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Proton NMRCreated 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

Interpreting spectra quiz

ShieldingEquipment

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Magneticresonance

• 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

N

S

Externalmagnetic 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.

+1/2 -1/2

‘up’ ‘down’

N

S

Direction of field

Externalmagnetic 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

-1/2

B0

ΔE

‘Against’ is higher in energy

‘With’ is lower in energy

N

S

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

How does NMR use spin?

https://www.youtube.com/watch?v=1CGzk-nV06g

The principles of magnetic resonance, from the perspective of MRI:

https://www.youtube.com/v/1CGzk-nV06g

1. Apply magnetic field

2. Begin pulses3. Stop pulses

Outside of a field – random orientations

Field applied – separates spin up and down states

Try your own magnetic resonance experiment!

Fourier transformation

Detector

Radiofrequency pulse excites nuclei so they can spin flip

Excited spin state relaxes to original alignment. Radiofrequency emitted.

4. Restart

B0

B0

Population of energy levels

1. Apply magnetic field

2. Begin pulses3. Stop pulses

Outside of a field – random orientations

Field applied – separates spin up and down states

Try your own magnetic resonance experiment!

Fourier transformation

Detector

Radiofrequency pulse excites nuclei so they can spin flip

Excited spin state relaxes to original alignment. Radiofrequency emitted.

B0

B0

Population of energy levels

Equipment

The main principle of NMR, is the use of

A field is applied to a proton, which causes

alignment with, or against, the magnetic field A radiofrequency pulse, specific to the nucleus

involved, ‘flips’ the alignment Radiofrequency emissions are recorded, and are

unique to the nucleus being observed.

Magnetic resonance is the excitation of nuclei using radiofrequencies while a magnetic field is applied

x

magnetic resonance

How to run an NMR experiment:

https://www.youtube.com/v/kPx6BlJj5DU

Click on a label for more information:

Radiowavegenerator Detector

Magnetic field (B0)

Magnet

Produces pulses of radiowaves which change the alignment of protons

Generates applied magnetic field which creates an energy difference in spin states and causes alignment More

The detector picks up energy emitted as protons relax

Signal is then converted into NMR spectrum for analysis, using Fourier Transformation

A Basic NMR schematic

The magnets are superconductors, cooled to 4K (as close as possible), and submerged in liquid helium

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 ) represents a unique proton environment

An environment is a group of equivalent protons in a molecule.

Equivalent protons are identical All equivalent protons give the same signal

Multiplets are groups of peaks on a spectrum which collectively represent a single proton environment

Here, there are 2 proton environments: a CH3 group, and a CH2

group

multipletx

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

Why is TMS used?

This works by comparing the frequency required for resonance in the observed environment with the resonant frequency of TMS

Chemical shift is therefore a frequency value, relative to the standard

TMS has the following key properties:

- 12 hydrogens, all identical, provides a very clear, strong signal to compare against

- 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

x

NMR spectra have a 'downfield' and an 'upfield'.

As a peak's chemical shift increases in value, it moves downfield, having been

Downfield Upfield

Did you know?

Deshielding is the reduction of a 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 protons in TMS.

x

deshieldedSome protons are so well shielded, that they are more heavily shielded than TMS.This gives them a negative shift.

The protons labelled here are shielded by the delocalised electrons opposing the magnetic field.

x

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

2.3

4.6

2.3

1

2

1

Click here for some alternate peak values.Notice they follow the same 1:2:1 ratio between the peaks

Shielding

Shielding determines how far , or , a peak is shifted

As a proton becomes deshielded, it is shifted further downfield, as the magnetic field is

able to affect it more

Shielding determines how nuclei interact with the magnetic field and the of the signal

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

Downfield is an increasing chemical shift, as a proton is more deshielded

downfieldupfield

Upfield is the decreasing chemical shift, as a proton is more shielded

x

x

chemical shift

The resonant frequency of a proton, compared to the standard in NMR of tetramethylsilane (TMS)See section: Spectra

x

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

Downfield Upfield

Click to see how far each proton will be shifted

• 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

Splittingpatterns

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.

Singlet

Triplet

Doublet

Quartet

0 neighbouring protons

1 neighbouring proton

2 neighbouring protons

3 neighbouring protons

The CH3 is coupled

with the CH2. There

are 2 protons, so CH

3 is split into a

triplet

The CH2 is coupled

with the CH3. There

are 3 protons and therefore split CH

2

into a quadruplet

Intensity = 1:3:3:1Intensity = 1:2:1

Click on the group to see how it will appear on the spectrum:

Origin of the Pascal’s triangle pattern Starting simple: doublets

C CH H

Observing the signal of H, it will be split by H H interacts with H's magnetic field, H

0

The magnetic field interactions cause splitting between higher and lower energy levels

H0

There are two possible interactions that H's field can have with H's field (H

0)

One possibility will be with, and one against the interacting field

As these states are interacting with another field, the states have different energies

C CH H

There is a 1:1 ratio distributed between these levels

This gives the doublet

1:1H0

Down(Against)

Up(With)

As a more complicated example: quartet

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

C CH H3

H0

1

3 of equal energy

3 of equal energy

1

4 (n+1) peaks1:3:3:1 intensity pattern

Quartet splitting pattern

All with All against2 with, 1 against

1 with, 2 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.

Interpreting spectra quiz

(You may want pen and paper for this!)

With each structure, identify the correct NMRClick on the circle to confirm your answer

?

?

?

2

6

6

2

1 1

3 3

Good try, but have another go and see if you can work out the correct spectrum

Return to question

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

Well done!

Click for the next question

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

Note: In solvent, due to proton exchange, the OH group does not couple

?

?

?

2

1

3

21

3

21

3

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!

Click for the next question

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

Note: TMS reference signal occurs at a shift of 0 Hz when present

?

?

?

4 6 6

66

4

46 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!

Click for the end the quiz

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

Congratulations!This is the end of the quiz

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You can find some more advanced problems through the following link:http://sasc-specialists.ucdavis.edu/jim/118A/ProtonNMR.Probs.html