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A-level Chemistry > PL - NMR spectroscopy > Flashcards

PL - NMR spectroscopy Flashcards

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1
Q

What does NMR give you information about?

A

The structure of molecules.

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2
Q

That doe NMR stand for?

A

Nuclear magnetic resonance.

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3
Q

What is NMR?

A

An analytical technique that you use to work out the structure of an organic molecule.

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4
Q

How does NMR work?

A

1) A sample of a compound is placed in a strong magnetic field and exposed to a range of different frequencies of radio waves.
2) The nuclei of certain atoms within the molecule absorb energy from the radio waves.
3) The amount of energy that a nucleus absorbs at each frequency will depend on the environment that it is in.
4) The pattern of these absorptions gives you information about the positions of certain atoms within the molecule, and about how many atoms of that type the molecule contains.
5) You can piece these bits of information together to work out the structure if the molecule.

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5
Q

Where is a sample of a compound placed in NMR spectroscopy and what is it exposed to?

A

In a strong magnetic field and exposed to a range of different frequencies of radio waves.

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6
Q

What in the molecule absorbs the radio waves in NMR?

A

The nuclei of certain atoms within the molecule.

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7
Q

What does the amount of energy that a nucleus absorbs at each frequency in NMR depend on?

A

The environment that it’s in.

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8
Q

What can the pattern of the absorptions in NMR give you information about?

A

The positions of certain atoms within the molecule, and about how many atoms of that type the molecule contains.

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9
Q

If you piece the information from NMR together what can you work out?

A

The structure if the molecule.

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10
Q

What are the 2 types of NMR spectroscopy?

A

Carbon-13 NMR and high resolution proton NMR.

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11
Q

What does carbon-13 (or 13C) NMR give information about?

A

The number and types of different carbon environments that are in a molecule.

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12
Q

What does high resolution proton NMR give information about?

A

The number of hydrogen atoms that are in a molecule, and the environments that they’re in.

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13
Q

What is a nucleus partly shielded from?

A

The effects of external magnetic fields.

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14
Q

How is the nucleus partly shielded from the effects of external magnetic fields?

A

By its surrounding electrons.

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15
Q

Other than the surrounding electrons, what else will also affect the amount of electron shielding of a nucleus?

A

Any other atoms and groups of atoms that are around the nucleus - e.g. if a carbon atom bonds to a more electronegative atoms (like oxygen) the amount of electron shielding around its nucleus will decrease.

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16
Q

Why might the amount of electron shielding around a nucleus decrease?

A

E.g. if a carbon atom bonds to a more electronegative atoms (like oxygen) the amount of electron shielding around its nucleus will decrease.

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17
Q

Why do nuclei in different environments experience different effects of the magnetic field depending on their environments?

A

The nucleus partly shielded from the effects of external magnetic fields by its surrounding electrons. Any other atoms and groups of atoms that are around the nucleus - e.g. if a carbon atom bonds to a more electronegative atoms (like oxygen) the amount of electron shielding around its nucleus will decrease.

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18
Q

What will nuclei in different environments do differently?

A

Absorb different amounts of energy at different frequencies.

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19
Q

What are you looking for in NMR sepctroscopy?

A

Differences in absorptions of energy between environments.

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20
Q

What does an atom’s environment depend on?

A

All the groups it’s connected to, going right along the molecule - not just the atoms it’s actually bonded to.

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21
Q

What must there be for two atoms to be in the same environment?

A

The two atoms must be joined to exactly the same things.

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22
Q

What is chemical shift measured relative to?

A

Tetramethylsilane.

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23
Q

What do nuclei in different environments absorb?

A

Different frequencies.

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24
Q

What does NMR measure?

A

The differences in frequencies absorbed by nuclei in different environments relative to a standard substance - the different is called the chemical shift.

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25
Q

What is (the) chemical shift?

A

The difference recorded by/in NMR of the different frequencies absorbed by nuclei in different environments.

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26
Q

What is the standard substance that is used in NMR?

A

Tetramethylsilane (TMS).

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27
Q

What is the formula of tetramethylsilane (TMS)?

A

Si(CH3)4

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28
Q

Is tetramethylsilane reactive?

A

No, it is inert so it doesn’t react with the sample. It is also non-toxic, and volatile so it’s easy to remove from the sample.

29
Q

How many environments are there in tetramethylsilane?

A

It has 12 hydrogen atoms all in identical environments.

30
Q

Because tetramethylsilane has 12 hydrogen atoms all in identical environments, what does it produce in NMR?

A

A single absorption peak, well away from most other absorption peaks.

31
Q

What is chemical shift measured in?

A

Parts per million (or ppm) relative to tetramethylsilane (TMS).

32
Q

What is the single peak produced by TMS given a chemical shift value of and why?

A

0 (ppm) because chemical shift is measured in parts per million (ppm) relative to TMS.

33
Q

What is the symbol for chemical shift?

A

δ

34
Q

Where will you often see a peak on spectra as a result of TMS being added to the test compound for calibration purposes?

A

δ = 0

35
Q

Why will you often see a peak at δ = 0 on spectra?

A

Because TMS is added to the test compound for calibration purposes and TMS has a chemical shift value of 0.

36
Q

Why is TMS added to the test compound in NMR?

A

For calibration purposes.

37
Q

What will happen to the spectra as a result of adding TMS to the test compound for calibration purposes?

A

You will often see a peak at δ = 0 on spectra and TMS has a chemical shift value of 0.

38
Q

What do 13C NMR spectra tell you about?

A

Carbon environments.

39
Q

How do you interpret carbon-13 spectra?

A

1) Count the number of carbon environments:
- First, count the number of peaks in the spectrum - this is the number of carbon environments in the molecule. If there is a peak at δ = 0, don’t count it as it is the reference preak from TMS.

2) Look up the chemical shifts in a shift diagram:
- Use the diagram on the data sheet which shows the different chemical shifts experienced by carbon nuclei in different environments.
- The boxes show the range of shift values a carbon in that environment could have.
- You need to match up the peaks in the spectrum with the chemical shifts in the diagram to work out which carbon environments they could represent - this may not always be straightforward as chemical shifts can overlap.

3) Try to work out possible structures:
- Select different types of molecules with different functional groups (e.g. an aldehyde and a ketone) with the same molecular formula to try to work out which molecule was analysed - check the number of carbon environments the molecule has and make sure it matched the number of peaks and the chemical shifts (this molecule should be the right one).

40
Q

How do you know how many carbon environments are in a molecule from a spectrum?

A

The number of peaks = the number of carbon environments.

If there is a peak at δ = 0, don’t count it as it is the reference preak from TMS.

41
Q

What do 1H spectra tell you about?

A

Hydrogen environments.

42
Q

What are proton NMR spectra also called?

A

1H NMR spectra (hydrogen spectra).

43
Q

How do you interpret proton NMR spectra?

A

1) Each peak represents one hydrogen environment.
2) Look up the chemical shifts on the data sheet to identify possible environments - this is a separate diagran from the 13C NMR spectra chemical shift diagram.
3) In 1H NMR, the relative area under each peak tells you the relative number of H atoms in each environment - e.g. if the area under two peaks is in the ratio 2:1, there will be two H atoms in the first environment for every one in the second environment.
4) Areas can be shown using numbers above the peaks or with an integration trace.

44
Q

What does each peak represent in proton NMR spectra?

A

One hydrogen environment.

45
Q

What is the line that is sometimes drawn on 1H NMR spectra?

A

The integration trace.

46
Q

What are the height increases in the integration trace proportional to?

A

The area under each peak.

47
Q

In 1H NMR what does the area under each peak tell you?

A

The relative number of H atoms in each environment - e.g. if the area under two peaks is in the ratio 2:1, there will be two H atoms in the first environment for every one in the second environment.

48
Q

What can you use to work out the relative number of H atoms in each environment?

A

The relative area under each peak.

49
Q

What is the big different between carbon-13 NMR and proton NMR spectra?

A

The peaks in 1H spectrum split according to how the hydrogen environments are arranged.

50
Q

How do the peaks in 1H spectrum split?

A

According to how the hydrogen environments are arranged.

51
Q

What does an integration trace show?

A

It is another way of showing the area underneath each peak

52
Q

What does spin-spin coupling do?

A

Splits the peaks in a proton NMR spectrum.

53
Q

What splits the peaks in a proton NMR spectrum?

A

Spin-spin coupling.

54
Q

What is the splitting of a peak that represents a hydrogen environment in proton NMR spectrum caused by?

A

Caused by the influence of hydrogen atoms that are bonded to neighbouring carbons - these are carbons one along in the carbon chain from the carbon the hydrogen’s attached to.

55
Q

What is the effect that causes the splitting of peaks that represent a hydrogen environment caused by the influence of hydrogen atoms bonded to neighbouring carbons called?

A

Spin-spin coupling.

56
Q

What is the spin-spin coupling effect?

A

The effect that causes the splitting of peaks that represent a hydrogen environment caused by the influence of hydrogen atoms bonded to neighbouring carbons.

57
Q

Which hydrogen nuclei have the effect of spin-spin coupling?

A

Only hydrogen nuclei that are on adjacent carbon atoms and in different environments affect each other.

58
Q

What are split hydrogen environment peaks called?

A

Multiplets.

59
Q

What are multiplets?

A

Split hydrogen environment peaks.

60
Q

How much do the hydrogen environment peaks split into multiplets because of spin-spin coupling?

A

They always split one more than the number of hydrogens on the neighbouring carbon atoms - it’s called the n + 1 rule.

E.g. If there are 2 hydrogens on the adjacent carbon atoms, the peak will be split into 2 + 1 = 3.

61
Q

What rule relates to the splitting of hydrogen environment peaks into multiplets?

A

The n + 1 rule where they always split one more than the number of hydrogens on the neighbouring carbon atoms.

E.g. If there are 2 hydrogens on the adjacent carbon atoms, the peak will be split into 2 + 1 = 3.

62
Q

How can you work out the number of neighbouring hydrogens?

A

By looking at how many the peak splits into.

63
Q

If the peak splits into 2 what type of multiplet is it?

A

A doublet

64
Q

If the peak splits into 3 what type of multiplet is it?

A

A triplet

65
Q

If the peak splits into 4 what type of multiplet is it?

A

A quartet

66
Q

If the peak splits into 2 (a doublet) then there’s how many hydrogens on the neighbouring carbon atoms?

A

1

67
Q

If the peak splits into 3 (a triplet) then there’s how many hydrogens on the neighbouring carbon atoms?

A

2

68
Q

If the peak splits into 4 (a quartet) then there’s how many hydrogens on the neighbouring carbon atoms?

A

3

69
Q

What is the rule for working out how many hydrogens there are on neighbouring carbon atoms when looking at how many the peak splits into?

A

Take one away from the number of peaks that it has split into.

n - 1 rule.

A-level Chemistry (51 decks)

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