Proton
1
( H)
NMR Spectroscopy
Proton nuclear magnetic resonance
Click on the link on the icon below to view a
video introducing NMR spectroscopy.
How are spectra created?
The compass
needle aligns
with the Earth’s
magnetic field
and points
north.
Nuclei behave
like tiny magnets
– similar to a
compass needle.
The needle will
swing back as soon
as the energy is
removed.
When energy is put in,
the needle can be
made to align in a
direction opposite to
the Earth’s magnetic
field.
• When an external magnetic field is applied, hydrogen
nuclei can align with the external field or against it.
Nucleus aligned
opposed to
magnetic field –
high-energy state.
External
magnetic
field
∆E radio waves
Nucleus aligned
with magnetic field
– low-energy state.
• As nuclei relax back to the low-energy alignment, energy
in the radio wave frequency is released. This energy is
detected and recorded as peaks on a spectrum.
Proton environments
• In a compound, the protons are bonded to other
atoms and so there are more electrons in the
region of the protons.
• These electrons affect the external magnetic field
experienced by the proton.
• The energy gap between high- and low-energy
states will be slightly different.
• The frequency of radiation emitted as the nuclei
relax back to the low-energy state will also be
different.
• Protons emitting radiation of the same frequency
are said to be in the same proton environment.
Determining proton environments
Example: How many proton environments are
there in ethanol?
H
H
H
C
C
The OH proton
is in a third
environment.
O
H
The three CH3
protons are in one
environment.
H
H
The two CH2
protons are in a
second
environment.
Preparing a sample
• To obtain the 1H NMR spectrum of a sample it is
usually necessary to dissolve the sample in a
solvent.
• Solvents must not contain protons that will
interfere with the sample being measured.
• A solvent must:
– contain no hydrogen atoms, eg tetrachloromethane,
CCl4
or
– have the hydrogen atoms replaced with deuterium
(2H), eg CDCl3 or CD3OD.
Explaining spectra
The scale runs from right
to left and is called the
chemical shift. It is
measured in parts per
million (ppm). Peaks to the
left of TMS peak are said to
be downfield of TMS.
TMS (tetramethylsilane) is
added to the solvent and
provides a reference peak.
The protons in TMS are
assigned the value
0 ppm and the rest of the
spectrum is calibrated
relative to this.
Interpreting low-resolution
1H NMR spectra
Points to note:
• Each peak in a low-resolution spectrum
represents one proton environment.
• The type of proton environment can be identified
by looking up the chemical shift in a correlation
table (data book).
• The area under the peak relates to the number of
protons in the environment.
Example 1
Methyl ethanoate
H
H
C
H
O
O
H
C
C
H
H
The area under the
peak gives information
about how many
protons are in each
environment.
The area is given as
an integration curve
and the height of the
curve can be
measured.
By comparing the
heights of the
integration curves we
can determine the
ratio of protons in
each environment.
In this spectrum the
height ratio is 1:1 so
there is an equal
number of protons
in each
environment.
Solving problems
• It is helpful to lay out your interpretations in a
table.
Peak shift (ppm)
Integration height (mm)
Ratio
2.1
30
1
3.8
30
1
High resolution
High-resolution spectra are run using a higher radio
frequency and the peaks have more detail.
Compare the spectra below for methyl propanoate.
O
H
H
C
C
C
H
H
O
H H
C
H
H
Low-resolution NMR for
methyl propanoate.
High-resolution NMR for
methyl propanoate.
This spectrum is for pentan-3-one. The peaks
show that there are two proton environments.
Can you assign these peaks to the structure?
O
H
H
C
C
H
H
C
CH3
C
H H
H
H
C
H
H
CH2
When the spectrum is expanded it can be seen
that each peak is made up of a number of peaks.
These are called multiplets.
Quartet:
four peaks in
the group.
Other multiplets include
singlets and doublets.
Triplet:
three peaks in
the group.
n + 1 rule
• The number of peaks in a multiplet can give
additional information about the structure.
• The splitting of peaks is caused by the
neighbouring carbon’s hydrogen atoms.
• Protons in the same environment are said to be
equivalent and as such behave as one proton.
• This follows the n + 1 rule.
– n is the number of hydrogen atoms attached to the
next-door carbon
– n + 1 is how many peaks will be seen in the cluster.
CH3CH2COCH2CH3
CH3
CH2
Split by 3
protons on nextdoor carbon so
n = 3,
n + 1 = 4 peaks.
Split by 2
protons on nextdoor carbon, so
n = 2,
n + 1 = 3 peaks.
Example 2 – Assign the peaks and suggest a structure
Molecular formula C3H7Br
H
CH2
A triplet due
to CH2
group
adjacent.
H H
H
C C
C
H H
H
This is not a simple
quartet. There are
extra splittings due
to CH3 and CH2
neighbouring
groups.
Br
CH3
CH2
A triplet due to
CH2 group
adjacent.