy does have some advantages over carbon-13 spectroscopy:

  • Most hydrogen atoms are the isotope hydrogen-1, whereas only about 10 percent of carbon atoms are the isotope carbon-13. This means that hydrogen-1 spectra give clearer, more distinct results.
  • The size of peaks in hydrogen-1 spectra is proportional to the number of hydrogen-1 nuclei in that particular environment, which isn’t the case for carbon-13 NMR peaks. This is shown using an integration trace.
  • Hydrogen-1 peaks show something called spin-spin coupling. This is where they split into smaller peaks depending on how many hydrogen atoms are in adjacent environments, and it gives us further information about the molecule's structure.

NMR spectroscopy hydrogen-1 spectrum spin-spin coupling StudySmarterThis hydrogen-1 spectrum for ethanol shows spin-spin coupling. Some peaks have split into multiple smaller peaks. Anna Brewer, StudySmarter Originals

You’ll learn more about carbon-13 and hydrogen-1 NMR in Carbon -13 NMR and Hydrogen -1 NMR respectively.

Using NMR spectroscopy

NMR spectroscopy has many applications in modern science. As we’ve explored, its primary function is analysing molecule structure and shape. However, it is also used for the following purposes:

  • Determining protein folding.
  • Drug screening and design.
  • Finding out how molecules interact in chemical reactions.
  • Determining the proportion of solids and liquids in lipids.

Pros and cons of NMR spectroscopy

NMR Spectroscopy has both pros and cons. Let's consider them below.

Benefits

  • Its spectra are unique, well-resolved, and generally predictable for smaller molecules.
  • It produces distinguishable signals for different functional groups.
  • It can even be used to distinguish the same functional group in different environments.

Limitations

  • It requires large sample sizes of between 2-50mg.
  • Machines are costly to purchase and run. Less expensive machines are available, but these give lower resolution spectra, in which the chemical shift peaks overlap and blur together.
  • It can generally only be used for soluble substances, as analysing solids requires a dedicated machine and gives lower resolution spectra.
  • It is a slow process, and can’t be used for fast reactions.

NMR Spectroscopy - Key takeaways

  • NMR spectroscopy is an analytical technique used to determine molecule shape and structure.
  • To carry out NMR spectroscopy, you dissolve your sample in a suitable solvent, add a small amount of the reference molecule TMS, place it in an external magnetic field and fire radio waves at the sample. A detector produces a graph of energy absorbed against a property called chemical shift.
  • A nucleus’s environment is all the different atoms and groups of atoms surrounding it. Different nuclei have different chemical shift values depending on their environment.
  • You can compare chemical shift values to ranges in a data book to find the functional groups present in a molecule.
  • The two most common types of NMR spectroscopy are carbon-13 NMR and hydrogen-1 NMR. Hydrogen-1 NMR spectroscopy provides more information about the environments of the nuclei it detects through integration traces and spin-spin coupling.
  • NMR spectroscopy produces unique, well-resolved spectra. It can be used to distinguish between different functional groups and environments in a molecule. However, it is expensive and slow.

NMR Spectroscopy

NMR spectroscopy is an analytical technique used to determine molecule shape and structure. It is based on the behaviour of certain nuclei in an external magnetic field.

You read NMR spectra by comparing peaks, which show chemical shift, to values in a data book. These tell you the functional groups present in your sample molecule.

NMR spectroscopy measures the chemical shift of nuclei. This is a property related to magnetic resonance frequency, the energy needed to flip a nucleus from its antiparallel state to its parallel state.

NMR spectroscopy is used to find molecule structure and shape, determine how proteins fold, identify molecules, and help design drugs.

Chemical shift is a property related to magnetic resonance frequency, which is the energy needed to flip a nucleus from its antiparallel state to its parallel state. Nuclei from atoms of the same element have different chemical shift values depending on the other chemical groups attached to them.

Final NMR Spectroscopy Quiz

Question

What does NMR spectroscopy stand for?

Show answer

Answer

Nuclear magnetic resonance spectroscopy.

Show question

Question

Give three uses of NMR spectroscopy.

Show answer

Answer

  • Analysing proteins
  • Medical MRI scans 
  • Determining molecule structure

Show question

Question

What is the collective term for protons and neutrons?


Show answer

Answer

Nucleons

Show question

Question

 Which atoms have net spins of ½?


Show answer

Answer

Those with odd mass numbers.

Show question

Question

If you place nuclei with spin in an external magnetic field, they can take two states: _____ and ______.

Show answer

Answer

Parallel and antiparallel

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

A nucleus’s parallel state is higher energy than its antiparallel state. True or false? 

Show answer

Answer

False

Show question

Question

What is resonance frequency?


Show answer

Answer

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

Show question

Question

Predict which carbon atom has the highest resonance frequency. Justify your answer.


Show answer

Answer

C=O

Show question

Question

An atom that is more shielded from the external magnetic field has a _______ resonance frequency.


Show answer

Answer

Lower

Show question

Question

What is chemical shift?


Show answer

Answer

 A quantity related to resonance frequency.

Show question

Question

What is the symbol for chemical shift?


Show answer

Answer

δ

Show question

Question

What are the units for chemical shift?


Show answer

Answer

ppm

Show question

Question

Name the reference compound used in NMR spectroscopy.


Show answer

Answer

Tetramethylsilane, or TMS.

Show question

Question

Give some reasons why we use TMS as a reference in NMR spectroscopy.


Show answer

Answer

  • It is cheap.
  • It is inert.
  • It is easy to remove.
  • It produces a clear peak.

Show question

Question

State the numbers of each type of subatomic particle in carbon-13.


Show answer

Answer

Six protons, six electrons, seven neutrons.

Show question

Question

Compare proton NMR to carbon-13 NMR.


Show answer

Answer

  • Both are types of NMR spectroscopy used to analyse organic molecules. 
  • Both produce graphs of chemical shift against energy absorbed and use TMS as a reference. 
  • Carbon-13 spectroscopy uses carbon-13 atoms and produces chemical shift values ranging from 0-200.
  • Proton spectroscopy uses protons, produces chemical shift values between 0-10, and produces peaks proportional to the number of hydrogen nuclei in that environment.

Show question

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?


Show answer

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.


Show answer

Answer

Higher

Show question

Question

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


Show answer

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.


Show answer

Answer

Tetramethylsilane, or TMS.

Show question

Question

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

Show answer

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?


Show answer

Answer

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

Show question

Question

What are the units for chemical shift?

Show answer

Answer

Parts per million, ppm

Show question

Question

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

Show answer

Answer

0-200 ppm

Show question

Question

What does each peak in an NMR spectrum show?


Show answer

Answer

A different carbon environment.

Show question

Question

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


Show answer

Answer

False

Show question

Question

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


Show answer

Answer

Two

Show question

Question

Give another name for hydrogen-1 NMR spectroscopy.

Show answer

Answer

Proton NMR spectroscopy.

Show question

Question

What property must atoms have to show up in NMR?

Show answer

Answer

Spin

Show question

Question

 Nuclei with spin must have __________________.


Show answer

Answer

An even mass number.

Show question

Question

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


Show answer

Answer

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

Show question

Question

What sort of energy is used in NMR?


Show answer

Answer

Radio waves.

Show question

Question

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


Show answer

Answer

TMS

Show question

Question

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


Show answer

Answer

Parallel, antiparallel.

Show question

Question

What is magnetic resonance frequency?


Show answer

Answer

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

Show question

Question

What are the units for chemical shift?


Show answer

Answer

Parts per million, ppm.

Show question

Question

What does an integration trace show?


Show answer

Answer

The number of hydrogen atoms in each environment.

Show question

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?


Show answer

Answer

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

Show question

Question

Give another name for spin-spin coupling.


Show answer

Answer

Spin-spin splitting.

Show question

Question

What is the n+1 rule?


Show answer

Answer

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

Show question

Question

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


Show answer

Answer

Quartet.

Show question

Question

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

Show answer

Answer

There are three hydrogen atoms in adjacent environments.

Show question

Question

What does NMR spectroscopy stand for?

Show answer

Answer

Nuclear magnetic resonance spectroscopy.

Show question

Question

What does an NMR spectrum show?


Show answer

Answer

Energy absorbed by the sample against chemical shift.

Show question

Question

What is chemical shift?


Show answer

Answer

A property related to magnetic resonance frequency, which is the energy required to flip a nucleus from its parallel to its antiparallel state.

Show question

Question

Give the units for chemical shift.


Show answer

Answer

Parts per million, ppm.

Show question

60%

of the users don't pass the NMR Spectroscopy quiz! Will you pass the quiz?

Start Quiz

Discover the right content for your subjects

No need to cheat if you have everything you need to succeed! Packed into one app!

Study Plan

Be perfectly prepared on time with an individual plan.

Quizzes

Test your knowledge with gamified quizzes.

Flashcards

Create and find flashcards in record time.

Notes

Create beautiful notes faster than ever before.

Study Sets

Have all your study materials in one place.

Documents

Upload unlimited documents and save them online.

Study Analytics

Identify your study strength and weaknesses.

Weekly Goals

Set individual study goals and earn points reaching them.

Smart Reminders

Stop procrastinating with our study reminders.

Rewards

Earn points, unlock badges and level up while studying.

Magic Marker

Create flashcards in notes completely automatically.

Smart Formatting

Create the most beautiful study materials using our templates.

Sign up to highlight and take notes. It’s 100% free.