3.15 Nuclear Magnetic Resonance Spectroscopy Flashcards

1
Q

Name 2 types of NMR

A
  • 1H (or proton) NMR
  • 13C NMR
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2
Q

What type of atomic nuclei spin?

A

Atomic nuclei with an odd number of nucleons (protons and neutrons)

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3
Q

What does nuclear spin cause an atom to have?

A

a weak magnetic field

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4
Q

Why do hydrogen nuclei have spin?

A

Hydrogen nuclei are single protons

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5
Q

Why doesn’t carbon have nuclear spin?

A

C usually has 6 protons and 6 neutrons

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6
Q

1% of carbon atoms are isotope 13C (___ protons and ____ neutrons) so does have ____

A

But about 1% of carbon atoms are isotope 13C (6 protons and 7 neutrons) so does have spin

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7
Q

What partly shields nuclei from the effects of external magnetic fields?

A
  • Its surrounding electrons
  • Any atoms and groups of atoms around the nucleus
    • Affect its amount of electron shielding
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8
Q

State what will happen to the amount of electron shielding around its nucleus, if carbon atoms bond to a more electronegative atom (e.g. oxygen)?

A

Will decrease

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9
Q

Why do nuclei in different environments absorb different amounts of energy at different frequencies?

A

∵ in different environments, nuclei will feel different magnetic fields

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10
Q

What does NMR spectroscopy essentially look for?

A

Differences in energy absorption between environments

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11
Q

For an atom to be in same environment, what must happen?

A

2 atoms must be joined to exactly the same things

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12
Q

What substance is used as a standard in NMR spectroscopy?

A

Tetramethylsilane (TMS), Si(CH3)4

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13
Q

What is chemical shift (δ)?

A

The differences (in energy absorbed) NMR spectroscopy measures relative to the standard substance (TMS)

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14
Q

Tetramethylsilane has 12 H atoms in ____ ________

A

identical environments

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15
Q

Name 5 reasons why TMS is used as a standard

A
  • So produces single absorption peak
    • Well away from most other absorption peaks
  • Inert (doesn’t react with sample)
  • Non-toxic
  • Volatile (easy to remove from sample)
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16
Q

What is chemical shift measured in?

A

parts per million (ppm) relative to TMS

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17
Q

State the chemical shift of the single peak produced by TMS

A

0

18
Q

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

A

∵ TMS is added to test compound for calibration purposes

19
Q

Draw tetramethylsilane (TMS)

A
20
Q

What does the number of peaks in 13C NMR tell you?

A

no. of different carbon environments in molecule

21
Q

State the number of carbon environments in cyclohexane-1,3-diol

A
22
Q
A
23
Q

What does 1H NMR tell you?

A

Tells you how many environments and how many hydrogens are in each

(1H NMR is about how hydrogen nuclei react to magnetic field)

24
Q

What is each peak in 1H NMR spectrum due to?

A

one or more hydrogen nuclei (protons) in a particular environment

25
Q

1H NMR

What does the numbers above the peaks on a spectrum tell you?

A
  • The ratio of areas under peaks
    • This relative area under each peak = relative number of H atoms in each environment
26
Q

1H NMR: Splitting Patterns

What is meant by spin-spin coupling?

A

When peaks are split into smaller peaks

27
Q

1H NMR

What is the n+1 rule?

A

Peaks always split into number of hydrogens on the neighbouring carbon, plus one

28
Q
A
29
Q
A
30
Q

1H NMR

What are used instead of peak ratios to show the area more clearly?

A

Integration Traces

31
Q

Explain how integration traces work

A

The increases in height are proportional to areas under each peak

32
Q

Describe how you can calculate a integration ratio from integration traces

A

Can use ruler to measure height of each vertical bit of trace and then use heights to work out ratio of peak areas

33
Q

State the integration ratio for this spectrum and what it means

A
  • 1:2
  • Means there’s 1 H atoms in 1st environment for every 2 H atoms in 2nd environment
34
Q

1H NMR

What solvents are used to dissolve samples?

A

Deuterated solvents

35
Q

What are the hydrogen atoms in deuterated solvents replaced by?

A

D or 2H

36
Q

What is deuterium?

A

Isotope of H that has 2 nucleons (proton and neutron)

37
Q

Why does deuterium not have a spin (and doesn’t create a magnetic field)?

A

∵ deuterium has even no. of nucleons

38
Q

1H NMR

Name a solvent that is used to dissolve samples

A

CCl4

39
Q

Explain why CCl4 can be used as a solvent to dissolve a sample

A

∵ doesn’t contain any 1H atoms

40
Q

Two isomers of C6H12O2 are both esters. Each isomer has only two peaks in their 1H NMR spectrum. The integration ratio for both esters is 3:1. Draw the two esters. (2)

A
41
Q

Another isomer of C6H12O2 is a carboxylic acid with a chiral centre. This isomer has five peaks in its 13C NMR spectrum. Draw the structure of this isomer.

A
42
Q

What solvent should you use when obtaininng the 1H NMR spectrum of an amino acid?

A

D2O