3.3.15 Nuclear Magnetic Resonance Spectroscopy

Question 1

Which elements flip under radio waves?

Answer 1

Any which has an odd number of nucleons e.g. hydrogen

Question 2

What happens when a nuclei flip from the beta position to the alpha position?

Answer 2

(Can only happen once the radio waves have been removed)

The energy is released

Question 3

What happens when atoms with an odd number of nucleons are exposed to radio waves?

Answer 3

They will line up and the nuclei align with the magnetic field
If you supply enough radio energy, the atom in the sample will flip and stay in the wrong orientation (beta)

Question 4

How can nuclear magnetic resonance be used?

Answer 4

The amount of energy given out is proportional to the number of hydrogen atoms
The larger the chemical shift, the more energy is given out

Question 5

How does electronegativity affect shielding?

Answer 5

The stronger the electronegativity, the more deshielded the nucleus is due to a decreased electron density

Question 6

What effect does increased deshielding have on nuclear magnetic spectroscopy?

Answer 6

Once deshielded, the atom is less electronegative so requires more energy to be flipped by the magnets

Question 7

How is an NMR machine structured?

Answer 7

See card

Question 8

Are nuclei with more electrons around them more or less shielded?

Answer 8

More

Question 9

What is a proton also known as?

Answer 9

Hydrogen

Question 10

What is a hydrogen atom also known as?

Answer 10

Proton

Question 11

How does the electron density relate to the resonating frequency of an atom?

Answer 11

The greater the electron density
The smaller the magnetic field felt by the nucleus
The lower the frequency at which it resonates

Question 12

How does a more deshielded nucleus affect the chemical shift?

Answer 12

The nucleus feels a greater magnetic field

Therefore there is a higher chemical shift

Question 13

What is being examined in proton NMR?

Answer 13

The H+ nucleus that is being examined

Question 14

What is the chemical shift?

Answer 14

It tells us the strength of the magnetic field that the nucleus feels
Related to the difference between the resonating nucleus to zero

Question 15

What occurs to the chemical shift when a hydrogen atom is further from an electronegative atom?

Answer 15

The smaller the shift

Question 16

What does the area under the proton NMR relate to?

Answer 16

The number of hydrogen atoms

Question 17

What does the integration trace give?

Answer 17

The area of the peaks

Question 18

What is spin-spin splitting?

Answer 18

When the magnetic field felt by a hydrogen atom is affected by magnetic fields of neighbouring hydrogen atoms

Question 19

What is shielding?

Answer 19

Where electrons in an atom can shield each other from the pull of the nucleus
The more electron shells there are, the greater the shielding effect experienced by the outermost electron

Question 20

What do the peaks look like when there is one hydrogen atom on the adjacent atom?

Answer 20

1 : 1

Question 21

What do the peaks look like when there are two hydrogen atoms on the adjacent atom?

Answer 21

1 : 2 : 1

Question 22

What do the peaks look like when there are three hydrogen atoms on the adjacent atom?

Answer 22

1 : 3 : 3 : 1

Question 23

If there are n hydrogen atoms on an adjacent carbon atom, how many peaks will that be?

Answer 23

n + 1

Question 24

Why is Deuterium used as an NMR solvent?

Answer 24

It is an isotope of hydrogen which doesn’t produce an NMR signal in the same range as hydrogen

Question 25

What are the two types of NMR?

Answer 25

13C NMR and proton NMR

Question 26

Why is no splitting seen in 13C NMR?

Answer 26

Because there is a low concentration of 13C compared to 12C, it is unlikely that two 13C atoms would be next to eachother
Therefore the signal is not changed

Question 27

How do you work out the number of peaks in H NMR?

Answer 27

Add the number of hydrogens on adjacent carbons then add one

Question 28

What are the first five ratios for splitting peaks?

Answer 28

1
1 : 1
1 : 2 : 1
1 : 3 : 3 : 1
1 : 4 : 6 : 4 : 1

Question 29

Why does carbon 12 not respond to NMR?

Answer 29

It’s nucleus don’t have a spin

Question 30

How is shielding linked to chemical shift?

Answer 30

Shielding decreases the nucleus’ pull on other electrons
This causes a lower electronegativity
Therefore the nucleus feels a weaker magnetic field
Therefore there is a reduced chemical shift