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AQA Chemistry > 3.15 NMR Spectroscopy > Flashcards

3.15 NMR Spectroscopy Flashcards

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1
Q

What does NMR stand for?

A

Nuclear Magnetic Resonance

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2
Q

What are the basic principles of NMR?

A

Finding the structures of complex molecules by placing them in a magnetic field and applying radio waves
If waves of right frequency are absorbed, the nuclei flips from parallel
Energy change can be monitored and recorded

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3
Q

How is NMR spectroscopy carried out?

A

Dissolve liquid sample in suitable solvent
Put in a tube with small amount of TMS
Spin sample to even out imperfections in magnetic field and zero spectrometer against TMS
Different radio frequencies with constant magnetic field applied to sample
Absorptions detected

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4
Q

Give a use of NMR spectroscopy.

A

MRI scans

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5
Q

What nuclei does NMR work with?

A

Those with uneven number of nucleons to spin e.g. ¹H ¹³C

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6
Q

What percentage of carbon atoms are ¹³C?

A

1%

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7
Q

What defines the resonant frequency of a ¹³C atom?

A

Its chemical environment (amount of electron shielding)

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8
Q

What graph is produced by NMR spectroscopy?

A

Energy absorbed against chemical shift

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9
Q

What is chemical shift?

A

The resonant frequency of nuclei compared to that of a ¹H atom in TMS

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10
Q

What is the symbol for chemical shift?

A

δ

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11
Q

What are the units of chemical shift?

A

Parts per million (ppm)

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12
Q

What is the range of chemical shift for ¹³C NMR?

A

0-200ppm

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13
Q

What means ¹³C atoms show a different chemical shift value?

A

Having different chemical environments

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14
Q

What environment leads to a greater chemical shift in ¹³C NMR?

A

A C atom next to more electronegative atom

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15
Q

What does the number of signals represent in ¹³C NMR?

A

One signal for each carbon environment

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16
Q

What does the chemical shift represent in ¹³C NMR?

A

Greater chemical shift from atoms closer to electronegative atoms or C=C

17
Q

What does the area under the peak represent in ¹³C NMR?

A

Nothing

18
Q

What does the number of splitting in ¹³C NMR?

A

There isn’t any splitting

19
Q

What is the range of chemical shift ¹H NMR?

A

0-10ppm

20
Q

What does the number of signals represent in ¹H NMR?

A

One signal for each hydrogen environment

21
Q

What does the chemical shift represent in ¹H NMR?

A

Greater chemical shift from atoms closer to electronegative atoms or C=C

22
Q

What does the area under the peak represent in ¹H NMR?

A

Proportional to number of ¹H atoms represented by peak

23
Q

What does the splitting pattern represent in ¹H NMR?

A

The number of hydrogen atoms on the neighbouring carbon atom + 1

24
Q

What is the integration trace in ¹H NMR?

A

A stepped line that makes it easier to measure area under curve

25
Q

What is TMS?

A

Tetramethylsilane

26
Q

How is TMS structures?

A

CH₃
|
H₃C - Si - H₃C
|
CH₃

27
Q

What state is TMS at room temperature?

A

Liquid

28
Q

Why is TMS used?

A

Calibrate NMR equipment
Provides peak at exactly 0ppm
Reference point for all chemical shifts

29
Q

What are the advantages of using TMS?

A

Inert
Non-toxic
Volatile - easy to remove from sample

30
Q

When does splitting occur in ¹H NMR?

A

When neighbouring hydrogen atoms affect the magnetic field of ¹H atoms

31
Q

What would be a neighbouring H atom?

A

3 or fewer bonds away
On adjacent carbon

32
Q

What is the n+1 rule?

A

If there are n ¹H atoms on the neighbouring carbon then the peak will split into n+1 smaller peaks

33
Q

Why must solvents used for ¹H NMR not contain any hydrogen atoms?

A

Signals from solvent would swamp signals from sample as there is more solvent than sample

34
Q

What solvents are used for ¹H NMR?

A

Deuterated solvents - CDCl₃, D₂O, C₆D₆
CCl₄ - tetracholoromethane

35
Q

Why does the peak from O-H bonds disappear if D₂O is used as solvent?

A

Forms O-D bond

AQA Chemistry (8 decks)

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