MD - NMR

Question 1

What does NMR stand for?

Answer 1

Nuclear Magnetic Resonance

Question 2

Why do hydrogen nuclei have spin?

Answer 2

Because nuclei with odd numbers of nucleons have spin

Question 3

What is spin?

Answer 3

The intrinsic angular momentum of a subatomic particle, causing particles with spin to have a weak magnetic field

Question 4

Describe the alignment of protons with and without being acted upon by an external magnetic field

Answer 4

• Without: the protons are spinning in random directions so their magnetic fields cancel out
• With: the protons align themselves either in the direction of the field or in the opposite direction to the field

Question 5

Explain why protons in an external magnetic field absorb or emit radiowaves

Answer 5

• The protons in the magnetic field either become aligned with or against the magnetic field, where the protons aligned against the magnetic field are at a higher energy level
• Protons can move between the energy levels by emitting or absorbing radio waves

Question 6

Explain why there is an overall absorption of energy for molecules in a magnetic field

Answer 6

Because there tends to be more protons aligned with the magnetic field than against it, so more radio waves are absorbed than emitted to change energy levels

Question 7

What causes protons to absorb different amounts of energies and why?

Answer 7

Protons are shielded from the external magnetic field by surrounding electrons, which differs due to the different environments they are in
The protons in different environments are affected differently by the magnetic field so absorb different amounts of energy at different frequencies

Question 8

What is the standard substance that chemical shift is relative to? Give its molecular formula

Answer 8

Tetramethylsilane (TMS)

Si(CH₃)₄

Question 9

Why is TMS used as the standard substance?

Answer 9

Because it has 12 protons with identical environments, so only produces a single absorption peak, well away from most other molecules

Question 10

What does the number of peaks show about the molecule?

Answer 10

The number of proton environments

Question 11

What does the relative area under each peak show about the molecule?

Answer 11

The number of protons in that environment

Question 12

What type of solvents must be used to dissolve samples for NMR and why?

Answer 12

Solvents which have no single protons - so they don’t show up on the spectrum

Question 13

What are 2 common solvents for NMR?

Answer 13

1) Deuterated solvents - hydrogen atoms have been replaced by deuterium
2) CCl₄

Question 14

What type of NMR must be used to notice spin-spin coupling?

Answer 14

High resolution NMR

Question 15

What causes spin-spin coupling and what does this show about the molecule?

Answer 15

Spin-spin coupling is due to the magnetic fields of neighbouring single protons on adjacent carbon atoms interacting with each other
It shows the number of protons on adjacent carbon atoms?

Question 16

State the n+1 rule for NMR

Answer 16

Multiplets are always split into the number of neighbouring protons plus one (n+1)

Question 17

What is the name given for a single NMR signal split into:

i) 1 peak
ii) 2 peaks
iii) 3 peaks
iv) 4 peaks
v) >4 peaks

Answer 17

i) Singlet
ii) Doublet
iii) Triplet
iv) Quartet
v) Multiplet

Question 18

What 2 environments do proton sources not split in and what type of peak do they give?

Answer 18

OH and NH

Singlet

Question 19

What 3 things does an NMR spectrum show about a molecule and how?

Answer 19

1) The different proton environments (from the chemical shifts)
2) The relative number of protons in each environment (from the relative peak area)
3) The number of protons adjacent to a particular proton (from the splitting pattern)