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

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Magneticresonance

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Click on the topic you wish for information on:Magnetic resonanceSpectra Splitting patternsInterpreting spectra quizShieldingEquipment

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One of the main features of NMR is the use of magnetic fieldsMoving 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 alignmentWithout the other field, the magnet can lie in any direction

NSExternalmagnetic field

S

NA 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.

+1/2-1/2updownNSDirection of fieldExternalmagnetic field5When a magnetic field is applied (B0), one spin state becomes higher in energy, and one lowerStates are separated in energy by an amount, EThe fields of the nuclei align with or against the applied magnetic fieldThe stronger the field, the greater the energy gap between up and down statesLarger separation means more nuclei are in the lower energy stateMagnetic resonance is using radiowaves to swap nuclei between different spin states; changing the alignment of nuclei within the field

+1/2-1/2

B0

EAgainst is higher in energyWith is lower in energyNS6In NMR, radiofrequency pulses can change alignments to an excited energy state, the higher energy stateThe frequency required is unique to the particular energy gap, depending on a protons environmentAfter a pulse, protons release energy as they settle back to their natural state; this is relaxationRadiofrequency emissions during relaxation are recorded against timeA mathematical process, called a Fourier Transformation, converts the signal into the NMR spectrum

How does NMR use spin?7https://www.youtube.com/watch?v=1CGzk-nV06g

The principles of magnetic resonance, from the perspective of MRI:https://www.youtube.com/v/1CGzk-nV06g1. Apply magnetic field2. Begin pulses3. Stop pulses

Outside of a field random orientationsField applied separates spin up and down states

Try your own magnetic resonance experiment!

Fourier transformation DetectorRadiofrequency pulse excites nuclei so they can spin flipExcited spin state relaxes to original alignment. Radiofrequency emitted.4. RestartB0B0Population of energy levels9

Splittingpatterns

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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

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How to run an NMR experiment:https://www.youtube.com/v/kPx6BlJj5DU13Click on a label for more information:

RadiowavegeneratorDetectorMagnetic field (B0)MagnetProduces pulses of radiowaves which change the alignment of protonsGenerates applied magnetic field which creates an energy difference in spin states and causes alignment MoreThe detector picks up energy emitted as protons relaxSignal 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 14

Fourier TransformationThe Fourier Transformation is a mathematical functionIt converts the signal produced by relaxing nuclei into the NMR spectrum which is analysedThe time based signal is converted into a frequency based signalMore environments result in a more complicated signal as characteristic frequencies superimpose

Chemical shift/ ppmTime / sEach peak (or ) represents a unique proton environmentAn 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

multipletx17The spectra have 'chemical shift' along the x-axisChemical shift is related to a protons resonant frequency and gives information about a proton's environment, such as the electronegativity of neighbouring atomsIntensity, on the y-axis, is rarely marked on NMR spectra, but gives information on the number of protons within an environment

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In order to produce a value for chemical shift, tetramethylsilane (TMS) is usedTMS is a standard, assigned a chemical shift of 0All 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 TMSChemical shift is therefore a frequency value, relative to the standardTMS has the following key properties:

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

- Protons in TMSs 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

xNMR spectra have a 'downfield' and an 'upfield'. As a peak's chemical shift increases in value, it moves downfield, having been

DownfieldUpfield

Did you know? Deshielding is the reduction of a protons 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.

xdeshieldedSome 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.

x20Proton NMR gives the number of hydrogen atoms presentThe size of the peaks gives the relative number of protons in each environmentWhen a peak has been split, the area under a group of peaks is taken. This is an integrated valueTherefore, 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

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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 toThis may be:Beside a line indicating which peaks have been includedBelow the peakOn a normalised y-axis

2.34.62.3121Click here for some alternate peak values.Notice they follow the same 1:2:1 ratio between the peaks

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 signalThe electron cloud surrounding a nucleus opposes the applied fieldThe 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 deshieldeddownfieldupfieldUpfield is the decreasing chemical shift, as a proton is more shieldedxxchemical shiftThe resonant frequency of a proton, compared to the standard in NMR of tetramethylsilane (TMS)See section: Spectrax24

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 seeHow much is a weakly withdrawing atom going to affect the cloud?Click to see

The electron cloud is pulled away from the protonThe electron cloud is left mostly unchanged25

As the protons 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 relaxationThis results in a greater chemical shift and the signal appears further downfield

DownfieldUpfield

Click to see how far each proton will be shifted

Signals tend to fall within particular ranges, depending on the environment the proton is inThis helps identify the signal based on the chemical shift

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.

29Splitting is caused by the interaction between inequivalent protons, called couplingProton-proton coupling usually takes place 3 bonds away from each otherThe number of peaks, or multiplicity, comes from the number of protons interacting

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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 multiplicityThe intensity of the peaks in a multiplet follows the pattern of Pascals triangleeg. If a group of protons has 2 neighbouring equivalent protons, it will be split into a triplet.

SingletTripletDoubletQuartet

0 neighbouring protons1 neighbouring proton2 neighbouring protons3 neighbouring protons31

The CH3 is coupled with the CH2. There are 2 protons, so CH3 is split into a tripletThe CH2 is coupled with the CH3. There are 3 protons and therefore split CH2 into a quadrupletIntensity = 1:3:3:1Intensity = 1:2:1Click on the group to see how it will appear on the spectrum:

32Origin of the Pascals triangle patternStarting simple: doublets

CCHHObserving the signal of H, it will be split by HH interacts with H's magnetic field, H0The magnetic field interactions cause splitting between higher and lower energy levelsH033There 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 fieldAs these states are interacting with another field, the states have different energies

CCHHThere is a 1:1 ratio distributed between these levels

This gives the doublet1:1H0Down(Against)Up(With)34As a more complicated example: quartet

This time, H interacts with H3 three equivalent protonsEach of the spins of the three protons can be aligned with or against H0Due to the interaction being between different magnetic fields, the different possible alignments have different energies, some of them are equivalent

CCHH3H013 of equal energy

3 of equal energy

14 (n+1) peaks1:3:3:1 intensity patternQuartet splitting pattern

All withAll against2 with, 1 against1 with, 2 againstComplete alignment with is the most favourable, so is lowest in energyThe 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.35

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

Return to questionTry this:Identify the equivalent protons and their environmentsLook 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 molecule38

Good try, but have another go and see if you can work out the correct spectrumTry this:Work out how many different environments there areLook 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 question41Well done!Click for the end the quiz

You should have noticed that:There are three environments, excluding the TMSs 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