3.15 Nuclear Magnetic Resonance Spectroscopy

Question 1

Name 2 types of NMR

Answer 1

• ¹H (or proton) NMR
• ¹³C NMR

Question 2

What type of atomic nuclei spin?

Answer 2

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

Question 3

What does nuclear spin cause an atom to have?

Answer 3

a weak magnetic field

Question 4

Why do hydrogen nuclei have spin?

Answer 4

Hydrogen nuclei are single protons

Question 5

Why doesn’t carbon have nuclear spin?

Answer 5

C usually has 6 protons and 6 neutrons

Question 6

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

Answer 6

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

Question 7

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

Answer 7

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

Question 8

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)?

Answer 8

Will decrease

Question 9

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

Answer 9

∵ in different environments, nuclei will feel different magnetic fields

Question 10

What does NMR spectroscopy essentially look for?

Answer 10

Differences in energy absorption between environments

Question 11

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

Answer 11

2 atoms must be joined to exactly the same things

Question 12

What substance is used as a standard in NMR spectroscopy?

Answer 12

Tetramethylsilane (TMS), Si(CH₃)₄

Question 13

What is chemical shift (δ)?

Answer 13

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

Question 14

Tetramethylsilane has 12 H atoms in ____ ________

Answer 14

identical environments

Question 15

Name 5 reasons why TMS is used as a standard

Answer 15

• 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)

Question 16

What is chemical shift measured in?

Answer 16

parts per million (ppm) relative to TMS

Question 17

State the chemical shift of the single peak produced by TMS

Answer 17

0

Question 18

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

Answer 18

∵ TMS is added to test compound for calibration purposes

Question 19

Draw tetramethylsilane (TMS)

Answer 19

Question 20

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

Answer 20

no. of different carbon environments in molecule

Question 21

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

Answer 21

Question 22

Answer 22

Question 23

What does ¹H NMR tell you?

Answer 23

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

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

Question 24

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

Answer 24

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

Question 25

**¹H NMR** What does the numbers above the peaks on a spectrum tell you?

Answer 25

* The ratio of areas under peaks * This relative area under each peak = relative number of H atoms in each environment

Question 26

**¹H NMR: Splitting Patterns** What is meant by spin-spin coupling?

Answer 26

When peaks are split into smaller peaks

Question 27

**¹H NMR** What is the n+1 rule?

Answer 27

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

Question 28

Answer 28

Question 29

Answer 29

Question 30

**¹H NMR** What are used instead of peak ratios to show the area more clearly?

Answer 30

Integration Traces

Question 31

Explain how integration traces work

Answer 31

The increases in height are proportional to areas under each peak

Question 32

Describe how you can calculate a integration ratio from integration traces

Answer 32

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

Question 33

State the integration ratio for this spectrum and what it means

Answer 33

* 1:2 * Means there's 1 H atoms in 1^st environment for every 2 H atoms in 2^nd environment

Question 34

**¹H NMR** What solvents are used to dissolve samples?

Answer 34

Deuterated solvents

Question 35

What are the hydrogen atoms in deuterated solvents replaced by?

Answer 35

D or ²H

Question 36

What is deuterium?

Answer 36

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

Question 37

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

Answer 37

∵ deuterium has even no. of nucleons

Question 38

**¹H NMR** Name a solvent that is used to dissolve samples

Answer 38

CCl₄

Question 39

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

Answer 39

∵ doesn't contain any ¹H atoms

Question 40

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)

Answer 40

Question 41

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.

Answer 41

Question 42

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

Answer 42

D₂O