Bridging between
A-level
and
Undergraduate
Proton NMR
Created by Chris Phillips
whilst a final year MChem
student in the Department
of Chemistry at the
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Magnetic
resonance
Spectra
Splitting
patterns
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Shielding
Interpreting
spectra quiz
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Magnetic
resonance
•
•
•
•
•
One of the main features of NMR is the use of magnetic fields
Moving charges produce their own magnetic fields. This
means nuclei have a field.
Nuclei align in a magnetic field, in a similar way to bar
magnets aligning when next to other magnets.
You would have to apply force to move the magnet against
the fields of the other magnets, making the alignment shown
below, the most stable alignment
Without the other field, the magnet can lie in any direction
S
N
External
magnetic field
S
N
•
•
•
•
A nucleus’ alignment in a field is described through the quantum
property named ‘spin’.
The quantum property of spin can be represented by saying a
nucleus can align in 2 directions (‘up’ and ‘down’), as a bar magnet
can in another magnetic field.
For protons, these 2 directions or spin states are described by
quantum numbers +1/2 and -1/2.
Without a field, the spin state does not matter, they are equally
energetically favourable, like a bar magnet without other magnets
nearby.
External
magnetic field
‘up’
‘down’
N
S
+1/2
-1/2
Direction
of field

When a magnetic field is applied (B0), one spin state becomes higher
in energy, and one lower

States are separated in energy by an amount, ΔE

The fields of the nuclei align with or against the applied magnetic field



The stronger the field, the greater the energy gap between ‘up’ and
‘down’ states
Larger separation means more nuclei are in the lower energy state
‘Magnetic resonance’ is using radiowaves to swap nuclei between
different spin states; changing the alignment of nuclei within the field
-1/2
N
ΔE
B0
S
‘Against’
is higher
in energy
+1/2
‘With’ is
lower in
energy
How does NMR use spin?





In NMR, radiofrequency pulses can change
alignments to an excited energy state, the higher
energy state
The frequency required is unique to the particular
energy gap, depending on a proton’s environment
After a pulse, protons release energy as they settle
back to their natural state; this is relaxation
Radiofrequency emissions during relaxation are
recorded against time
A mathematical process, called a Fourier
Transformation, converts the signal into the NMR
spectrum
The principles of magnetic resonance, from the perspective of MRI:
https://www.youtube.com/watch?v=1CG
zk-nV06g
https://www.youtube.com/v/1CGzk-nV06g
Try your own magnetic resonance experiment!
3.Restart
Stop
Apply
4.1.
2.
Begin
pulses
magnetic
pulsesfield
Outside of a field –
random orientations
B0
Field
applied
–
Excited
Radiofrequency
spin state
relaxes
separates
spinnuclei
up
to
pulse
original
excites
alignment.
and
down
states
Radiofrequency
so they
can
spinemitted.
flip
Population of
energy levels
Detector
B0
Fourier transformation
Try your own magnetic resonance experiment!
3.Apply
Stop
1.
2.
Begin
pulses
magnetic
pulsesfield
Outside of a field –
random orientations
B0
Field
applied
–
Excited
Radiofrequency
spin state
relaxes
separates
spinnuclei
up
to
pulse
original
excites
alignment.
and
down
states
Radiofrequency
so they
can
spinemitted.
flip
Population of
energy levels
Detector
B0
Fourier transformation
Equipment




The main principle of NMR, is the use of
magnetic resonance
x
A field is applied to a proton, which
causes
alignment with, or against, the magnetic field
Magnetic resonance is the excitation of
nuclei using radiofrequencies while a
magnetic field is applied
A radiofrequency pulse, specific to the nucleus
involved, ‘flips’ the alignment
Radiofrequency emissions are recorded, and are
unique to the nucleus being observed.
How to run an NMR experiment:
https://www.youtube.com/v/kPx6BlJj5DU
A Basic NMR schematic
Click on a label for more information:

Magnetic field (B0)

Generates applied magnetic field
which creates an energy
difference in spin states and
causes alignment More
The magnets are superconductors,
cooled to 4K (as close as possible),
and submerged in liquid helium
Magnet
Radiowave
generator

Produces pulses of
radiowaves which change
the alignment of protons
Detector


The detector picks up energy
emitted as protons relax
Signal is then converted into
NMR spectrum for analysis,
using Fourier Transformation
Fourier Transformation
•
The Fourier Transformation is a mathematical function
•
It converts the signal produced by relaxing nuclei into the NMR
spectrum which is analysed
•
The time based signal is converted into a frequency based
signal
•
More environments result in a more complicated signal as
characteristic frequencies superimpose
Chemical shift/ ppm
Time / s
Spectra


Each peak (or multiplet) represents a unique
proton environment Multiplets are groups of peaks on a x
spectrum which collectively represent
a single proton environment
An environment is a group of equivalent protons
in a molecule.

Equivalent protons are identical

All equivalent protons give the same signal

Here, there are 2
proton environments: a
CH3 group, and a CH2
group



The spectra have 'chemical shift' along the x-axis
Chemical shift is related to a proton’s resonant
frequency and gives information about a proton's
environment, such as the electronegativity of
neighbouring atoms
Intensity, on the y-axis, is rarely marked on NMR
spectra, but gives information on the number of
protons within an environment

In order to produce a value for chemical shift,
tetramethylsilane (TMS) is used

TMS is a standard, assigned a chemical shift of 0

All other signals are therefore compared to TMS


This works by
TMScomparing
has the following the
key x
properties: for
frequency required
resonance in
observed
- 12the
hydrogens,
all identical,
a very clear, strong
environmentprovides
with
the resonant
signal to compare against
frequency of TMS
- Protons in TMS’s C-H bonds
have the greatest electron
cloud of almost all other C-H
bonds, ensuring all other
signals are to the left of the
TMS signal
Chemical shift is therefore a
frequency value, relative to the
standard
Why is TMS used?

NMR spectra have a
'downfield' and an
'upfield'.
As a peak's chemical
shift increases in
value, it moves
downfield,
Some protons are so well
shielded, that x having
they are more heavily shielded
than
TMS.
been
deshielded
This gives them a negative shift.
x
Deshielding
is the reduction of a
Upfield
proton’s electron cloud. One example
of this is a neighbouring
electronegative atom withdrawing
electron density.
As a signal is found further downfield,
it means the electron cloud around
the proton is less than the cloud
around
in by
TMS.
The protons labelled here
areprotons
shielded
the delocalised electrons opposing the
magnetic field.
Downfield
Did you know?





Proton NMR gives the number of hydrogen
atoms present
The size of the peaks gives the relative
number of protons in each environment
When a peak has been split, the area under a
group of peaks is taken. This is an integrated
value
Therefore, the peak will give the correct
number of protons within the environment,
even if the intensity has been reduced by
being split into a multiplet
•
The values don't always have to be integers, the relative sizes are
what's important!
•
These values can sometimes be found in several locations, but
are always near the peak they are assigned to
•
This may be:
•
Beside a line indicating which peaks have been included
•
Below the peak
•
On a ‘normalised’ y-axis
Click here for some alternate peak values.
Notice they follow the same 1:2:1 ratio between the peaks
4.6
2
2.3
1
1
2.3
Shielding





Shielding determines how nuclei interact with
the magnetic field and the chemical shift of the
signal
x
The resonant frequency of a
proton, compared to the
standard in NMR of
tetramethylsilane (TMS)
See section: Spectra
The electron cloud surrounding a nucleus
opposes the applied field
The more electrons, the greater the shielding
around a proton, so the field is more strongly
opposed
Shielding determines how far downfield, or
upfield , a peak is shifted Downfield is an increasing
chemical shift, as a proton is
more deshielded
x
As
a proton
becomes
x deshielded, it is shifted
Upfield
is the decreasing
chemical shift, as a proton is
further
downfield, as the magnetic field is able
more shielded
to affect it more


Electronegative atoms (eg. oxygen or chlorine) will
draw electron density away from the protons,
deshielding them.
Less electronegative atoms like carbon will not pull
electron density, leaving the proton shielded, so it is
not shifted.
How does an
electronegative atom
affect the cloud?
Click to see
How much is a weakly
withdrawing atom going
to affect the cloud?
Click to see
• The electron cloud is
pulled away from the
proton
• The electron cloud is
left mostly unchanged
• As the proton’s electron cloud reduces, the nucleus is
exposed to more of the external magnetic field (B0)
• The energy gap increases causing magnetic
resonance frequency to change. This changes the
emissions frequency from relaxation
• This results in a greater chemical shift and the signal
appears further downfield
Click to see how
far each proton
will be shifted
Downfield
Upfield
•
Signals tend to fall within particular ranges, depending on
the environment the proton is in
•
This helps identify the signal based on the chemical shift
Splitting
patterns
In the spectrum below, there is a signal for
each environment, however, they appear split.
 These split peaks are multiplets.
 The number of peaks in a multiplet provides
information about neighbouring protons.




Splitting is caused by the interaction between
inequivalent protons, called coupling
Proton-proton coupling usually takes place 3
bonds away from each other
The number of peaks, or multiplicity, comes
from the number of protons interacting




The number of peaks follows an n+1 pattern,
where n is the number of protons in the interacting
environment
The number of peaks observed is the multiplicity
The intensity of the peaks in a multiplet follows the
pattern of Pascal’s triangle
eg. If a group of protons has 2 neighbouring
equivalent protons, it will be split into a triplet.
0 neighbouring protons
1 neighbouring proton
2 neighbouring protons
3 neighbouring protons
Singlet
Doublet
Triplet
Quartet
Click on the group to see how it will appear on the spectrum:
The CH3 is coupled
with the CH2. There
are 2 protons, so
CH3 is split into a
triplet
Intensity = 1:2:1
The CH2 is coupled
with the CH3. There
are 3 protons and
therefore split CH2
into a quadruplet
Intensity = 1:3:3:1
Origin of the Pascal’s triangle pattern

Starting simple: doublets
H
C
C
H

Observing the signal of H, it will be split by H

H interacts with H's magnetic field, H0

The magnetic field interactions cause splitting
between higher and lower energy levels
H0
H



C
H
C
There are two possible interactions that H's field can have
with H's field (H0)
One possibility will be with, and one against the interacting
field
As these states are interacting with another field, the
states have different energies

Down
(Against)
1:1
H0
Up
(With)
There is a 1:1 ratio
distributed between these
levels

This gives the doublet

As a more complicated example: quartet
H
C
C
H3

This time, H interacts with H3 – three equivalent protons

Each of the spins of the three protons can be aligned with or against H0

Due to the interaction being between different magnetic fields, the different
possible alignments have different energies, some of them are equivalent
All with
1 with,
2 against
2 with,
1 against
All against
• Complete alignment ‘with’ is the most favourable, so is lowest in energy
• The next most favourable is 2 with. But there are 3 possible combinations
which give 2 ‘with’ alignments and 1 ‘against’. These are equal in energy.
1
3 of equal energy
H0
4 (n+1) peaks
1:3:3:1 intensity pattern
3 of equal energy
1
Quartet splitting pattern
Interpreting spectra quiz
(You may want pen and paper for this!)
With each structure, identify the correct NMR
Click on the circle to confirm your answer
2
?
6
6
?
2
3 3
1 1
?
Good try, but have another go and see if you can
work out the correct spectrum
Try this:
 Identify the equivalent protons and their
environments
 Look to see which environment is coupling
with others, how many hydrogens is it
coupling to (remember: n+1)?
 Check to see that the integrated intensity
matches the group on the molecule
Return to question
Well done!
You should have noticed that:
 There are two groups of equivalent protons;
the single protons, and the methyl groups
 The single proton split the methyl signal into
a doublet
 The methyl group split the single proton
signal into a quartet
 The single proton signal is closer to the
double bond, making the signal more
downfield
Click for the next question
Note: In solvent, due to proton exchange,
the OH group does not couple
3
2
1
?
3
2
1
?
3
2
1
?
Good try, but have another go and see if you can
work out the correct spectrum
Try this:
 Work out how many different environments
there are
 Look to see which environment is coupling
with others, how many hydrogens is it
coupling to (remember: n+1)?
 Check to see that the integrated intensity
matches the group on the molecule
Return to question
Well done!
You should have noticed that:
 There are three environments; the methyl
group, the OH group, and the pair of
protons on the centre carbon
 The methyl group will be split into a triplet
by the pair or centre protons
 The pair of protons will give a signal which
is split into a quartet by coupling with the
methyl group
Click for the next question
Note: TMS reference signal occurs at a shift of 0 Hz
when present
6
6
4
?
4
6
6
?
4
6
6
?
Good try, but have another go and see if you can
work out the correct spectrum
Try this:
 With the integrated peaks, remember the
ratio of number of protons is provided, not
an absolute number of protons. Try
changing the numbers while maintaining the
ratio.
 Consider likely chemical shifts for the
signals and see how it compares with what
is present
Return to question
Well done!
You should have noticed that:
 There are three environments, excluding
the TMS’s signal at 0 ppm.
 The ratio of peak intensities can be easily
simplified to 3:2:3, fitting the number of
protons in the molecule.
 The pair of protons will be more downfield
due to being closer to the ester functional
group, than the methyl group it couples with
Click for the end the quiz
Congratulations!
This is the end of the quiz
You can find some more advanced problems through the following link:
http://sasc-specialists.ucdavis.edu/jim/118A/ProtonNMR.Probs.html
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