Unit One Part 9:nuclear magnetic resonance spectroscopy

gjr-–-

• Describe what an NMR spectrum is & how we record one• Recognise what information it gives us about equivalent hydrogens• Predict the position of absorptions based on functional groups• Relate the size of an absorption to the number of hydrogen responsible

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dr gareth rowlands; g.j.rowlands@massey.ac.nz; science tower a4.12http://www.massey.ac.nz/~gjrowlan

Spectroscopygjr-–-

• Electromagnetic radiation can interact with matter (e.g. sunburn...ouch)• Different kinds of radiant energy interact with different parts of a molecule• We can use this to investigate the structure of molecules...

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ENERGY

Electromagnetic spectrumgjr-–-

• We either measure the energy absorbed or the energy emitted• The frequency of radiation absorbed corresponds to the frequency of

energy associated with a molecular effect...

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molecule in energy state E2

E1

E2absorption of energy

emission of energy

molecule in energy state E1

E1

E2energy

in

E1

E2

energy out

UV

uv-visexcitation of an

electron

infraredbond vibration

nmrnuclear spin

Spectroscopic techniquesgjr-–-

• From NMR we learn about the carbon-hydrogen framework• From IR we learn about functional groups• From UV-vis we learn about conjugated π-bond systems

From MS we learn about size & formular (but it hasn't really got anything to do with EM radiation...

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techniqueradiation source

spectral region

energy (kJmol–1)

molecular effect

nuclear magnetic radiation

radio waves0 – 10 ppm (1H)

0 – 200 ppm (13C)10–4

spin flipping of nuclei

infrared (IR) infrared light 600 – 4000 cm–1 10 – 50 bond vibrations

ultraviolet-visible (UV)

ultraviolet or visible light

200 – 1100 nm 600 – 1000electronic transitions

mass (MS)high energy electrons

– 70 eV ionisation

Nuclear magnetic resonance spectroscopygjr-–-

• Anyone who has watched House M.D. will have seen a technique called MRI - magnetic resonance imaging

• This allows imaging of the body• It works just like (& is in fact identical to) NMR...

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• People just don't like going in machines with the word "nuclear" in them!

1H NMR spectroscopygjr-–-

α-spin state

β-spin state

• NMR is the most useful for of spectroscopy to the organic chemist (me)• Normally look at 1H (all I'll cover) but can look at 13C, 14N, 19F & 31P

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• Normally nuclei are randomly orientated• But placed in a magnetic field the spin of the nuclei either become aligned...α) or opposed (β) to the magnetic field

MA

GN

ET

IC F

IELD

EN

ER

GY

• The α-spin state is lower in energy but nuclei can be promoted to the β...state by the absorption of radiofrequency radiation• The nuclei return to the α-state, emitting energy equal to the difference...between α & β• Spectrometer plots intensity vs. frequency (energy) & we interpret results

energy to spectrometer

An NMR machinegjr-–-

• Basically, its a radio in a large superconducting magnet connected to a computer...

• But a little bit more complex

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What does it tell us?gjr-–- 8

caffeine

N

N N

N

OCH3

H3CO CH3

H

CH3Si

CH3H3CH3CTMS

H

aromatic

CH

Halkenyl

X CH

X = O, N or halide

C CH

allylic

C CH

alkyl

8 6 4 2 0low field high field δ scale

• 1H NMR spectrum reported with chemical shift (δ) in ppm along x axis

• Typical 1H NMR is between 0–10 ppm with TMS at 0 ppm

• Chemical shift of a proton depends on electronic environment

• Electrons shield proton from applied magnetic field and lead to shielded or high field shifts

• Protons in electron poor areas are deshielded & low field

deshielded shielded

Correlation tablegjr-–-

• Table gives just a rough idea of common chemical shifts but these are altered by many, many, many factors...

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Type of hydrogen δ (ppm)

Type of hydrogen δ (ppm)

Type of hydrogen δ (ppm)

C CH3 0.70–1.30 CH2C Ph 2.60 C CH2 4.60–5.00

CH2C C 1.20–1.35 C

H2C I 3.10–3.30

C CH

C 5.20–5.70

C CH

CC 1.40–1.65 C

H2C Br 3.40 CHCl2 5.80–5.90

H3C C C 1.60–1.90 CH2C Cl 3.50 Ph H 6.60–8.00

H3C C O 2.10–2.60 H2C O 3.50–3.80 C CO

H9.50–9.70

H3C N 2.10–3.00 H3C O 3.50–3.80 C CO

OH10.00–13.00

Ph CH3 2.20–2.50 Ar OH 4.00–8.00 R OH 0.50–5.50

C C H 2.40–2.70 CH2C F 4.30–4.40

Chemically equivalent hydrogensgjr-–-

SCH3

OH

HH

SCH3

OH

HH

SCH3

OH

HH

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H3CSCH3

O• Each H that has a distinct electronic

environment / chemical environment will give a distinct signal in 1H NMR spectrum

• A group of identical H will give one signal

• In dimethyl sulfoxide all hydrogen are the same so one peak / signal

• If we look at the conformation of dimethyl sulfoxide we should see that each H is equivalent...

Each H is indistinguishable

Chemically equivalent hydrogens IIgjr-–-

CH3H

H3CH

CH3

H

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CH3N

CH3H3C

CH3H

H3CH

CH3

H

• Mesityl has two distinct sets of chemically equivalent hydrogen

• The three identical methyl groups & the three aromatic hydrogen

• Therefore, we get two signals

• Trimethylamine has nine hydrogen• But all are equivalent• Therefore only one peak in spectrum

Chemically equivalent hydrogens IIIgjr-–-

OCH3

O

H3CH H

• We have three distinct chemical environments so we observe three peaks • The two H of –CH2– are relatively downfield as both the Oxygen &

carbonyl are electron-withdrawing• Other two groups only have one electron-withdrawing group each• A knowledge of which groups are electron-withdrawing allows to assign

spectrum

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OCH3

O

H3CH H

δ 3.42 ppm

δ 4.03 ppm

δ 2.15 ppm

Integration (hydrogen counting)gjr-–-

• The area under each signal is the integral & is proportional to the number of H

• Again it helps assign peaks...the peaks are in the ratio of 2:3:3 so the first must be the –CH2– position

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OCH3

O

H3CH H

2x3x 3x

Integration (hydrogen counting) IIgjr-–- 14

CH3H

H3CH

CH3

H

3x

9x

H3C O CH3

4x6x

• Remember, the integral is just a ratio• We known the structure (left) is 3:9

but if we did not know which molecule it was the integral could/would tell us it was 1:3

• It is the same with diethyl ether; the ratio is 2:3 but the structure is 4:6

Signal splittinggjr-–-

• If we look at the spectrum of diethyl ether in detail we see that our two peaks are further split

• The peak at δ 3.37 ppm is 4 peaks & the peak at δ 1.14 ppm is 3 peaks• The integral is still in the ratio 2:3 or 4:6 • Signal splitting is very common & tells us (me?) a lot about the structure• At the moment you are lucky & just need to know it occurs (& not the whole

can of worms of how, why, what etc)

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H3C O CH3

4x6x

The real deal...gjr-–-

• As you can see a 1H NMR spectrum can be quite complicated• But is gives us a considerable amount of information• In this example we can actually identify 15 out of 22 H without trouble (& we

know where the rest are...honest)

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SO

Tol

H

H

Overviewgjr-–-

What have we learnt?• That useful things can be very complicated...• NMR is a powerful technique• More detail will be given in 123.202 (well now you know...)

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What's next?• Thankfully, not much• Continue our look at analysis and spectroscopy (& spectrometry)• Look at the information that can be ascertained from infrared...spectroscopy & mass spectrometry

dr gareth rowlands; g.j.rowlands@massey.ac.nz; science tower a4.12http://www.massey.ac.nz/~gjrowlan