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• Nuclear magnetic resonance spectroscopy is a powerful analytical techniqueused to characterize organic molecules by identifying carbon-hydrogenframeworks within molecules.

• Two common types of NMR spectroscopy are used to characterize organicstructure: 1H NMR is used to determine the type and number of H atoms in amolecule; 13C NMR is used to determine the type of carbon atoms in themolecule.

• The source of energy in NMR is radio waves which have long wavelengths, andthus low energy and frequency.

• When low-energy radio waves interact with a molecule, they can change thenuclear spins of some elements, including 1H and 13C.

Introduction to NMR Spectroscopy

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All atoms, except those that have an even atomic number and an even mass

number, have a property called spin.

These are some atoms that possess spin.

Nuclei with spin are active in nuclear magnetic resonance (n.m.r.) spectroscopy.

The nucleus of a H atom is a proton. Hydrogen atoms are present in most organic

compounds, so proton n.m.r is a useful way to study them.

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Nuclei with spin behave as if they were tiny bar magnets. They can respond to an

applied magnetic field.

They can align with the magnetic field.

Nuclei aligned with a magnetic field are in a relatively low energy state.

Nuclei with spin can also align against the magnetic field.

Nuclei aligned against the magnetic field are in a higher energy state than nuclei

aligned with the field.

Energy is needed to move a nucleus to the higher energy state. The amount of

energy needed depends upon the chemical environment of the atom.

Let’s look at an n.m.r. spectrum for ethanol, CH CH OH.3 2

Notice that zero is on the right on the horizontal axis.

The horizontal axis represents the chemical shift. This is given the symbol

δ (delta) and it is measured in parts per million (ppm).

This is a simplified low-resolution spectrum for ethanol.

The three hydrogen atoms in the CH group produce this peak.3

The two hydrogen atoms in the CH group produce this peak.2

The hydrogen atom in the OH group produces this peak.

A proton n.m.r. spectrum can give us a lot of useful information about a molecule.

It can tell us how many different chemical environments there are in the

molecule. Hydrogen atoms in different environments are non-equivalent.

It can’t tell us how many hydrogen atoms the molecule contains, but it can tell us

the ratio of the number of hydrogen atoms in each chemical environment.

It can give us information about the nature of the different chemical environments.

It can also give us information about adjacent non-equivalent hydrogen atoms in

different chemical environments.

This is the displayed formula for ethanol. How many different chemical

environments does it have?

There are three equivalent hydrogen atoms in this chemical environment.

There are two equivalent hydrogen atoms in this chemical environment.

There is just one hydrogen atom in this chemical environment.

What peaks would we expect in a low-resolution n.m.r. spectrum of ethanol?

The CH group contains three hydrogen atoms, which form a large peak3

The CH group contains two hydrogen atoms, which form a smaller peak.2

The OH group contains one hydrogen atom, which forms a small peak.

The OH group contains one hydrogen atom, which forms a small peak.

The position of each peak on the n.m.r. spectrum gives us information about the

corresponding chemical environment.

The hydrogen atom in the OH group is attached to an oxygen atom, which is very

electronegative. A hydrogen atom like this is deshielded.

The peak it produces is shifted downfield in the spectrum.

The hydrogen atoms in the CH group are far from the oxygen atom. They are

shielded.3

The peak they produce is upfield in the spectrum, close to 0 ppm.

Chemical shifts are measured relative to the peak produced by a standard

substance, called TMS. By definition, δ is 0 for TMS.

This is TMS, tetramethylsilane. Why is it chosen for the reference peak?

These are some reasons why TMS is chosen.

It produces a single, intense peak.

Its n.m.r. peak is upfield of most other peaks.

It will not react with the sample material.

End of section one, return to menu by clicking "Reset".

How many peaks should appear in the proton n.m.r. spectrum of methoxyethane?

Methoxyethane has a methyl group here.

It also has a methyl group here, but it is in a different chemical environment.

It has a methylene group, CH .3

There are three different chemical environments, so there are three peaks in the

spectrum.

If we count up each equivalent hydrogen atom in the three chemical environments,

we expect peak areas in the ratio 3:2:3.

Propan-1-ol is an isomer of methoxyethane. How many peaks should appear in its

proton n.m.r. spectrum?

Propan-1-ol has a hydrogen atom in its hydroxyl group.

It has two methylene groups, CH , but they are in different chemical environments.3

It has a methyl group here.

There are four different chemical environments, so there are four peaks in the

spectrum.

If we count up each equivalent hydrogen atom in the four chemical environments,

we expect peak areas in the ratio 1:2:2:3.

Propan-2-ol is an isomer of methoxyethane and propan-1-ol. How many peaks

should appear in its proton n.m.r. spectrum?

Propan-2-ol has a methyl group here.

It also has a methyl group here.

Both methyl groups are in the same chemical environment. Their hydrogen atoms

are all equivalent and will produce a single peak in the n.m.r. spectrum.

Propan-2-ol has a hydrogen atom in the hydroxyl group.

It has another hydrogen atom here, but this is in a different chemical environment

to the one in the hydroxyl group.

There are three different chemical environments, so there are three peaks in the

spectrum.

If we count up each equivalent hydrogen atom in the three chemical environments,

we expect peak areas in the ratio 6:1:1.

End of section two, return to menu by clicking "Reset".

Some of the peaks are split into clusters of smaller peaks in high-resolution

proton n.m.r. spectra, because of spin-spin coupling.

This happens if non-equivalent hydrogen atoms are adjacent to each other. No

splitting occurs otherwise.

Without spin-spin coupling a single peak forms, called a singlet, just as in a low-

resolution spectrum.

If hydrogen atoms in one chemical environment are adjacent to one hydrogen atom

in another chemical environment, the peak they produce will split into two.

This is called a doublet, with a ratio of peak areas of 1:1.

If hydrogen atoms in one chemical environment are adjacent to two hydrogen

atoms in another chemical environment, the peak they produce will split into three.

This is called a triplet, with a ratio of peak areas of 1:2:1.

If hydrogen atoms in one chemical environment are adjacent to three hydrogen

atoms in another chemical environment, the peak they produce will split into four.

This is called a quartet, with a ratio of peak areas of 1:3:3:1.

This is ethyl ethanoate.

Its low-resolution proton n.m.r. spectrum would show three peaks in the ratio 3:2:3.

The hydrogen atoms in this methyl group have no adjacent non-equivalent

hydrogen atoms.

In a high-resolution spectrum, they would still produce a single peak.

The hydrogen atoms in this methyl group have two adjacent non-equivalent

hydrogen atoms.

In a high-resolution spectrum, they would produce a triplet of peaks, with a ratio of

1:2:1.

The hydrogen atoms in this methylene group have three adjacent non-equivalent

hydrogen atoms.

In a high-resolution spectrum, they would produce a quartet of peaks, with a ratio

of 1:3:3:1.

This is a simplified high-resolution proton n.m.r. spectrum of ethyl ethanoate.

These are the equivalent hydrogen atoms in the different chemical environments

and the peaks they produce.

These are the equivalent hydrogen atoms in the different chemical environments

and the peaks they produce.

These are the equivalent hydrogen atoms in the different chemical environments

and the peaks they produce.