B - Spectroscopy - NMR

Question 1

What happens to nuclei in an external magnetic field, B₀?

Answer 1

Nuclei align either parallel or anti-parallel. The parallel position is slightly lower in energy and so is favoured.

Question 2

What is the Larmor frequency, V_L?

Answer 2

The nuclear spin axis is tilted with respect to the external field direction, so it precesses around B₀.

The frequency at which this occurs is the Larmor frequency.

Question 3

What is V_L determined by?

Answer 3

The gyromagnetic ratio, γ.

This is constant for each nucleus, but different depending on the nucleus in question and B₀.

Question 4

How can spin-flips occur?

Answer 4

If the nucleus is irradiated with EM radiation in the radio wave region of the correct frequency, the nuclei will absorb the energy. The lower energy state spin-flips to the higher energy state

Question 5

How is the total nuclear spin quantum number, I, determined?

Answer 5

It is determined by the number and relative proportions of protons and neutrons.

Usually only nuclei with an odd mass number are NMR active, and I can equal = 0, 1/2, 1, 3/2, 2, 5/2 etc

Question 6

How is the number of quantised spin states worked out?

Answer 6

2I + 1

therefore spin 1/2 -> 2 x 1/2 + 1 = 2

so spin 1/2 nuclei can have 2 possible spin states, spin up or spin down

Question 7

What is the equation for the difference in energy between two spin states?

Answer 7

E = γ x B_​0 x h / 2π

Question 8

What is the equation for Lamor frequency, V_L?

Answer 8

As E = h x V_L

V_L = γ x B_​0 / 2π

Question 9

What is the solvent used in NMR?

Answer 9

Deuterated chloroform, CDCl₃ (trichloromethane)

Question 10

What is the standard used in NMR, and why is it used?

Answer 10

Trimethyl silane, TMS, Si(CH₃)₄

• unreactive
• liquid and mixes with common solvents
• gives a single NMR peak, which occurs at a lower frequency than most others
• highly volatile, so it can be evaporated off to recover the sample

Question 11

What effect do electron clouds have on absorbing radio waves?

Answer 11

Electrons cause a small magnetic field that opposes B_​0.

This is felt by the nucleus, meaning the NMR frequency is slightly lowered.

This is known as shielding.

Question 12

What is the equation for shielding?

Answer 12

Shielding = σ x B_​0, where σ = shielding constant

Question 13

What is the equation for the effective field felt by a shielded nucleus?

What rules allow us to calculate this?

Answer 13

B_eff = B₀ ( 1 - σ )

Slater’s rules

Question 14

What is the equation for the Larmor frequency of a shielded nucleus?

Answer 14

V_L = γ x B_eff / 2π

= γ x B₀ ( 1 - σ ) / 2π

Question 15

What is the effect of shielding on the NMR frequency?

Answer 15

Shielding causes a chemical shift of the NMR frequency.

Nuclei in different bonding environments experience different levels of shielding and, therefore, experience different magnetic fields, so they will absorb different frequencies.

The frequency is usually smaller than expected for an unshielded nucleus in the same magnetic field.

Question 16

What is the inductive effect?

Answer 16

Protons bound in molecules will experience more or less shielding depending on the electron donating/withdrawing effect of neighbouring groups.

Electron donating = increases electron shielding around the proton -> NMR frequency goes down

Electron withdrawing = decreases electron shielding around the proton -> NMR frequency goes up

Shielding and NMR frequency are inversely proportional; low shielding means a high frequency / greater chemical shift

Question 17

What is the equation to work out chemical shift?

Answer 17

Chemical shift, δ = ( ν_sample - ν_ref) / ν_ref x 10⁶

Question 18

Where do protons attached to -CH₂ or -CH₃ groups appear?

Answer 18

These protons are highly shielded and so lie close to the TMS peak.

They appear δ 1 - 4 ppm.

They may be affected by neighbouring atoms eg. O

Question 19

Where do protons attached to O and N appear?

Answer 19

O -> protons are less shielded due to the electronegativity of O

2 - 5 ppm.

N -> appear in a similar range to -OH. but the peak is often broadened due to magnetic interactions with 14N, 15N nuclear spins

Question 20

Where do protons attached to -CHO and -COOH appear?

Answer 20

These protons are highly deshielded due to the neighbouring O atoms, so appear δ 9 - 10 ppm

Question 21

Where do protons in aromatic systems appear?

Answer 21

Electrons travelling around the ring in π orbitals generate a ring current; this usually adds to B₀.

They will appear δ 8 - 9 ppm.

In some highly conjugated or aromatic molecules, H atoms may be inside the ring; the ring current then opposes B_​0, causing negative δ values

Question 22

What is the NMR fine structure?

Answer 22

Absorption peaks consist of multiple very narrow lines that are best observed in high-resolution NMR.

Question 23

What is spin-spin coupling?

Answer 23

Adjacent protons are in different shielding environments, so their magnetic environments are non-equivalent.

When one proton flips its spin, the other proton can be in its up or down state; this causes the magnetic field felt by the original proton to be slightly different, causing two closely spaced lines to be observed.

Question 24

What does the total integrated area under each peak correspond to?

Answer 24

It corresponds to the number of protons producing that signal.

Question 25

What is the coupling constant, J?

Answer 25

J measures the gap between splitting peaks.

The amount of splitting depends upon the strength of the magnetic interaction between the protons, which corresponds to the magnitude of J.

Question 26

What is 2D NMR and what is it used for?

Answer 26

It uses radio wave pulses and computer processing to produce contour maps.

This is used to show the correlation between ¹H NMR and ¹³C NMR.

This is often used to identify large biomolecules.

Question 27

What is MRI and how is it used?

Answer 27

Magnetic Resonance Imaging

This looks at protons in H₂0 molecules.

It uses pulsed NMR to observe the relaxation time of protons orientated by the magnetic field.

Different regions are distinguished by the mobility of H₂0 molecules, so it can be used to look at soft tissue rather than bone.

Multiple layers can be looked at to produce a 3D image.

Question 28

What if fMRI?

Answer 28

Functional Magnetic Resonance Imaging

fMRI looks at the magnetic spin state of Fe in oxygenated Hb during increased O₂ consumption.

This is able to map brain activity in real time.

Question 29

What is a PET scan?

Answer 29

Positron emission tomography

Detects the decay of radioactive 15O -> 122s

Emits e+

Question 30

How does a multiplet arise?

Answer 30

It arises due to magnetic spin-spin coupling between the spins of protons that are on adjacent carbons in a structure, and that are in magnetically non-equivalent environments.

Multiplet patterns follow the n+1 rule, where n is the number of ¹H nuclei on the neighbouring carbon atom

Question 31

Why do NMR signals from protons attached to O or N always appear as broad singlets?

Answer 31

The magnetic coupling from which the multiplet arises does not occur across heteroatoms such as O or N

Protons attached to heteroatoms may be easily exchanged and can H-bond.

The chemical exchange which is also responsible for the broadening of the signal is mediated and accelerated by traces of water, acid and/or base.

When the exchange is very rapid on the NMR timescale, the spin states of protons on the heteroatom are averaged out and the signal appears as a broad singlet.

The rate of chemical exchange can be slowed down by cooling and removing all traces of the aforementioned substances.

Question 32

What causes the broadening of an OH NMR peak?

Answer 32

Intermolecular and intramolecular H-bonding and chemical exchange.

Question 33

How does NMR work?

Answer 33

Nuclei like 1H and 13C have a property called ‘spin’.

The spin results in the nucleus behaving like a tiny magnet, with N and S poles.

If the molecule is placed in an external magnetic field, the spins line up with the field direction.

If the sample is irradiated with EM radiation, it can cause the spins to ‘flip’.

The spin flips have a particular energy that depends on: - the size of the external field - the type of nucleus - the local environment of each atom.

The spin-flip energies occur in the radio wave region

We use radio transmitters and receivers to generate and detect the spin flip energies, forming an NMR spectrum

Different nuclei within a given molecule resonate at different positions

They can also interact with other magnetic nuclei in different ways, to produce characteristic patterns